Weed Control with Pathogens - ACS Publications - American

Chapter 18

Weed Control with Pathogens

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on May 14, 2018 | https://pubs.acs.org Publication Date: September 25, 1990 | doi: 10.1021/bk-1990-0439.ch018

Future Needs and Directions George E . Templeton Department of Plant Pathology, University of Arkansas, Fayetteville, AR 72701

An increasing emphasis on biological alternatives to chemicals for pest control is expected because of the persistent lack of confidence in synthetic chemical pesticides by a discerning society. Chemical residue contaminations of food, water, soil and the environment from use or misuse of pesticides are undeniably strong emotive issues that lead to an even more stringent regulatory climate and consequently to cost increases for chemical pesticide development. The trend toward biologicals is further strengthened by rapid development of pest resistance to the safer, metabolically specific chemical pesticides. The shift from chemical based to biologically based pest control technology will require substantially more public and private sector input into target and agent biology at the ecosystem, organismal and molecular levels than is now committed, and significant re-direction of genetic engineering research efforts to strain improvement of potential biological pesticides is required. Discovery a n d development of biological alternatives to chemical pesticides i s an e s s e n t i a l agenda for agricultural s c i e n t i s t s f a c e d w i t h t h e d a u n t i n g t a s k s o f p r o t e c t i n g c r o p s and l i v e s t o c k on a l a r g e s c a l e from t h e v i d e range a n d e v e r s h i f t i n g s p e c t r u m o f a g r i c u l t u r a l p e s t s s u c h as weeds, i n s e c t s , d i s e a s e s a n d nematodes. Chemical p e s t i c i d e s are e x c e l l e n t t o o l s i n t o d a y ' s crop p r o t e c t i o n technology b u t they a r e n o t the o n l y n o r n e c e s s a r i l y the b e s t means f o r e v e r y p e s t p r o b l e m . A l t h o u g h c h e m i c a l s have p r o v e n s a f e , e f f e c t i v e a n d e c o n o m i c a l f o r c o n t r o l o f most p e s t s , heavy r e l i a n c e upon a s i n g l e c o n t r o l s t r a t e g y h a s c r e a t e d f o r m i d a b l e problems(1,2,3,4,5,6,7,8,9,10). 0097-6156V90A)439-O320$06.00A) © 1990 American Chemical Society

Hoagland; Microbes and Microbial Products as Herbicides ACS Symposium Series; American Chemical Society: Washington, DC, 1990.


