Pathogens with Potential for Weed Control - ACS Symposium Series

Chapter 7

Pathogens

with Potential for Weed Control R. Charudattan

Plant Pathology Department, University of Florida, Gainesville, F L 32611

Biological control with plant pathogens is an effective, safe, selective, and practical means of weed management that has gained considerable impor tance. The recent success in using certain rust and smut fungi as classical biocontrol agents, the registration and commercial use of two facultatively parasitic fungi as mycoherbicides in the United States, and the potential registration of two more mycoherbicides in the USA and Canada have further stimulated worldwide interest in biological weed control with plant pathogens. Approximately 45 species of fungi are reported to be under evaluation or development in 19 countries and 44 locations against about 65 weeds. Included are 14 of the top 18 world's worst weeds, and several others of regional and specific importance. Among pathogens with potential, several species have been found that attack more than one weed species. Efforts should be made to develop these pathogens as broad-spectrum mycoherbicides through strain selection or genetic engineering. Certain weed-pathogen systems have been shown to be suitable for low-input weed control technology that could be developed with public support and dispensed through user's efforts. Many others may be suitable for specialty or niche markets in which the target weed lacks suitable chemical or other means of control. It is anticipated that public institutions and private industries will continue to support research and development, respectively, of classical and myco herbicidal candidates and thus promote this field of alternative weed control methods. NOTE: This chapter Is Florida Agricultural Experiment Stations Journal Series No. 9986.

0097-6156/90A)439-0132$06.75/0 © 1990 American Chemical Society

7. CHARUDATTAN

Pathogens wiûi PoUnualJor Weed

133

T h i s c h a p t e r f o c u s e s on pathogens t h a t h o l d most p o t e n t i a l f o r p r a c t i c a l use i n t h e near f u t u r e , and t h e r e s e a r c h and i n d u s t r i a l t r e n d s t h a t appear t o i n f l u e n c e t h i s p o t e n t i a l i t y . S i n c e 1970, considera b l e p r o g r e s s has been made towards p r a c t i c a l use o f p l a n t pathogens as s a f e and s e l e c t i v e a g e n t s o f weed management ( 1 - 3 ) . The i n t e r e s t i n u s i n g p l a n t pathogens i s i n c r e a s i n g ( 4 , 5 ) p a r t l y because t h i s a p p r o a c h o f f e r s an e x p l o i t a b l e b i o t e c h n o l o g y and i s an e f f e c t i v e supplement t o c o n v e n t i o n a l weed c o n t r o l based on c h e m i c a l and mechan i c a l methods. Often the l a c k of s u i t a b l e chemical h e r b i c i d e s s e r v e s as an impetus f o r b i o l o g i c a l c o n t r o l s . This could r e s u l t from t h e u n a v a i l a b i l i t y o f c h e m i c a l h e r b i c i d e s , u n s u i t a b l e p e r f o r mance o f e x i s t i n g c h e m i c a l s i n terms o f e f f i c a c y o r e n v i r o n m e n t a l s a f e t y , t h e i r h i g h c o s t to r e t u r n r a t i o , or the b u i l d u p of r e s i s t a n t weed b i o t y p e s . Under s u c h s i t u a t i o n s , pathogens c o u l d o f f e r s p e c i f i c i t y i n c o n t r o l , e n v i r o n m e n t a l s a f e t y , and l e s s d r a s t i c changes i n t h e ecosystem compared t o c h e m i c a l h e r b i c i d e s . They may a l s o be l e s s c o s t l y t o d e v e l o p a s h e r b i c i d e s t h a n c h e m i c a l s (6). Microbial pathogens o f f e r s e v e r a l a d d i t i o n a l b i o l o g i c a l and t e c h n i c a l a d v a n t a g e s w h i c h a r e r e v i e w e d i n C h a p t e r 18 i n t h i s book. Examples o f weed c o n t r o l o b t a i n a b l e w i t h pathogens a r e l i s t e d i n T a b l e I . I n c l u d e d i n t h i s t a b l e a r e a g e n t s t h a t a r e p r e s e n t l y used o r a r e e x p e c t e d t o be used as c o m m e r c i a l h e r b i c i d a l p r o d u c t s as w e l l as d i f f e r e n t t y p e s o f p a t h o g e n s , weeds, and h a b i t a t s . S e l f - d i s s e m i n a t i n g pathogens ( e . g . , r u s t s , smuts, and c e r t a i n d r y - s p o r e d f o l i a r f u n g i ) a s w e l l as p o o r l y d i s s e m i n a t i n g pathogens such as s o i l - b o r n e and raucoid-spored f u n g i a r e b e i n g d e v e l o p e d f o r use. The former a r e g e n e r a l l y i n t e n d e d t o be used a s c l a s s i c a l agents capable of causing s e l f - s u s t a i n i n g epidemics f o l l o w i n g i n o c u lative releases. T y p i c a l l y , they a r e s p r e a d by s p o r e s o r o t h e r i n f e c t i v e p r o p a g u l e s t h a t a r e d i s s e m i n a t e d by w i n d , w a t e r , o r i n s e c t vectors. The l a t t e r a r e commonly pathogens t h a t have c o - e v o l v e d w i t h t h e i r weed h o s t s i n p a r t i c u l a r r e g i o n s r e s u l t i n g i n h o m e o s t a t i c r e l a t i o n s h i p s c h a r a c t e r i s t i c o f endemic d i s e a s e s . N o r m a l l y , they do not produce s e l f - s u s t a i n i n g e p i d e m i c s o f d e s t r u c t i v e p o t e n t i a l s u f f i c i e n t t o a f f o r d weed c o n t r o l . But by a p p l y i n g i n u n d a t i v e d o s e s o f i n o c u l u m , as b i o h e r b i c i d e s , t h e y can be used t o c r e a t e a temporary but h i g h l y d e s t r u c t i v e l e v e l o f d i s e a s e ( 3 ^ . I t i s also possible to use n a t i v e r u s t f u n g i i n an augmentative s t r a t e g y t o i n c r e a s e t h e l e v e l o f e p i d e m i c and weed c o n t r o l (25) o r t o i n c r e a s e b i o t i c s t r e s s on t h e weed t h e r e b y augmenting c r o p c o m p e t i t i o n towards t h e weed (26). Inoculative

(Classical Biocontrol)

Agents

G e n e r a l l y , t h e f o l l o w i n g c o n s i d e r a t i o n s d e t e r m i n e the d e c i s i o n t o seek and d e p l o y c l a s s i c a l ( i n o c u l a t i v e ) a g e n t s : (1) The weed i s an e x o t i c , l a c k i n g n a t i v e p a t h o g e n s ; (2) no a l t e r n a t i v e c o n t r o l s a r e a v a i l a b l e ; (3) s u f f i c i e n t economic i n c e n t i v e e x i s t s t o j u s t i f y b i o c o n t r o l e f f o r t s ; (4) e p i d e m i o l o g i c a l l y , t h e e c o - c l i m a t i c f a c t o r s i n t h e r e g i o n o f i n t r o d u c t i o n i n d i c a t e r e a s o n a b l e chances o f s u c c e s s o f t h e c a n d i d a t e p a t h o g e n s ; (5) no c o n f l i c t o f i n t e r e s t e x i s t s , i.e., everyone i s i n f a v o r o f c o n t r o l l i n g the weed; and (6) the i n t r o d u c e d pathogens a r e e x t r e m e l y h o s t s p e c i f i c . Historically, inoculative a g e n t s have been used a g a i n s t e x o t i c weeds growing i n a r e a s t h a t a r e

134

MICROBES AND MICROBIAL PRODUCTS AS HERBICIDES

Table I .

Pathogen

Disease/Damage Pathogen Type

Alternaria

cassiae

Alternaria

sp.