18. TEMPLETON

321


Even w i t h proper use, c h e m i c a l s may b e h a z a r d o u s . Toxic r e s i d u e s can accumulate i n food, s o i l , ground and surface water (1,3,4,5,6,9,10). I n j u r y to n o n - t a r g e t crops and endangered species c a n o c c u r a n d t o x i c m e t a b o l i t e s may o c c u r i n f o o d c h a i n s (1,5). R e s i s t a n c e may d e v e l o p i n t h e t a r g e t p e s t s , r e q u i r i n g i n c r e a s e d numbers o f t r e a t m e n t s a t h i g h e r r a t e s and r e s u l t i n g i n i n c r e a s e d exposure and p o l l u t i o n of the environment(8). B e n e f i c i a l organisms a r e o f t e n e l i m i n a t e d , r e s u l t i n g i n p e s t r e s u r g e n c e t o more s e r i o u s l e v e l s o r p r e v i o u s l y m i n o r p e s t s b e c o m i n g a m a j o r p r o b l e m due t o d e s t r u c t i o n o f t h e i r n a t u r a l enemies (2). Widespread use o f c h e m i c a l p e s t i c i d e s i n t h e home a n d i n a g r i c u l t u r e has l e d i n e v i t a b l y to g r e a t e r misuse and g r e a t e r r i s k s t o u s e r s and consumers, l i k e p e s t i c i d e - t r e a t e d g r a i n m i s u s e d as hog feed, and c o n t a m i n a t i o n o f a regional milk supply (6,9,10). C o n t a m i n a t i o n o f human f o o d s u p p l y o n a n a t i o n a l s c a l e was t h e c a s e r e c e n t l y when C a l i f o r n i a w a t e r m e l o n s were c o n t a m i n a t e d b y u n a p p r o v e d treatment with a systemic nematicide/insecticide (9,10). S i g n i f i c a n t e n v i r o n m e n t a l a n d human h e a l t h h a z a r d s h a v e a r i s e n a t s p e c i f i c n o n - a g r i c u l t u r a l s i t e s as a r e s u l t o f our r e q u i r e m e n t for synthetic chemical pesticides (4,5). Contamination of ground a n d s u r f a c e w a t e r s u p p l i e s f r o m t o x i c - w a s t e dumps o r a c c i d e n t a l s p i l l s h a s a f f e c t e d many r e g i o n s r e m o t e f r o m a r e a s o f p e s t i c i d e u s e . Storage, t r a n s p o r t and d i s p o s a l of toxic m a t e r i a l s and waste p r o d u c t s a r e an i n c r e a s i n g r i s k t o the c o u n t r y as our dependence upon chemical pesticides grows. The social and economic c o n s e q u e n c e s c a n be n e i t h e r known p r e c i s e l y n o r p r e d i c t e d w i t h c e r t a i n t y , b u t the r i s k s are s u b s t a n t i a l and i n c r e a s i n g l y r e c o g n i z e d as a n a l y t i c a l t e c h n i q u e s improve and o u r u n d e r s t a n d i n g grows about the fate in the environment of synthetic chemical, their manufacturing contaminants and t h e i r breakdown p r o d u c t s . The r i s k s are of concern to p e s t i c i d e producers, users, and government r e g u l a t o r s and to the p u b l i c i n g e n e r a l . Alternative pest control measures are c l e a r l y needed. P r u d e n c e r e q u i r e s t h a t we f i n d a n d e m p l o y s a f e r t y p e s o f p e s t i c i d e s , a g e n t s w h o s e mode o f a c t i o n i s something other than innate t o x i c i t y (1). B i o l o g i c a l c o n t r o l o f p e s t s w i t h t h e i r n a t u r a l enemies can reduce our dependence upon t o x i c s y n t h e t i c c h e m i c a l s . In this p r e s e n t a t i o n I w i l l give an overview of b i o l o g i c a l c o n t r o l of p e s t s : 1. S t r a t e g i e s u s e d i n b i o l o g i c a l c o n t r o l , i t s s a f e t y a n d b a r r i e r s f o r development; 2. R e s e a r c h o p p o r t u n i t i e s f o r b i o l o g i c a l h e r b i c i d e s i n weed c o n t r o l and s e l e c t i o n o f i m p r o v e d s t r a i n s o f b i o l o g i c a l c o n t r o l a g e n t s ; and 3. F u t u r e needs and d i r e c t i o n i n b i o l o g i c a l c o n t r o l of weeds. B i o l o g i c a l C o n t r o l S t r a t e g i e s . S a f e t y and B a r r i e r s

for

Development.

The s t r a t e g y o f b i o l o g i c a l c o n t r o l o f p e s t s w i t h t h e i r n a t u r a l enemies i s done b y use o f t h e f o l l o w i n g t a c t i c s : a . The C l a s s i c a l B i o l o g i c a l C o n t r o l T a c t i c - I m p o r t a t i o n o f e x o t i c n a t u r a l enemies f o r r e l e a s e , d i s s e m i n a t i o n , and s e l f - p e r p e t u a t i o n on the t a r g e t p e s t . b . The I n t e g r a t e d P e s t Management T a c t i c (IPM) - C o n s e r v a t i o n and enhancement o f i n d i g e n o u s n a t u r a l enemies. c . The B i o l o g i c a l P e s t i c i d e T a c t i c - A u g m e n t a t i o n o f