0

A r a u j l a mosaic v i r u s Cercospora rodmanii Chondrostereum purpureum Colletotrichum orbiculare ( B e r k , e t Mont.) v . A r x . Ç. g l o e o s p o r i o i d e s f . s p . aeschynomene" C. gloeosporioides f . s p . malvae" Entyloma compositarum Fusarium culmorum F.

solani f.sp. cucurbitae Phvtophthora p a l m i v o r a ^ Puccinia chondrillina

Examples o f Weed

and

F o l i a r b l i g h t , seedling k i l l , i m p e r f e c t fungus F o l i a r b l i g h t , seedling k i l l , i m p e r f e c t fungus Systemic mosaic, reduced p l a n t productivity, virus L e a f - s p o t , reduced p l a n t p r o d u c t i v i t y , i m p e r f e c t fungus W i l t , wood-rot, plant k i l l , basidiomycete Stem and l e a f l e s i o n s , a n t h r a c n o s e , s e e d l i n g death Anthracnose, shoot and p l a n t k i l l i m p e r f e c t fungus Anthracnose, shoot and p l a n t k i l l , i m p e r f e c t fungus Leaf b l i g h t , d e f o l i a t i o n smut fungus C h l o r o s i s , p l a n t k i l l and l y s i s , i m p e r f e c t fungus Vascular w i l t , plant k i l l , i m p e r f e c t fungus Root r o t , p l a n t death d e a t h , phycomycete F o l i a r p u s t u l e s , reduced p l a n t p r o d u c t i v i t y , r u s t fungus

'Recalculated or compiled from the data in the references cited. Control « reduction in weed density, weed problem, or cost savings; avenges or the range of reported values are presented here. ^Registered or expected soon to be registered as commercial mycoherbkkSes. Eight years after release of the fungus. Laboratory data only, Four years after release of the fungus. c

d

c

7. CHARUDATTAN

Pûtkogau with Patmtialfor Weed Omtrtd

135

C o n t r o l Obtained w i t h P l a n t Pathogens

Weed

Plant Type

% Control

Cassia o b t u s i f o l i a L . (slcklepod) Carduus pycnocephalus L . (Italian thistle) Morrenia odorata (stranglervine) Eichhornia crassipes (waterhyacinth) Prunus s e r o t i n a (black cherry) Xanthium spinosum L . (spiny cocklebur) Aeschynomene v i r g i n i c a (northern jointvetch) Malva p u s i l l a ( r o u n d - l e a v e d mallow) Ageratina r i p a r i a (Hamakua-pamakani) Hydrilla verticillata (hydrilla) C u c u r b i t a texana (Texas gourd) Morrenia odorata (stranglervine) C h o n d r i l l a -juncea (skeletonweed)

Annual herb

80

7

Annual herb

0-100

8

Perennial

26

9

vine

c

8

Ref.

Floating aquatic macrophyte Tree

11-99

10

90

11

Annual herb

70

i l

Annual h e r b , semiaquatic Annual herb

76-100

12.

73

15

Annual herb

50-95°

16,

Submerged a q u a t i c macrophyte Annual v i n e

100

18

20-97

li.

20

Perennial

90-100

21,

22

88

23,

24

vine

P e r e n n i a l herb

e

d

14

17

136


r e l a t i v e l y unmanaged, i n a c c e s s i b l e , o r o f low economic r e t u r n . The f o l l o w i n g a r e examples o f s u c c e s s f u l c l a s s i c a l i n t r o d u c t i o n s which are s t i l l e f f e c t i v e . S i n c e s u c c e s s w i t h a c l a s s i c a l agent i n one r e g i o n o f t h e w o r l d t e n d s t o promote t h e use o f t h e same agent i n o t h e r r e g i o n s a g a i n s t t h e same o r c l o s e l y r e l a t e d t a r g e t weed, t h e s e pathogens may f i n d a d d i t i o n a l use i n o t h e r a r e a s i n t h e f u t u r e . P u c c i n i a c h o n d r i l l i n a Bubak. & S y d . , a r u s t fungus from E u r o p e , d e t e r m i n e d t o be a v i r u l e n t , h o s t - s p e c i f i c , and s a f e b i o c o n t r o l a g e n t , was i n t r o d u c e d i n t o A u s t r a l i a i n 1971 t o c o n t r o l C h o n d r i l l a juncea L . , r u s h skeletonweed ( 2 7 , 2 8 ) . I n 1976 i t was i n t r o d u c e d i n t o C a l i f o r n i a and l a t e r i n t o o t h e r w e s t e r n s t a t e s i n t h e U . S . (29-31). The b i o l o g y o f t h i s p a t h o g e n , weed c o n t r o l s u c c e s s , and assessment o f t h e economic impact o f c o n t r o l l i n g r u s h skeletonweed i n A u s t r a l i a have been r e v i e w e d ( 2 3 , 24, 2 7 - 3 1 ) . I n summary, t h e r u s t has been r e m a r k a b l y s u c c e s s f u l ( T a b l e I ) i n c o n t r o l l i n g one o f t h r e e b i o t y p e s o f t h e weed, t h e n a r r o w - l e a f t y p e , w i t h consequent b e n e f i c i a l r e t u r n s t o t h e l o c a l economy and e c o l o g y ( 2 4 ) . Recent e x p l o r a t i o n i n t h e M e d i t e r r a n e a n r e g i o n has y i e l d e d two r a c e s o f t h e r u s t t h a t a r e v i r u l e n t on t h e i n t e r m e d i a t e - l e a f t y p e . These have a l s o been i n t r o d u c e d i n t o A u s t r a l i a ( 3 2 ) . A d d i t i o n a l surveys are underway t o f i n d r a c e s v i r u l e n t on t h e b r o a d - l e a f t y p e a s w e l l ( 3 2 ) . As i n A u s t r a l i a , t h e r u s t i s t h e most e f f e c t i v e o f t h r e e i n t r o duced b i o c o n t r o l a g e n t s i n t h e U n i t e d S t a t e s ( 2 9 - 3 1 ) ; t h e o t h e r two a r e a g a l l midge, C y s t i p h o r a s c h m i d t i Rubsaamen, and a g a l l m i t e , Aceria chondrillae Canestrini. Only minor problems have been e n c o u n t e r e d f o l l o w i n g t h e i n t r o d u c t i o n and e s t a b l i s h m e n t o f t h e r u s t i n new a r e a s o f t h e w o r l d . These b e i n g t h e r e d u c e d economic r e t u r n s due t o t h e p r e v a l e n c e o f t h e r e s i s t a n t weed b i o t y p e s i n A u s t r a l i a and t h e r e l a t i v e l y modest e f f i c a c y o f t h e r u s t i n t h e U n i t e d S t a t e s . Phragmidium v i o l a c e u m ( S c h u l t z ) W i n t . , a European r u s t pathogen o f b l a c k b e r r i e s , Rubus s p p . , was i n t r o d u c e d i n t o C h i l e i n 1973 t o c o n t r o l Rubus c o n s t r i c t u s L e f . e t M . and R. u l m i f o l i u s S c h o t t . The two h o s t s d i f f e r e d i n t h e i r s u s c e p t i b i l i t y t o t h e r u s t ; R. u l m i f o l i u s was more s u s c e p t i b l e . The r u s t caused e x t e n s i v e d e f o l i a t i o n , a s i g n i f i c a n t d e c r e a s e i n seed p r o d u c t i o n , and i n c r e a s e d t h e i n v a s i o n o f canes by s e c o n d a r y p a t h o g e n s . About f i v e y e a r s a f t e r i t s i n t r o d u c t i o n , t h e pathogen had s p r e a d t o o v e r n e a r l y 800 km i n a n o r t h south d i r e c t i o n along the C h i l e a n coast p r o v i d i n g high l e v e l s of weed c o n t r o l ( 3 3 ) . I n 1984 t h i s fungus was r e p o r t e d t o have been i n t r o d u c e d u n l a w f u l l y i n t o A u s t r a l i a , presumably f o r c o n t r o l l i n g weedy European b l a c k b e r r i e s o f t h e R. f r u t i c o s u s L . a g g r e g a t e . The r u s t i s now s p r e a d i n g r a p i d l y and p r o v i d i n g c o n s i d e r a b l e b i o c o n t r o l (34). M e a n w h i l e , A u s t r a l i a n s c i e n t i s t s have d e t e r m i n e d i t t o be h o s t s p e c i f i c t o Rubus s p p . w i t h i n t h e Rosaceae ( 3 5 ) . Although i t i s t o o e a r l y t o a s s e s s t h e b i o c o n t r o l e f f i c a c y o f t h i s f u n g u s , and t h e r e i s t h e danger o f i n f e c t i n g c u l t i v a t e d b l a c k b e r r i e s and o t h e r e c o n o m i c a l l y i m p o r t a n t Rubus s p p . , t h e p r o s p e c t s f o r l e g i t i m a t e u s e of t h i s r u s t i n A u s t r a l i a a r e s a i d t o be good based on t h e o v e r a l l r i s k - b e n e f i t considerations (35). P u c c i n i a carduorum J a c k y , a r u s t fungus from T u r k e y , was r e l e a s e d i n 1987 on a l i m i t e d b a s i s i n V i r g i n i a t o c o n t r o l musk t h i s t l e , Carduus t h o e r m e r i Weinm. (=C. nutans L . s s p . l e i o p h y l l u s [ P e t r o v i c ] S t o j . & S t e f . ) , a problem i n about 15% o f a l l c o u n t i e s i n the U n i t e d S t a t e s and i n C a n a d a . Detailed studies i n quarantine