322

MICROBES AND MICROBIAL PRODUCTS AS HERBICIDES

i n d i g e n o u s n a t u r a l enemies w i t h r e a r e d o r c u l t u r e d agents by a p p l i c a t i o n to pest p o p u l a t i o n s . Each t a c t i c i s s c i e n t i f i c a l l y sound and has been demonstrated to work. U n l i k e t o x i c s y n t h e t i c c h e m i c a l s , b i o l o g i c a l t a c t i c s have h a d no known n o n t a r g e t e f f e c t s ( 1 , 1 1 , 1 2 , 1 3 ) . They a l l a r e dependent on fundamental knowledge of biological interactions at the o r g a n i s m a l and ecosystem levels. Intensive, usually long-term r e s e a r c h i s r e q u i r e d to f i n d and u n d e r s t a n d the unique b i o l o g y of specific agent/target combinations, evaluate potential of a c a n d i d a t e a g e n t a n d d e t e r m i n e e m p i r i c a l l y i t s u t i l i t y i n managed ecosystems. The e x p e n s e o f t h i s r e s e a r c h must u s u a l l y be b o r n e b y the p u b l i c sector. T h e r e i s no p r o f i t i n c e n t i v e f o r t h e p r i v a t e s e c t o r t o r e s e a r c h and d e v e l o p most b i o c o n t r o l t a c t i c s , w i t h t h e e x c e p t i o n o f c e r t a i n agents u s e d as b i o l o g i c a l p e s t i c i d e s i n l a r g e markets. We a r e t h u s f a c e d w i t h t h e p a r a d o x : biological control, a l t h o u g h e n v i r o n m e n t a l l y sound and advantageous g e n e r a l l y , i s not economically a t t r a c t i v e to the p r i v a t e sector. I n t h e p a s t we o p t e d f o r c h e m i c a l c o n t r o l , a s h o r t - t e r m economic s o l u t i o n a t the expense o f l o n g - t e r m e n v i r o n m e n t a l a n d human s a f e t y c o n s i d e r a t i o n s - - t h u s our too-heavy dependence upon c h e m i c a l p e s t i c i d e s . I n the f u t u r e we m u s t i n v e s t m o r e p u b l i c f u n d s i n b i o l o g i c a l c o n t r o l r e s e a r c h a n d development to ensure g r e a t e r u t i l i z a t i o n of t h i s e n v i r o n m e n t a l l y c o m p a t i b l e p e s t c o n t r o l technology and to b r i n g b i o l o g i c a l pest c o n t r o l up t o a p p r o p r i a t e l e v e l s i n o u r p e s t c o n t r o l e f f o r t s . The need i s i n t e n s i f i e d by the f a c t t h a t fewer and fewer chemical p e s t i c i d e s are a v a i l a b l e from the p r i v a t e s e c t o r . The c o s t of r e s e a r c h a n d d e v e l o p m e n t o f a new c h e m i c a l p e s t i c i d e , i n c l u d i n g c o s t of r e g i s t r a t i o n and l i a b i l i t y , is currently estimated to be approximately 28.1 m i l l i o n d o l l a r s (14). Such c o s t s mandate t h a t o n l y p e s t s t h a t r e p r e s e n t l a r g e m a r k e t p o t e n t i a l be t a r g e t e d for development by the p r i v a t e s e c t o r . C o n s e q u e n t l y , many p e s t p r o b l e m s are c u r r e n t l y l e f t unaddressed. Economic b e n e f i t s from p u b l i c investment i n b i o l o g i c a l c o n t r o l r e s e a r c h accrue d i r e c t l y to the producer and u l t i m a t e l y to the consumer as p e s t c o n t r o l expense i s d i m i n i s h e d o r e l i m i n a t e d from cost of production. The p u b l i c also benefits i n d i r e c t l y by reduction i n cost of r e g u l a t i n g and m o n i t o r i n g f o r pesticide r e s i d u e s i n f r e s h and processed food, ground w a t e r , and h a b i t a t s . P u b l i c investment i n b i o l o g i c a l c o n t r o l r e s e a r c h and development i s not o n l y e s s e n t i a l f o r s a f e t y and environmental c o n s i d e r a t i o n s but a l s o e c o n o m i c a l l y sound from a n a t i o n a l p e r s p e c t i v e . There i s a s u b s t a n t i a l body o f knowledge about p e s t s and t h e i r n a t u r a l enemies, and the theory and p r a c t i c e of classical biological control, i n t e g r a t e d p e s t management, and b i o l o g i c a l p e s t i c i d e s s u f f i c i e n t t o ensure that greater p u b l i c investment i n t h i s research e f f o r t would be p r o d u c t i v e and expand e f f e c t i v e n e s s o f n o n - c h e m i c a l p e s t c o n t r o l . B a r r i e r s t o t h e more g e n e r a l d e v e l o p m e n t o f b i o p e s t i c i d e s a r e c h i e f l y e c o n o m i c , n o t human h a z a r d o r e n v i r o n m e n t a l r i s k s . The p r i v a t e s e c t o r i s not w i l l i n g to i n v e s t i n r e s e a r c h to d i s c o v e r and develop these environmentally compatible p e s t i c i d e s because t h e i r h i g h pest s p e c i f i c i t y u s u a l l y l i m i t s market p o t e n t i a l . Public sector research is r e q u i r e d to overcome the d i s i n c e n t i v e s to commercialization, to demonstrate for each pest class that