7. CHARUDATTAN

Pathogens with Potentialfor Weed Control

137

greenhouse r e v e a l e d t h a t t h e r u s t r a c e c o n s i d e r e d f o r i n t r o d u c t i o n i n t o t h e U n i t e d S t a t e s was c a p a b l e o f i n f e c t i n g g l o b e a r t i c h o k e ( C y n a r a scolymus L . ) and some n a t i v e , N o r t h American C i r s i u m s p p . However, t h e r u s t does n o t a t t a c k a r t i c h o k e under i t s n a t i v e c o n d i t i o n s i n E u r a s i a where i t c o e x i s t s w i t h a r t i c h o k e and s e v e r a l o t h e r members o f t h e C o m p o s i t a e . I t i s possible that the l i m i t e d suscept i b i l i t y o f a r t i c h o k e and t h e r e a c t i o n o f C i r s i u m s p p . , i s a g r e e n house anomaly ( 3 6 ) . Hence, t h e r u s t i s c o n s i d e r e d t o be s u f f i c i e n t l y h o s t s p e c i f i c and s a f e f o r u s e i n t h e U n i t e d S t a t e s ( 3 6 , 3 7 ) . I n i t i a l o b s e r v a t i o n s c o n f i r m t h a t t h e r u s t has o v e r w i n t e r e d and e s t a b l i s h e d i t s e l f i n V i r g i n i a ( B a u d o i n , A . B . and B r u c k a r t , W. L . p e r s o n a l c o m m u n i c a t i o n s ) , but i t i s t o o e a r l y t o a s s e s s i t s e f f e c t on t h i s t l e c o n t r o l . Entyloma compositarum F a r l o w , a smut f u n g u s , was i n t r o d u c e d from J a m a i c a i n t o H a w a i i i n 1974 t o c o n t r o l Hamakua pamakani, A g e r a t i n a r i p a r i a ( R e g e l ) K i n g & Robinson (=Eupatorium r i p a r i u m R e g e l ) , a weed i n p a s t u r e s , f o r e s t s , and r a n g e l a n d s ( 1 6 , 1 7 ) . I t i s capable o f c a u s i n g c h l o r o t i c t o r e d d i s h - b r o w n a n g u l a r l e s i o n s on t h e upper l e a f s u r f a c e and massive w h i t e s p o r u l a t i o n on t h e lower l e a f s u r face. Inoculum becomes a i r b o r n e d u r i n g c o o l , m o i s t ( >98% r e l a t i v e humidity) n i g h t s . Two t o t h r e e months a f t e r t h e p a t h o g e n ' s release, d e v a s t a t i n g e p i d e m i c s were r e c o r d e d i n dense s t a n d s o f t h e weed i n h i g h r a i n f a l l s i t e s i n Oahu, H a w a i i , and M a u i , r e d u c i n g weed p o p u l a t i o n s up t o 80%. E i g h t y e a r s a f t e r r e l e a s e , t h e smut i s r e p o r t e d t o have p r o v i d e d up t o 50% c o n t r o l even i n a r e a s w i t h low r a i n f a l l but c o o l temperatures (Table I ) ( 1 6 ) . No e n v i r o n m e n t a l o r n o n t a r g e t damage has r e s u l t e d from t h i s i n t r o d u c t i o n ( 1 6 ) . S e v e r a l o t h e r o b l i g a t e and f a c u l t a t i v e p a r a s i t e s have been r e p o r t e d a s p o t e n t i a l i n o c u l a t i v e b i o c o n t r o l a g e n t s ; t h e s e a r e summ a r i z e d i n T a b l e I I and r e v i e w e d p r e v i o u s l y ( 5 0 , 5 1 ) . I n u n d a t i v e Agents

(Microbial

Herbicides)

The key c o n s i d e r a t i o n i n t h e d e c i s i o n t o d e v e l o p m y c o h e r b i c i d e s i s the weed c o n t r o l e f f i c a c y o f t h e pathogen ( 5 2 ) . E f f i c a c y s h o u l d be a s s e s s e d by t h e s p e e d , amount, and ease o f weed c o n t r o l ( 5 2 ) . Fact o r s such a s t h e ease o f d i s e a s e i n i t i a t i o n and e p i d e m i c b u i l d - u p independent o f v e r y s p e c i a l i z e d e p i d e m i o l o g i c a l r e q u i r e m e n t s a l s o impinge upon t h e m y c o h e r b i c i d a l p o t e n t i a l o f p a t h o g e n s . Following e f f i c a c y , h o s t s p e c i f i c i t y o f t h e agent r a n k s next i n i m p o r t a n c e . A l t h o u g h pathogens w i t h i l l - d e f i n e d o r u n r e s t r i c t e d h o s t range a r e not recommended f o r b i o c o n t r o l u s e , h o s t s p e c i f i c i t y t o an extreme degree i s n o t needed i n t h e c a s e o f n a t i v e pathogens t h a t a r e t o be used i n s i t e - d i r e c t e d i n u n d a t i v e a p p l i c a t i o n s ( 5 3 ) . In f a c t , as w i l l be d i s c u s s e d l a t e r , a l e v e l o f h o s t n o n s p e c i f i c i t y i s d e s i r a b l e t o d e v e l o p m i c r o b i a l h e r b i c i d e s c a p a b l e o f c o n t r o l l i n g more t h a n one e c o n o m i c a l l y i m p o r t a n t weed. Several other considerations influence the p o t e n t i a l success of a mycoherbicide. F o r example, t h e organism s h o u l d grow w e l l i n c u l t u r e and y i e l d i n f e c t i v e m y c e l i a o r s p o r e s f o r i n o c u l u m . The i n o c u l u m must have good v i a b i l i t y and s h e l f l i f e , and y i e l d i n f e c t i o n s and d i s e a s e c y c l e s o v e r a range o f e n v i r o n m e n t a l c o n d i t i o n s , whether f o r m u l a t e d o r n o t . The pathogen a l s o must be g e n e t i c a l l y s t a b l e t o p r e c l u d e l o s s o f v i r u l e n c e o r a change i n h o s t r a n g e . S u r v i v a l of i n u n d a t i v e agents i s not a requirement. In f a c t , l a c k

Syd.

& Holw. (Jackson)

(Schub.)

See a l s o

ref.

50 and

51.

P . obtegens ( L i n k ) T u l . P. punctiformis ( S t r . ) Roehl P . x a n t h i i Schw. f . s p . a m b r o s i a trifidae Uromyce8 h e l i o t r o p i i S r e d i n s k i U . r u m i c i s (Schum.) W i n t . Uromycladium t e p p e r i a n u m ( S a c c . ) McAlp.

Melampsora e u p h o r b i a e Cast. P u c c i n i a abrupta D i e t . var. partheniicola Parmelee P. cardui-pycnocephali P . expansa L i n k P. jaceae Otth

(common

Biocontrol

tree)

38 %8 49

H . europaeum (common h e l i o t r o p e ) Rumex s p p . ( c u r l y d o c k s ) A c a c i a s a l i g n a ( L a b i l l . ) Wendl. ( a c a c i a

40

39

38

Réf.

41 42 43 44 45 46 47

( f a l s e ragweed)

Agents

Carduus p y c n o c e p h a l u s ( I t a l i a n t h i s t l e ) S e n e c i o jacobaea L . ( r a g w o r t ) Centaurea s o l s t i t i a l i s L . (yellow s t a r t h i s t l e ) C . d i f f u s a Lam, ( d i f f u s e knapweed) C i r s i u m a r v e n s e ( L . ) S c o p . (Canada t h i s t l e ) Ç . a r v e n s e (Canada t h i s t l e ) A m b r o s i a t r i f i d a ( g i a n t ragweed)

L.