18. TEMPLETON


323

biological agents are available, effective, economical and c o n s i s t e n t w i t h the n a t i o n a l i n t e r e s t to improve environmental q u a l i t y a n d l e s s e n human h e a l t h h a z a r d s .


Research Opportunities

- Biological Herbicides

for

Weed C o n t r o l .

N a t u r a l e n e m i e s o f w e e d s t h a t may b e u s e d a s b i o l o g i c a l p e s t i c i d e s i n c l u d e f u n g i , b a c t e r i a , v i r u s e s , nematodes, and i n s e c t s . Fungi have the g r e a t e s t p o t e n t i a l because they can g a i n e n t r y i n t o the h o s t p l a n t w i t h o u t t h e a s s i s t a n c e o f a v e c t o r , a n d many may b e m a s s produced i n fermentation tanks w i t h e x i s t i n g technology then d r i e d and m a r k e t e d j u s t as b r e a d y e a s t o r b r e w i n g y e a s t . F u n g a l d i s e a s e s o f w e e d s a r e common, b u t u n t i l r e c e n t l y t h e y h a v e n o t r e c e i v e d much r e s e a r c h a t t e n t i o n : p l a n t p a t h o l o g i s t s have emphasized r e s e a r c h to c o n t r o l diseases of economic crops. Weed d i s e a s e s have o f t e n b e e n n o t e d b y weed s c i e n t i s t s as confounding f a c t o r s i n weed r e s e a r c h p l o t s and b y g r o w e r s f a c e d w i t h s e v e r e weed i n f e s t a t i o n s i n crops for which chemical herbicides are either u n a v a i l a b l e o r inadequate t o c o n t r o l a p a r t i c u l a r weed. Only r e c e n t l y has r e s e a r c h focused on use of f u n g a l pathogens as biological pesticides for weed control. Three fungi, m y c o h e r b i c i d e s , a r e c u r r e n t l y u s e d i n t h e U . S . , a n d two o t h e r s a r e e x p e c t e d t o be a p p r o v e d f o r c o m m e r c i a l u s e i n t h e n e a r f u t u r e . One i s a v a i l a b l e i n The P e o p l e ' s R e p u b l i c o f C h i n a , a n d a c t i v e p r o j e c t s a r e known t o be underway i n 14 o t h e r countries. Sufficient e x p l o r a t i o n has been conducted i n the U . S . and abroad to i l l u s t r a t e t h a t , a l t h o u g h t h e r e a r e numerous f u n g a l p a t h o g e n s o f weeds w i t h p o t e n t i a l , t h e y a r e n o t u n i f o r m l y d i s t r i b u t e d w i t h t h e weed h o s t s and they do n o t a l l have e q u i v a l e n t p o t e n t i a l as biological herbicides. Some h a v e f a s t i d i o u s n u t r i t i o n a l r e q u i r e m e n t s for g r o w t h a n d r e p r o d u c t i o n i n a r t i f i c i a l c u l t u r e , a n d some a r e w i d e l y d i s t r i b u t e d a n d commonly e n c o u n t e r e d b u t e x h i b i t l o w v i r u l e n c e e v e n when a p p l i e d as i n u n d a t i v e i n o c u l u m t o young p l a n t s . Some h a v e s t r i c t environmental requirements t h a t l i m i t t h e i r use to p a r t i c u l a r times or within geographic limits that are impractical in comprehensive pest c o n t r o l systems. Strain