( l e a f y spurge)

Parthenium hysterophorus

Euphorbia esula L .

H e l i o t r o p i u m europaeum L . heliotrope)

sp.

Cercospora

w i t h P o t e n t i a l as I n o c u l a t i v e

Weed

Pathogens

Pathogen

Table I I .

7. CHARUDATTAN

Pathogène with Potentialfor Weed Control

139

of prolonged s u r v i v a l i s d e s i r a b l e f o r s a f e t y . Short-term s u r v i v a l would m i n i m i z e i n o c u l u m b u i l d u p i n t r e a t e d s i t e s and t h e p o t e n t i a l f o r long-term genetic i n t e r a c t i o n s i n the f i e l d . F i n a l l y , economic c o n s i d e r a t i o n s , such a s t h e l a c k o f o t h e r s u i t a b l e c o n t r o l s f o r t h e t a r g e t weed; a p r o f i t a b l e market s i z e , and p a t e n t p r o t e c t i o n ; t e c h n i c a l f e a s i b i l i t y o f l a r g e - s c a l e p r o d u c t i o n ; and m a r k e t a b i l i t y w i l l d e t e r m i n e t h e u l t i m a t e d e c i s i o n t o r e g i s t e r and c o m m e r c i a l i z e p o t e n t i a l inundative agents. More d i s c u s s i o n on t h e s e p o i n t s c a n be found i n C h a p t e r 17 o f t h i s volume and i n Bowers ( 5 4 ) , C h u r c h i l l ( 5 5 ) , and Bannon ( 5 6 , 5 7 ) . P r e s e n t l y two m y c o h e r b i c i d e s a r e r e g i s t e r e d f o r c o m m e r c i a l use i n North America. These a r e : DeVine ( A b b o t t L a b o r a t o r i e s , N o r t h C h i c a g o , I L 60064) and C o l l e g o (Ecogen I n c . , L a n g h o r n e , PA 19047) used r e s p e c t i v e l y a g a i n s t s t r a n g l e r v i n e , M o r r e n i a o d o r a t a ( H . & A . ) L i n d l . , and n o r t h e r n j o i n t v e t c h , Aeschynomene v i r g i n i c a ( L . ) B . S . P . Two o t h e r s , C a s s t (Mycogen C o r p . , San D i e g o , CA 92121) and B i o M a l ( P h i l o m B i o s , S a s k a t o o n , Saskatchewan, Canada S7N 2 X 8 ) , a r e u n d e r g o i n g e v a l u a t i o n s f o r p o s s i b l e r e g i s t r a t i o n i n t h e near f u t u r e f o r c o n t r o l l i n g s i c k l e p o d , C a s s i a o b t u s i f o l i a L . , and r o u n d - l e a v e d m a l low, Malva p u s i l l a S m . , r e s p e c t i v e l y . In a d d i t i o n , several other pathogens have been e v a l u a t e d t o v a r i o u s e x t e n t s , i n c l u d i n g many t h a t have undergone f i e l d t e s t i n g and e v a l u a t i o n s under t h e E x p e r i m e n t a l Use P e r m i t (EUP e v a l u a t i o n s ) i s s u e d by t h e U . S . E n v i r o n m e n t a l P r o t e c t i o n Agency ( 5 8 , 5 9 ) . D e V i n e , d i s c o v e r e d and d e v e l o p e d by R i d i n g s , B u r n e t t , and o t h e r s , was t h e f i r s t r e g i s t e r e d m y c o h e r b i c i d e ( 6 0 - 6 2 ) . I t was r e g i s t e r e d i n 1981 f o r use a s a p o s t - e m e r g e n t d i r e c t e d s p r a y and i s m a r k e t e d a s a l i q u i d c o n c e n t r a t e o f chlamydospores o f a p a t h o t y p e o f Phytophthora palmivora ( B u t l e r ) B u t l e r n a t i v e t o F l o r i d a . The p a t h ogen i s c a p a b l e o f k i l l i n g s e e d l i n g s and a d u l t s t r a n g l e r v i n e , a weed o f South American o r i g i n found i n F l o r i d a ' s c i t r u s g r o v e s . Extens i v e h o s t range and e f f i c a c y t r i a l s done i n t h e l a b o r a t o r y and f i e l d s u p p o r t e d t h e s a f e t y and b i o c o n t r o l p o t e n t i a l o f t h i s fungus ( 6 0 ) . S a f e t y t o n o n t a r g e t s u s c e p t i b l e p l a n t s was a d d r e s s e d by l a b e l r e s t r i c t i o n s which p r e s c r i b e s e l e c t i v e , s i t e - s p e c i f i e d a p p l i c a t i o n s of the mycoherbicide. DeVine i s n o t t o be used where s u s c e p t i b l e n o n t a r g e t p l a n t s a r e grown o r o c c u r ( 2 2 ) , and t o d a t e no a d v e r s e n o n t a r g e t e f f e c t s have been r e p o r t e d . The l e v e l o f weed c o n t r o l ( T a b l e I ) has been good t o e x c e l l e n t (21, 22, 6 1 , 6 2 ) . However, t h e market f o r DeVine i s s m a l l , s p e c i a l i z e d , and c o n c e n t r a t e d i n t h e c i t r u s - g r o w i n g areas of F l o r i d a . C o l l e g o , based on C o l l e t o t r i c h u m g l o e o s p o r i o i d e s ( P e n z . ) S a c c . f . s p . aeschynomene, an a n t h r a c n o s e - i n c i t i n g pathogen o f n o r t h e r n j o i n t v e t c h d i s c o v e r e d i n A r k a n s a s , was d e v e l o p e d by D a n i e l , T e m p l e t o n , S m i t h , T e B e e s t , and c o - w o r k e r s i n c o l l a b o r a t i o n w i t h t h e Upjohn Company and t h e U . S . Department o f A g r i c u l t u r e (^t ,6» 63# 6 4 ) · The fungus i s c a p a b l e o f k i l l i n g b o t h s e e d l i n g and mature n o r t h e r n j o i n t v e t c h , a h a r d - s e e d e d n o n - n o d u l a t i n g leguminous weed i n r i c e and soybean c r o p s . The c o m m e r c i a l p r o d u c t , r e g i s t e r e d i n 1982, i s a w e t t a b l e powder o f d r i e d s p o r e s produced by l i q u i d f e r m e n t a t i o n ( 5 4 , 55, 6 4 ) . I t i s a p p l i e d post-emergence, a e r i a l l y or with land-based sprayers (14). The h i s t o r y , development, r e g i s t r a t i o n , i n t e g r a t e d u s e , and p o s t - r e g i s t r a t i o n s t a t u s o f C o l l e g o have been r e v i e w e d (2^, 54, 5 5 , 6 5 ) . C o l l e g o has p r o v i d e d c o n s i s t e n t l y h i g h l e v e l s o f weed