S e l e c t i o n and Improvement

Research

C u r r e n t r e s e a r c h o n h e r b i c i d e p o t e n t i a l o f f u n g i h a s r e v e a l e d many opportunities f o r s t r a i n improvement r e s e a r c h (15,16,17). For example, many w e e d - d a m a g i n g f u n g i have fastidious nutritional requirements f o r growth and r e p r o d u c t i o n or p a r t i c u l a r p h y s i c a l requisites for reproduction i n a r t i f i c i a l culture. Orange r u s t and white r u s t of morningglory.(Ipomomea s p p . ) , exemplify o p p o r t u n i t i e s in fungal nutrition research. These two diseases routinely d e v a s t a t e m a t u r e p o p u l a t i o n s o f weedy m o r n i n g g l o r y i n grower f i e l d s , too l a t e , however, to b e n e f i t the growing crop. E f f e c t i v e use of t h e s e two f u n g a l p a t h o g e n s a s b i o l o g i c a l p e s t i c i d e s d e p e n d s upon having a supply of a r t i f i c i a l l y c u l t u r e d inoculum for a p p l i c a t i o n t o young s e e d l i n g s b e f o r e t h e y compete w i t h the c r o p . Neither f u n g a l p a t h o g e n c a n be grown i n a r t i f i c i a l c u l t u r e w i t h e x i s t i n g t e c h n o l o g y , s o r e s e a r c h i s n e e d e d t o o v e r c o m e t h i s b a r r i e r a n d make p o s s i b l e t h e u s e o f t h e s e two u b i q u i t o u s f u n g i a s m y c o h e r b i c i d e s .