140


c o n t r o l ( T a b l e I ) and i s w e l l a c c e p t e d by u s e r s ( 6 4 ) « Although the fungus has a w i d e r h o s t range than o r i g i n a l l y thought ( 6 5 ) , and was found t o be c a p a b l e o f i n f e c t i n g but not k i l l i n g s e v e r a l e c o n o m i c a l l y i m p o r t a n t legumes ( 6 6 ) , i t has not posed any danger t o n o n t a r get p l a n t s under f i e l d u s e . I t s p r e a d s p o o r l y due t o the s t i c k y n a t u r e o f i t s s p o r e s , and the chance o f u n c o n t r o l l a b l e e p i d e m i c i s nonexistent. The i n o c u l u m d e c l i n e s t o v e r y low l e v e l s i n s o i l and i r r i g a t i o n water f o l l o w i n g i n u n d a t i v e a p p l i c a t i o n s , but t h e pathogen may p e r s i s t on i n f e c t e d h o s t t i s s u e and o v e r w i n t e r ( 6 7 , 6 8 ) . This low s u r v i v a l r a t e , w h i c h may be t y p i c a l f o r i n u n d a t i v e l y a p p l i e d i n o c u l u m , i s an added s a f e t y f e a t u r e o f m y c o h e r b i c i d e s , but i t a l s o n e c e s s i t a t e s a n n u a l a p p l i c a t i o n s f o r e f f e c t i v e weed c o n t r o l (2* 3)· No e n v i r o n m e n t a l o r u s e r h a z a r d s have been found w i t h C b l l e g o d u r i n g n e a r l y two decades o f use (2^). A l t e r n a r i a c a s s i a e J u r a i r & Khan, a dematiaceous f u n g a l p a t h ogen i n d u c i n g f o l i a r b l i g h t , was d i s c o v e r e d i n M i s s i s s i p p i . Walker and c o - w o r k e r s (2, 6 9 , 70) showed i t t o be s a f e and e f f i c a c i o u s against sicklepod. Subsequently, i n a two-year region-wide c o l l a b o r a t i v e f i e l d t r i a l , i t was demonstrated t o be h i g h l y e f f e c t i v e i n c o n t r o l l i n g t h e weed i n soybean ( T a b l e I I I ) ( 7 1 ) . I t has a f a i r l y narrow h o s t r a n g e , and i s c a p a b l e o f c o n t r o l l i n g t h r e e e c o n o m i c a l l y i m p o r t a n t leguminous weeds ( s i c k l e p o d , c o f f e e senna [ C a s s i a o c c i d e n t a l i s L . , and showy c r o t a l a r i a [ C r o t a l a r i a s p e c t a b i l i s R o t h , ] ) w h i c h are important i n the southeastern U n i t e d S t a t e s (70, 72). The f u n gus i s c u r r e n t l y under c o m m e r c i a l development as C a s s t , a w e t t a b l e powder f o r m u l a t i o n , by Mycogen C o r p o r a t i o n . D e t a i l s o f t h i s d e v e l opment a r e d i s c u s s e d i n C h a p t e r 17 o f t h i s book and e l s e w h e r e by Bannon ( 5 6 ) . C o l l e t o t r i c h u m g l o e o s p o r i o i d e s ( P e n z . ) S a c c . f . s p . m a l v a e , an a n t h r a c n o s e - c a u s i n g pathogen o f r o u n d - l e a v e d m a l l o w , i s b e i n g d e v e l oped a s a m y c o h e r b i c i d e by P h i l o m B i o s , a C a n a d i a n b i o t e c h n o l o g y company ( 1 5 , G a n t o t t i , B . and M o r t e n s e n , K . , p e r s o n a l communications). The f u n g u s , n a t i v e t o m i d - w e s t e r n C a n a d a , i s h i g h l y v i r u l e n t , v e r y e f f i c a c i o u s , and easy t o c u l t u r e and a p p l y w i t h c o n v e n t i o n a l equipment ( M o r t e n s e n , K . , p e r s o n a l c o m m u n i c a t i o n ) . Regist e r e d c h e m i c a l h e r b i c i d e s do n o t g i v e s a t i s f a c t o r y c o n t r o l o f t h i s weed, so the market o u t l o o k f o r t h i s m y c o h e r b i c i d e i s g o o d . The fungus i s a l s o p a t h o g e n i c t o v e l v e t l e a f , A b u t i l o n t h e o p h r a s t i M e d i c , a l t h o u g h i t s p o t e n t i a l t o c o n t r o l t h i s weed i s y e t t o be confirmed (15). S a f f l o w e r (Carthamus t i n c t o r i u s L . ) , an e c o n o m i c a l l y i m p o r t a n t p l a n t , i s s u s c e p t i b l e t o t h i s pathogen under optimum growth chamber c o n d i t i o n s . The r i s k t o s a f f l o w e r i s b e i n g a d d r e s s e d t h r o u g h e x t e n s i v e h o s t range and p a t h o g e n i c i t y t r i a l s ( M o r t e n s e n , K . , p e r s o n a l communication) t o d e c i d e the q u e s t i o n o f r i s k s v s . benefit. As p r e v i o u s l y s t a t e d , numerous pathogens a r e b e i n g i n v e s t i g a t e d as p o t e n t i a l b i o c o n t r o l a g e n t s f o r many a n n u a l and p e r e n n i a l weeds and i t would be i m p o s s i b l e t o r e v i e w a l l o f the r e s e a r c h h e r e . F u n g i r e p r e s e n t i n g n e a r l y a l l o f the major f u n g a l c l a s s e s a r e b e i n g studied (Charudattan, unpublished d a t a ) . A small p r o p o r t i o n of t h e s e c a n d i d a t e s may r e a c h c o m m e r c i a l o r p r a c t i c a l use i n the near term. A few i n t h i s c a t e g o r y , C e r c o s p o r a r o d m a n i i Conway ( f o r c o n t r o l l i n g waterhyacinth, Eichhornia crassipes [Mart.] Solms), F u s a r ium culmorum (W. G . Smith) S a c c . and an u n i d e n t i f i e d s c l e r o t i a l

7. CHARUDATTAN

141


Table I I I . Average P e r c e n t a g e o f S i c k l e p o d ( C a s s i a obtusifolia) C o n t r o l Obtained with A l t e r n a r i a c a s s i a e i n a Region-wide F i e l d Trial Year 1982

1983

No. o f Applications 1 2 1 2

Arkansas F l o r i d a

Mississippi

N.Carolina S.Carolina

a

25 85

81 94

78 74

100 100

86 78

95 98

97 80

98 100

°ί o

b

71 82

Data from t h e 2 1 s t day a f t e r t h e f i r s t a p p l i c a t i o n o f i n o c u l u m . 0ne or two a p p l i c a t i o n s o f t h e fungus a t 1 χ 10 c o n i d i a p e r ml ( 9 . 4 kg c o n i d i a per h e c t a r e ) were made, t h e l a t t e r a t 7-day i n t e r v a l s , leather-related e x p e r i m e n t a l f a i l u r e was r e p o r t e d . From C h a r u d a t t a n e t a l . ( 7 1 ) . a

142


fungus ( f o r h y d r i l l a , H y d r i l l a v e r t i c i l l a t a [ L . f . ] R o y l e ) , and F u s a r i u m s o l a n i ( M a r t . ) A p p e l & Wollenw. f . s p , c u c u r b i t a e ( f o r Texas g o u r d , C u c u r b i t a t e x a n a [ S c h e e l e ] G r a y ) , a r e d i s c u s s e d i n more d e t a i l i n C h a p t e r s 8 and 9 i n t h i s volume. I n my v i e w , t h e pathogens most l i k e l y t o r e a c h c o m m e r c i a l o r p r a c t i c a l u s e , hence t h e most n o t e w o r t h y , a r e t h o s e t h a t a r e ( 1 ) c a n d i d a t e s a g a i n s t weeds o f economic i m p o r t a n c e on a v e r y l a r g e s c a l e ( e c o n o m i c a l l y a t t r a c t i v e w e e d s ) , (2) pathogens t h a t may be amenable f o r development a s b r o a d - s p e c t r u m m y c o h e r b i c i d e s , ( 3 ) m y c o h e r b i c i d e s f o r l o w - i n p u t t e c h n o l o g y , and (4) m y c o h e r b i c i d e s f o r s p e c i a l t y or niche markets. P o t e n t i a l Pathogens o f E c o n o m i c a l l y A t t r a c t i v e Weeds. T a b l e IV i n c l u d e s weeds t h a t a r e c o n s i d e r e d t o be o f major worldwide i m p o r tance. These weeds b e l o n g t o Group 1 o f t h e w o r l d ' s w o r s t weeds a c c o r d i n g t o Holm e t a l . ( 7 3 ) . Due t o t h i s r a n k i n g t h e y may be a t t r a c t i v e targets f o r mycoherbicide i n d u s t r i e s seeking economically viable products. I t i s noteworthy t h a t s e v e r a l c a n d i d a t e pathogens a r e a v a i l a b l e f o r t h i s group o f weeds. Among t h e s e , C o l l e t o t r i c h u m c o c c o d e s ( W a l l r . ) Hughes ( 7 4 , 75) and C e r c o s p o r a rodmânii ( 7 6 - 7 8 ) have undergone e x t e n s i v e f i e l d t e s t i n g a n d / o r EUP e v a l u a t i o n s . O t h e r s i n t h i s g r o u p , s u c h a s F u s a r i u m l a t e r i t i u m Nees ( 7 9 ) , Phomops i s c o n v o l v u l u s Ormeno ( 8 0 ) , and B i p o l a r i s s o r g h i c o l a ( N i s i k . & M i y a k e ) Shoem. (81) have a l s o undergone ( o r a r e u n d e r g o i n g ) some measure o f c o m m e r c i a l development and e v a l u a t i o n . P o t e n t i a l l y Broad-spectrum Mycoherbicides. Among pathogens r e p o r t e d t o h o l d promise a s m y c o h e r b i c i d e s a r e s t r a i n s o r p a t h o t y p e s o f t h e same s p e c i e s o r s u b s p e c i e s t h a t a r e p a t h o g e n i c t o more t h a n one weed target. Examples a r e l i s t e d i n T a b l e V . I n some c a s e s , i n d i v i d u a l s t r a i n s o r p a t h o t y p e s h o s t s p e c i f i c t o p a r t i c u l a r weeds have been developed as mycoherbicides ( e . g . , the s t r a n g l e r v i n e pathotype of P h y t o p h t h o r a p a l m i v o r a used a s DeVine [ 6 1 ] ) . In other instances, i n d i v i d u a l s t r a i n s p u t a t i v e l y h o s t s p e c i f i c t o d i f f e r e n t weeds have been r e p o r t e d , b u t no a t t e m p t has been made t o s c r e e n t h e s e s t r a i n s a g a i n s t a c o l l e c t i o n o f e c o n o m i c a l l y i m p o r t a n t weeds. Thus, i t might be w o r t h w h i l e t o s c r e e n t h e s t r a i n s o f F u s a r i u m l a t e r i t i u m r e p o r t e d from v e l v e t l e a f , g i a n t ragweed ( A m b r o s i a t r i f i d a L . ) , s p u r r e d anoda (Anoda c r i s t a t a [ L . ] S c h l e c h t . ) , p r i c k l y s i d a ( S i d a s p i n o s a L . ) , and pondweeds (Potamogeton s p p . ) ( T a b l e V) t o d e t e r m i n e whether a s t r a i n w i t h b r o a d - r a n g e c a p a b i l i t y f o r b i o c o n t r o l c o u l d be found. I n o t h e r i n s t a n c e s , s u b s p e c i e s o r formae s p é c i a l e s o f t h e same s p e c i e s ( e . g . , C o l l e t o t r i c h u m g l o e o s p o r i o i d e s [ P e n z . ] S a c c . ) have been found t o be e f f e c t i v e a g a i n s t d i f f e r e n t weeds. T h e r e has been no e x p l o r a t i o n o f t h e f e a s i b i l i t y o f r e c o m b i n i n g o r h y b r i d i z i n g t h e s e s u b s p e c i e s i n t o b r o a d h o s t - r a n g e p a t h o t y p e s w h i c h c o u l d be t h e n used a g a i n s t more t h a n one weed t a r g e t . B o t h s o m a t i c and g e n e t i c r e c o m b i n a t i o n t e c h n i q u e s c o u l d be used f o r t h i s purpose ( 1 3 , 58, 9 4 - 9 6 ) , and t h i s p o s s i b i l i t y s h o u l d be e x p l o r e d . Bioengineering c a n open n o t o n l y g r e a t e r market p o t e n t i a l f o r m y c o h e r b i c i d e s based on r e c o m b i n a n t o r g a n i s m s , b u t a l s o p r o v i d e u s a b e t t e r u n d e r s t a n d i n g and a b i l i t y t o m o n i t o r t h e s a f e t y , s p e c i f i c i t y , and s u r v i v a l o f t h e recombinants.

(Cke.) Sacc.

C o c h l i o b o l u s lunatus Nelson & Haasis

Pyricularia grisea

(velvetleaf)

(common l a m b s q u a r t e r s )

85

82

84 84 84 82 82 82

74, 79 82 83

Ref. 75

Continued on next page

C o n v o l v u l u s a r v e n s i s L . ( f i e l d bindweed) Cyperus esculentus (yellow nutsedge) Ç. rotundus L . (purple nutsedge) ( i n c e r t a i n cases) D i g i t a r i a s a n g u i n e u s ( L . ) Scop, (large crabgrass) Echinochloa c r u s - g a l l i ( L . ) Beauv., (barnyardgrass)

Chenopodium album L .

A m b r o s i a t r i f i d a ( g i a n t ragweed) Avena f a t u a L . ( w i l d o a t )

Abutilon theophrasti

C o l l e t o t r i c h u m coccodes Fuaarium l a t e r i t i u m Fusarium l a t e r i t i u m S e p t o r i a t r i t i c i Desm. f . s p . avenae Ascochyta c a u l i n a Sacc. Cercospora chenopodii F r e s . C. dubia (Riess) Wint. Phomopsis c o n v o l v u l u s Cercospora c a r i c i s Oud. P h y l l a c h o r a c y p e r i Rehm

8

E c o n o m i c a l l y A t t r a c t i v e Weeds

Weed

P o t e n t i a l Mycoherbicide Candidates of

Pathogen

Table IV.

a

These, arranged a l p h a b e t i c a l l y , Holm e t a l . (73).

a r e i n Group I o f

the W o r l d ' s

82 — 82

81

93

88 89 90 —' 92 92 26

by

91 —

87 76-78 82

86

Worst Weeds l i s t e d

A l t e r n a r i a e i c h h o r n i a e Nag E i c h h o r n i a crassipes (waterhyacinth) R a j & Ponnappa Cercospora p i a r o p i Tharp C. rodmanii B i p o l a r i s s e t a r i a e (Saw.) Eleusine indica (L.) Gaertn. Shoemaker (goosegrass) Pyricularia grisea D i c h o t o m o p h t h o r a i n d i c a Rao P o r t u l a c a o l e r a c e a L . (common p u r s l a n e ) D. portulacae M e h r l i c h & F i t z p a t r i c k ex M. B . E l l i s Curvularia sp. R o e t t b o l l i a cochinchinensis L . f. Phaeoseptoria sp. (itchgrass) Sphacelotheca h o l c i J a c k . Sorghum h a l e p e n s e ( L . ) P e r s . ( j o h n s o n g r a s s ) ( S . c r u e n t a (Kuehn) P o t t e r B i p o l a r i s halepense Chiang, L e o n a r d & Van Dyke Β· s o r g h i c o l a ( L e f e b v r e & Sherwin) A l c o r n Colletotrichum graminicola (Ces.) G . W. W i l s . Gloeocercospora s o r g h i D. B a i n & E d g .

Table IV. Continued

grisea

Sclerotinia sclerotiorum de B a r y

Pyricularia

(Lib.)

Amphobotrys r i c i n i (Buchw.) Hennebert Colletotrichum gloeosporioides various f.spp. C . dematium ( F r . ) Grove f . s p p . ( i n c l . f . t r u n c a t a s e n s u von A r x ; C. graminicola Fusarium l a t e r i t i u m Weeds i n L e g u m i n o s a e , M a l v a c e a e , Dodders, e t c . Weeds i n Leguminosae ref. 103) Weeds i n Gramineae Abutilon theophrasti (velvetleaf) A m b r o s i a t r i f i d a ( g i a n t ragweed) Anoda c r i s t a t a ( s p u r r e d anoda) Sida spinosa ( p r i c k l y sida) Potamogeton s p p . D i g i t a r i a sanguinalis (large crabgrass), Eleusine indica (goosegrass) S e v e r a l weeds a c r o s s taxonomic lines

Cassia obtusifolia (sicklepod) C . o c c i d e n t a l i s ( c o f f e e senna) C r o t a l a r i a s p e c t a b i l i s (showy crotalaria) Weeds i n E u p h o r b i a c e a e

cassiae

Alternaria

Mycoherbicides

Weed

P o t e n t i a l l y Broad-spectrum

Pathogen

Table V.

98

88

105, , C h a p t e r 10, t h i s book

82 79 82 79 79 104 82,

13, 15, 63 9 9 , 100 101, , 102

97,

6 9 - 72

Ref.

146


Mycoherbicides f o r Low-input Technology. Some m y c o h e r b i c i d e c a n d i d a t e s , even i f t h e i r e f f i c a c y and o t h e r b i o l o g i c a l a t t r i b u t e s a r e a c c e p t a b l e , may n o t be a t t r a c t i v e c o m m e r c i a l l y due t o a v e r y l o c a l i z e d p r e v a l e n c e o f t h e weed p r o b l e m , s m a l l market s i z e , c o m p e t i t i o n from c h e m i c a l h e r b i c i d e s , o r a p r o j e c t e d low r e t u r n on i n v e s t m e n t . Yet t h e y c o u l d be d e v e l o p e d by p u b l i c - s u p p o r t e d a g e n c i e s and d i s persed through i n d i v i d u a l u s e r s . S i n c e t h e a g e n t s may n o t be used w i d e l y , t h e y c o u l d be d e v e l o p e d w i t h l o w - i n p u t t e c h n o l o g y t h r o u g h small-scale or cottage i n d u s t r i e s . Examples a r e g i v e n i n T a b l e V I .