324


E x t e n s i o n o f s u c h new c u l t u r e t e c h n o l o g y t o o t h e r o r a n g e a n d w h i t e r u s t pathogens would enable the development of c o u n t l e s s o t h e r r u s t s p e c i e s t o c o n t r o l t h e i r s p e c i f i c weed h o s t s . The development of c e r t a i n weed p a t h o g e n s as biological herbicides i s c o n s t r a i n e d by innate low v i r u l e n c e or l o s s of virulence i n a r t i f i c i a l culture (15,16). S p e c i e s o f t h e common f u n g a l pathogens, A l t e r n a r i a . Cercospora. and HelBlnthoggPrluffl. f o r e x a m p l e , g e n e r a l l y a r e too w e a k l y v i r u l e n t t o k i l l t h e i r weed h o s t e v e n when a p p l i e d as inundative inoculum to seedling plants. I n f e c t e d p l a n t s may b e d i s e a s e d b u t n o t d e b i l i t a t e d e n o u g h t o r e m o v e them from c o m p e t i t i o n w i t h t h e c r o p . Research to understand and overcome t h i s c o n s t r a i n t would p e r m i t development o f mycoherbiciees f o r some o f t h e s e r i o u s w e e d p r o b l e m s now o n l y m a r g i n a l l y c o n t r o l l e d by chemical herbicides or those not currently controlled by chemicals at a l l . Many fungal pathogens of weeds are not developed as mycoherbicides because they l o s e v i r u l e n c e i n a r t i f i c i a l c u l t u r e and w i l l n o t i n f e c t t h e i r s p e c i f i c h o s t s when i n o c u l a t e d . The f u n g a l pathogen o f persimmon w i l t d i s e a s e , f o r example, c a n be u s e d t o r i d p a s t u r e s a n d r a n g e l a n d s o f w e e d p e r s i m m o n , ( D l o s p v r o s v i r g l n l a n a L. ) , only a f t e r l i m i t e d subculture i n the l a b o r a t o r y , y e t i t i s noni n f e c t i o u s when c u l t u r e c o n d i t i o n s a r e s c a l e d - u p f o r large-scale trials. I n f e c t i v i t y c a n be r e s t o r e d by r e - i s o l a t i o n o f c u l t u r e s f r o m n a t u r a l i n f e c t i o n s o r p a s s a g e o f weak i s o l a t e s t h r o u g h t h e h o s t - - procedures too i m p r a c t i c a l f o r l a r g e - s c a l e p r o d u c t i o n needed f o r c o n t r o l o f t h i s w i d e s p r e a d i n t r a n s i g e n t weed p r o b l e m . This type o f v i r u l e n c e l o s s i s c o m m o n l y e n c o u n t e r e d among f u n g a l p a t h o g e n s o f p l a n t s , i n s e c t s , o r nematodes; t h e r e f o r e , r e s e a r c h to u n d e r s t a n d the p h y s i o l o g i c a l o r g e n e t i c b a s i s f o r i t , so t h a t v i r u l e n c e c a n be s t a b i l i z e d , w o u l d e n h a n c e t h e b i o l o g i c a l c o n t r o l p o t e n t i a l o f many specific fungi for control of several classes of pests. I n some i n s t a n c e s t h e p o t e n t i a l o f s p e c i f i c f u n g a l s t r a i n s i s d i m i n i s h e d by s t r i c t requirements for p a r t i c u l a r environmental c o n d i t i o n s or sequences f o r i n f e c t i o n , c o l o n i z a t i o n , or r e p r o d u c t i o n i n the host p l a n t . T h e s t r a i n t h u s may b e l i m i t e d t o u s e in narrowly circumscribed geographic l o c a t i o n s or at p a r t i c u l a r times i n the season. The weed k i l l i n g f u n g u s , C o l l e t o t r i c h u m m a l v a r u m . f o r example, can k i l l i t s h o s t p l a n t i f i n o c u l a t e d when d a i l y maximum t e m p e r a t u r e s a r e b e l o w 32° C . U n f o r t u n a t e l y t h e weed h o s t , prickly sida, (Sida splnosa L.), emerges and becomes weedy t h r o u g h o u t m o s t o f i t s r a n g e w h e n maximum t e m p e r a t u r e s a r e a b o v e t h i s l i m i t ; t h e r e f o r e , use o f the fungus i s s e v e r e l y r e s t r i c t e d and impractical. E f f o r t s to c o l l e c t s t r a i n s o f the fungus throughout the range o f i t s h o s t have not r e v e a l e d any w i t h s u f f i c i e n t l y b r o a d environmental adaptation. P h y s i o l o g i c a l and genetic r e s e a r c h to understand the molecular b a s i s for t h i s type of environmental r e s t r a i n t o n p a t h o g e n s a n d means o f o v e r c o m i n g i t c o u l d a c c e l e r a t e efforts to u t i l i z e f u n g a l pathogens f o r a l l types o f biological pesticides. M o l e c u l a r g e n e t i c t e c h n i q u e s now a v a i l a b l e h o l d g r e a t p o t e n t i a l f o r improving s t r a i n s for b i o l o g i c a l p e s t i c i d e development. For e x a m p l e , l o w - v i r u l e n c e s t r a i n s may b e e n h a n c e d t o i m p r o v e e f f i c a c y . S p e c i e s d i f f i c u l t t o grow o r t h a t r e p r o d u c e p o o r l y i n a r t i f i c i a l c u l t u r e may b e g e n e t i c a l l y a l t e r e d t o make t h e m a v a i l a b l e i n l a r g e



18. TEMPLETON


325

q u a n t i t i e s and p r a c t i c a l f o r f i e l d u s e . Unstable s t r a i n s that lose v i r u l e n c e i n a r t i f i c i a l c u l t u r e may be g e n e t i c a l l y s t a b i l i z e d and thus a v a i l a b l e f o r widespread use. W h i l e much o f what c a n be e n v i s i o n e d i s s p e c u l a t i v e a t the moment, the power o f the new technology i s not. T e c h n o l o g y i s now a v a i l a b l e f o r the p h y s i c a l i s o l a t i o n o f genes from b a c t e r i a and f u n g i , f o r r e c o m b i n a t i o n o f t h e s e genes i n the l a b o r a t o r y , and f o r i n s e r t i n g them i n t o the o r g a n i s m from w h i c h they came o r i n t o o t h e r organisms ( 1 7 ) . The r e s e a r c h c h a l l e n g e i s to understand host-parasite interaction at the molecular level s u f f i c i e n t l y so t h a t m o l e c u l a r b i o l o g i c a l methods c a n be u s e d to a d d r e s s s t r a i n improvement o f b i o l o g i c a l c o n t r o l methods. For example, the enzymes, c u t i n a s e , p e c t i n l y a s e , and p o l y g a l a c t u r o n a s e have been found t o be o p e r a t i v e d u r i n g i n f e c t i o n , c o l o n i z a t i o n , and symptom development by C o l l e t o t r i c h u m . an i m p o r t a n t f u n g a l pathogen w i t h demonstrated mycoherbicide c a p a b i l i t y (18). I d e n t i f i c a t i o n of the gene t h a t c o n t r o l s p r o d u c t i o n o r a c t i v i t y o f t h e s e enzymes might be u s e d t o t r a n s f o r m o t h e r C o l l e t o t r i c h u m s p e c i e s o r even o t h e r f u n g i t h a t a r e s p e c i f i c f o r , b u t n o n - l e t h a l t o , i m p o r t a n t weed h o s t s and thus enhance m y c o h e r b i c i d e p o t e n t i a l o f t h e s e f u n g i . Genetic v e c t o r s a r e a v a i l a b l e and have been u s e d t o t r a n s f o r m C o l l e t o t r i c h u m with antibiotic and nutritional marker genes; therefore, t r a n s f o r m a t i o n o f t h i s fungus w i t h genes t h a t a r e i n f l u e n t i a l i n pathogenesis i s p l a u s i b l e . A s i m i l a r example c a n be cited for genes that control a p p r e s s o r i u m f o r m a t i o n , a s t r u c t u r e e s s e n t i a l i n many f u n g i f o r h o s t p e n e t r a t i o n . M o l e c u l a r g e n e t i c p r o b e s have been u s e d t o demonstrate t h a t t h e s e genes become o p e r a t i v e upon i n i t i a t i o n o f t h i s o r g a n i n a r u s t fungus ( 1 9 ) . I s o l a t i o n o f t h e s e genes and t r a n s f o r m a t i o n o f c e r t a i n f u n g i w i t h them would improve p e n e t r a t i o n e f f i c a c y of s t r a i n s and thus m y c o h e r b i c i d e p o t e n t i a l . A l e s s advanced b u t perhaps even more p l a u s i b l e example is t r a n s f o r m a t i o n o f f u n g a l s t r a i n s w i t h genes t h a t c o n t r o l p r o d u c t i o n of highly s p e c i f i c pathotoxins. Many f u n g i a r e known t o produce biochemically specific, low m o l e c u l a r w e i g h t m e t a b o l i t e s i n the d i s e a s e development p r o c e s s . Enhancement o f s t r a i n s w i t h genes t h a t c o n t r o l p r o d u c t i o n or a c t i v i t y of these metabolites is a clear r e s e a r c h a b l e g o a l t h a t would Improve m y c o h e r b i c i d e p o t e n t i a l o f many n o n - l e t h a l , h o s t - s p e c i f i c fungi (17). F u t u r e Needs