Table V I .

Mycoherbicides f o r Low-input

Pathogen

Technology

Weed R e f .

Cephalosporium d i o s p y r i

Diospyros v i r g i n i a n a (persimmon) Chondrostereum purpureum Prunus s e r o t i n a ( b l a c k cherry) Colletotrichum gloeosporioides Clidemia h i r t a f.sp. clidemiae (Koster's curse) Puccinia canaliculate Cyperus e s c u l e n t u s ( y e l l o w nutsedge)

106

11.

107

108,, 109 25,

110,

T h i s a p p r o a c h has been t a k e n i n d i s p e n s i n g C e p h a l o s p o r i u m d i o s p y r i C r a n d e l l , a w i l t - i n d u c i n g pathogen o f persimmon ( D i o s p y r o s v i r g i n i a n a L . ) , c o n s i d e r e d weedy i n r a n g e l a n d s o f Oklahoma ( 1 0 6 ) . The fungus i s p r o v i d e d t o r a n c h e r s who i n o c u l a t e clumps o f persimmon t r e e s w i t h t h e a i d o f s h o t - g u n p e l l e t s f i l l e d w i t h i n o c u l u m . The pathogen c a u s e s w i l t i n g and d e a t h o f i n o c u l a t e d t r e e s and s p r e a d s t o nearby t r e e s presumably t h r o u g h r o o t c o n t a c t s . Chondrostereum purpureum ( P e r s . : F r . ) P o u z a r , a w o u n d - i n f e c t i n g b a s i d i o m y c e t e , i s proposed t o be used a s a b i o c o n t r o l f o r b l a c k c h e r r y , Prunus s e r o t i n a E h r h . , a weedy t r e e i n t h e f o r e s t s o f t h e Netherlands (Table I) ( l _ l 107). A few m i l l i g r a m s o f m y c e l i a l i n o c u l u m p e r t r e e , a p p l i e d t o stumps, i s s u f f i c i e n t t o i n s u r e p l a n t k i l l and p r e v e n t r e s p r o u t i n g ( 1 1 ) . I n view o f t h e need t o i n o c u l a t e i n d i v i d u a l t r e e s t o o b t a i n c o n t r o l , t h i s fungus may a l s o be b e s t s u i t e d f o r dispensing through i n d i v i d u a l u s e r s . Colletotrichum gloeosporioides (Penz.) Sacc. f . s p . clidemiae, which i n c i t e s a l e a f b l i g h t and d e f o l i a t i o n o f young K o s t e r ' s c u r s e p l a n t s , C l i d e m i a h i r t a ( L . ) D . D o n , (108) i s s a i d t o be d i s t r i b u t e d t h r o u g h h i k e r s who a r e p r o v i d e d w i t h l i q u i d i n o c u l u m and i n s t r u c t i o n s t o a p p l y t h e fungus ( 1 0 9 ) . The weed i s a problem o v e r a w i d e s p r e a d a r e a i n H a w a i i a n f o r e s t s b u t t h e pathogen does n o t d i s s e m i n a t e r e a d i l y due t o t h e s t i c k y s p o r e s . I n t h e absence o f i n d u s t r i a l m a s s - p r o d u c t i o n and a p p l i c a t i o n , s p r e a d i n g t h i s b i o c o n t r o l agent t h r o u g h i n d i v i d u a l u s e r s a p p e a r s t o be a n o v e l and r a p i d means o f achieving p r a c t i c a l success. Phatak e t a l . (25) have demonstrated t h a t a n a t i v e r u s t f u n g u s , P u c c i n i a c a n a l i c u l a t a ( S c h w . ) L a g h . , c a n be used i n an augmentation f

7. CHARUDATTAN

Pathogène with PotentialJar Weed Control

147

s t r a t e g y t o c o n t r o l y e l l o w n u t s e d g e , Cyperus e s c u l e n t u s L . , i n c r o p s such a s c o r n . As l i t t l e a s 5 g o f u r e d o s p o r e s o f t h e fungus p e r h e c t a r e , a p p l i e d through a c e n t e r - p i v o t i r r i g a t i o n system, can lead t o h i g h l e v e l s o f d i s e a s e and weed c o n t r o l ( 1 1 0 ) . Presently, the o n l y means o f p r o d u c i n g i n o c u l u m o f t h i s r u s t i s from i n f e c t e d p l a n t s i n n u r s e r i e s o r greenhouses. The l a c k o f i n v i t r o methods o f i n o c u l u m p r o d u c t i o n and t h e need f o r r e l a t i v e l y s m a l l amounts o f i n o c u l u m s u g g e s t t h a t t h i s fungus i s a good c a n d i d a t e f o r l o w - i n p u t weed c o n t r o l t e c h n o l o g y ( 1 1 1 ) . Mycoherbicides f o r Niche Markets. C e r t a i n weeds a r e l i k e l y t o r e m a i n a t t r a c t i v e t a r g e t s f o r m y c o h e r b i c i d e s due t o p e c u l i a r i t i e s i n v o l v e d i n c o n t r o l and u s e r demands. F o r example, weeds o f r e g i o n a l i m p o r t a n c e may be s u i t a b l e f o r m y c o h e r b i c i d a l c o n t r o l i f no o t h e r method p r o v i d e s e c o n o m i c a l c o n t r o l . I n some c a s e s , c h e m i c a l h e r b i c i d e s may be p r e c l u d e d from u s e due t o e n v i r o n m e n t a l c o n c e r n s . The growing t r e n d i n o r g a n i c f a r m i n g may a l s o f a v o r b i o l o g i c a l o v e r chemical h e r b i c i d e s . I n d e e d , t h e m y c o h e r b i c i d e s mentioned e a r l i e r , D e V i n e , C o l l e g o , C a s s t , and B i o M a l , a r e a l l d e v e l o p e d f o r s p e c i a l needs o f n i c h e markets i n w h i c h c h e m i c a l h e r b i c i d e s were l a c k i n g o r unsatisfactory. H e n c e , d e s p i t e t h e s m a l l market s i z e and t h e s p e c i a l i z e d n a t u r e o f t h e s e markets ( e . g . , c i t r u s , r i c e , and i r r i g a t e d peanut) i n d u s t r i e s were w i l l i n g t o d e v e l o p t h e s e m y c o h e r b i c i d e s . T a b l e V I I i n c l u d e s o t h e r pathogen-weed c o m b i n a t i o n s t h a t may f i t this specialty designation. I n t h e s e examples, s p e c i f i c demands o f t h e weed c o n t r o l s y s t e m , i . e . , a need t o p r o t e c t v u l n e r a b l e a q u a t i c h a b i t a t s from c h e m i c a l p o l l u t i o n , o r t h e s e n s i t i v i t y o f c r o p p l a n t s t o c h e m i c a l s used f o r dodder ( C u s c u t a s p . , a p a r a s i t e o f t h e c r o p p l a n t s ) c o n t r o l , a r e e x p e c t e d t o s e r v e a s an impetus f o r t h e d e v e l opment o f m y c o h e r b i c i d e s . Problems i n P r a c t i c a l Use o f M y c o h e r b i c i d e s . D e s p i t e t h e promise h e l d by m y c o h e r b i c i d e s and t h e e x p e r i m e n t a l s u c c e s s e s a c h i e v e d t o d a t e , s e v e r a l b i o l o g i c a l , t e c h n o l o g i c a l , and economic problems e x i s t which may l i m i t t h e p r a c t i c a l u t i l i z a t i o n o f t h i s t e c h n o l o g y . T h e f o l l o w i n g a r e some i m p o r t a n t e x a m p l e s . F u n g a l p r o p a g u l e s must be h y d r a t e d i n o r d e r t o g e r m i n a t e and infect their hosts. Dew o r o t h e r forms o f f r e e m o i s t u r e on t h e p l a n t surface enable t h i s h y d r a t i o n . A pathogen t h a t needs p r o l o n g e d c o n t a c t w i t h f r e e m o i s t u r e ( r o u g h l y f o r more t h a n 6 h r ) , o r s e v e r a l c y c l e s o f w e t t i n g and d r y i n g , i s more s u b j e c t t o t h e v a g a r i e s o f n a t u r e and t h e r e f o r e i s l i k e l y t o be l e s s competent i n i n f e c t i n g t h e h o s t t h a n one t h a t needs s h o r t e r exposure t o m o i s t u r e . E m p i r i c a l knowledge w i t h many f o l i a r d i s e a s e s s u g g e s t s t h a t f u n g i w i t h d r y ( n o t s t i c k y ) and l a r g e s p o r e s a r e more a f f e c t e d by t h e m o i s t u r e f a c t o r t h a n t h o s e w i t h s t i c k y and s m a l l s p o r e s . The l a t t e r appear t o be more c a p a b l e t h a n t h e former a t a d h e r i n g , r e m a i n i n g , and h y d r a t i n g on t h e p l a n t s u r f a c e . Thus, t h e type o f propagule used a s i n o c u l u m and i t s e p i d e m i o l o g i c a l r e q u i r e m e n t s f o r g e r m i n a t i o n and i n f e c t i o n c a n pose a s e r i o u s l i m i t a t i o n . Pathogens t h a t a r e d i f f i c u l t t o mass-produce o f f e r a t e c h n o l o g i c a l challenge. Many f u n g i s u c h a s A l t e r n a r i a s p p . and some C o l l e t o t r i c h u m s p p . do n o t s p o r u l a t e under l i q u i d f e r m e n t a t i o n , t h e p r e f e r r e d method o f c o m m e r c i a l i n o c u l u m p r o d u c t i o n . Labor-intensive

American Chemical Sodefj Library 1155 16th St, N.W. Washington, OJL 20038

Cuscuta spp,

8

are also

(dodders)

weeds;

Many o t h e r c a n d i d a t e s under r e s e a r c h and development i n t h i s c a t e g o r y ; t h e above a r e m e r e l y e x a m p l e s .

Colletotrichum gloeosporioides (Penz.) Sacc, f . s p , cuscutae, Alternaria sp.

Parasitic

Potamogeton s p p , (pondweeds) Lemna s p p , (duckweeds)

Myriophyllum spicatum (eurasien watermilfoil)

Colletotrichum gloeosporioides, Mycoleptodiscus t e r r e s t r i s (Gerdemann) O s t a z e s k i P y t h i u m βρρ·

weeds;

8

119

117

fall

118,

13

116,

115

lâ 112. C h a p t e r 8 t h i s book 99, 113 114

Ref.

l i k e l y to

Markets

Hydrilla verticillata (hydrilla)

Aquatic

Weed

Mycoherbicides for S p e c i a l t y

F u s a r i u m culmorum, Rhizoctonia sp,?

Pathogen

Table VII·

in

7. CHARUDATTAN

Pathogens with Potentialβατ Weed Control

149

and c o s t l y methods o f i n o c u l u m p r o d u c t i o n may be i m p r a c t i c a l from a commercial p e r s p e c t i v e . Therefore, technological d i f f i c u l t i e s i n i n o c u l u m p r o d u c t i o n may p r e c l u d e development o f a pathogen t h a t i s o t h e r w i s e d e s i r a b l e from s a f e t y and e f f i c a c y s t a n d p o i n t s . I n t e g r a t i o n o f m y c o h e r b i c i d e s w i t h c h e m i c a l - b a s e d p e s t manage ment p r a c t i c e s c a n be c o m p l i c a t e d due t o i n c o m p a t i b i l i t y between t h e b i o l o g i c a l and c h e m i c a l p e s t i c i d e s . T h i s problem and p r a c t i c a l s o l u t i o n s t o i t have been d i s c u s s e d by S m i t h (14) from h i s e x p e r i e n c e w i t h t h e C o l l e g o - n o r t h e r n j o i n t v e t c h system i n r i c e and s o y b ean. S i m i l a r e m p i r i c a l s o l u t i o n s w i l l be r e q u i r e d f o r e v e r y mycoh e r b i c i d e t h a t i s t o be used i n i n t e g r a t i o n w i t h c h e m i c a l p e s t i c i d e s . The a b i l i t y o f a weed t o compensate f o r d i s e a s e - i n d u c e d g r o w t h l o s s and t h e r e b y overcome p a t h o g e n i c a t t a c k has been demonstrated i n the c a s e o f C e r c o s p o r a r o d m a n i i l e a f s p o t o f w a t e r h y a c i n t h , a p l a n t w i t h a dominant c l o n a l growth h a b i t ( 7 7 ) . S i m i l a r tendency has been r e p o r t e d w i t h v e l v e t l e a f , a v i g o r o u s t e r r e s t r i a l a n n u a l i n f e c t e d by C o l l e t o t r i c h u m coccodes (74, 7 5 ) . T h i s p r o b l e m , n o t t o o uncommon among w e e d s ; stems from t h e i n a b i l i t y o f t h e pathogen t o k i l l t h e host completely or severely c u r t a i l i t s regrowth. I n such s i t u a t i o n s , one o f t h e ways t o improve t h e p a t h o g e n ' s weed c o n t r o l a b i l i t y may be t o i n c l u d e a p l a n t growth r e g u l a t o r o r a s u b l e t h a l dose of a chemical h e r b i c i d e t o the mycoherbicide f o r m u l a t i o n (10, 75, 78). F o r example, Hodgson e t a l . (75) r e p o r t e d t h a t t h i d i a z u r o n ( N - p h e n y l - N ' - l , 2 , 3 - t h i a d i a z o l - 5 - y l u r e a ) , w h i c h promotes d e f o l i a t i o n and r e t a r d s p l a n t r e g r o w t h , might be used t o r e t a r d v e l v e t l e a f growth, thereby improving the e f f e c t i v e n e s s of C . coccodes. Similar a d j u v a n t e f f e c t o f t h i d i a z u r o n has been found i n t h e w a t e r h y a c i n t h - C . r o d m a n i i system a s w e l l ( C h a r u d a t t a n and D e V a l e r i o , u n p u b lished data). More on t h i s p o i n t may be found i n C h a p t e r 16 o f t h i s volume. S e v e r a l economic f a c t o r s a f f e c t c o m m e r c i a l i z a t i o n o f m y c o h e r b i c i d e s (54, 64), the u l t i m a t e c o n s i d e r a t i o n being the projected e c o nomic r e t u r n s from t h e m y c o h e r b i c i d e . A pathogen t h a t i s prone t o be u n p r o f i t a b l e o r u n p r e d i c t a b l e i n t h i s r e s p e c t , whether due t o s m a l l market s i z e , c o s t o f p r o d u c t i o n and m a r k e t i n g , c o m p e t i t i o n from o t h e r a l t e r n a t i v e c o n t r o l s , o r e f f i c a c y , i s u n l i k e l y t o be d e v e l o p e d o r o f f e r e d f o r s a l e by c o m m e r c i a l e n t e r p r i s e s . Conclusions B i o l o g i c a l c o n t r o l w i t h p l a n t pathogens o f f e r s a f e a s i b l e a l t e r n a t i v e and supplement t o c h e m i c a l weed c o n t r o l . Research, r e g u l a t o r y , and i n d u s t r i a l t r e n d s s u g g e s t t h a t some o f t h e pathogens r e v i e w e d h e r e may become u s a b l e t h r o u g h t h e e f f o r t s o f p r i v a t e i n d u s t r i e s o r public agencies. P r o s p e c t i v e pathogens t h a t do n o t meet c o m m e r c i a l c r i t e r i a f o r development c o u l d s t i l l be made a v a i l a b l e t o t h e u s e r s through low-input technology o u t l e t s . Economic and p u b l i c demands f o r a f f o r d a b l e , e f f e c t i v e , and s a f e h e r b i c i d e s a r e e x p e c t e d t o s t i m u l a t e and s u s t a i n t h e i n t e r e s t i n t h i s f i e l d . Future research e f f o r t s must f o c u s on t h e p r o b l e m a t i c a s p e c t s d i s c u s s e d h e r e . Acknowledgments I thank D r . Tom B e w i c k , V e g e t a b l e C r o p s , and D r . Ed Freeman, P l a n t Pathology, U n i v e r s i t y of F l o r i d a , f o r c r i t i c a l l y reviewing t h i s c h a p t e r and o f f e r i n g h e l p f u l s u g g e s t i o n s f o r improvement.

150


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