and D i r e c t i o n s

G r e a t e r r e s e a r c h emphasis needs t o be p l a c e d on e x p l o r a t i o n , b o t h d o m e s t i c and f o r e i g n , t o f i n d new f u n g i on weeds and new s t r a i n s o f known weed p a t h o g e n s . Surveys s h o u l d i n c l u d e p l a n t s now known to be weedy b u t a l s o p l a n t s t h a t have p o t e n t i a l o f becoming weedy by succession because of their tolerance to selective chemical h e r b i c i d e s now i n u s e . Specimens o f t h e s e f u n g i and i s o l a t e s in c u l t u r e s h o u l d be m a i n t a i n e d and renewed p e r i o d i c a l l y t o assure maintenance o f v i r u l e n c e and r e p r o d u c t i v i t y and t o a s s u r e t h a t a b r o a d spectrum o f the g e n e t i c d i v e r s i t y a v a i l a b l e i n the o r g a n i s m i s w e l l r e p r e s e n t e d l n the c o l l e c t i o n . New developments and p r o s p e c t s i n b i o t e c h n o l o g y g i v e cause f o r optimism a l s o . I t c a n be e x p e c t e d t h a t p o w e r f u l new genetic



326


engineering tools c a n be used to enhance existing biocontrol t a c t i c s , improve our a b i l i t y to manipulate the agent p r a c t i c a l l y or make p o s s i b l e e n t i r e l y new t a c t i c s b a s e d o n u n d e r s t a n d i n g m o l e c u l a r l e v e l i n t e r a c t i o n between n a t u r a l enemies and p e s t s t h a t c a n be manipulated to advantage. Some o f t h e s e d e v e l o p m e n t s h a v e p r o f i t potential and are a t t r a c t i v e to the p r i v a t e sector. Public investment i n biotechnology with biological control agents is r e q u i r e d , h o w e v e r , t o e n s u r e f u l l u t i l i z a t i o n o f t h e s e p o w e r f u l new techniques i n our e f f o r t to develop b i o l o g i c a l c o n t r o l as the f o u n d a t i o n of pest c o n t r o l i n the U . S . A s i g n i f i c a n t s h i f t away f r o m c h e m i c a l p e s t i c i d e s t o g r e a t e r r e l i a n c e upon b i o l o g i c a l c o n t r o l methods i s r e s t r a i n e d by l a c k o f knowledge. More i n f o r m a t i o n i s r e q u i r e d a t the o r g a n i s m a l and molecular levels. The uniqueness of the biology of each a g e n t / t a r g e t combination p l u s the complexity of t h e i r i n t e r a c t i o n w i t h other organisms i n the ecosystem r e q u i r e s i n t e n s i v e s p e c i a l i z e d r e s e a r c h p l u s complementary i n t e r d i s c i p l i n a r y teamwork. Molecular i n t e r a c t i o n s a r e e q u a l l y complex and r e q u i r e m u l t i d i s c i p l i n a r y teams of well-trained scientists. Such teams include geneticists, biochemists, molecular b i o l o g i s t s , microbiologists, physiologists, p l a n t p a t h o l o g i s t s , entomologists and e c o l o g i s t s . A comprehensive n a t i o n w i d e , p l u r a l i s t i c r e s e a r c h e f f o r t on b i o l o g i c a l c o n t r o l is n e e d e d t o r e m o v e r e s t r a i n t s t o d e v e l o p m e n t a n d make g r e a t e r u s e o f this pest control resource. A p l u r a l i s t i c approach i s e s s e n t i a l because the agents and mechanisms occur n a t u r a l l y a n d may b e d i s c o v e r e d i n l o c a l i z e d g e o g r a p h i c a r e a s , y e t t h e y c a n be o p e r a t i v e a n d m a n a g e a b l e i n e n t i r e l y s e p a r a t e r e g i o n s a n d e c o s y s t e m s (1,12). Research on molecular b i o l o g y must be b r o a d b a s e d also. Fundamental understanding of biological interactions at the m o l e c u l a r l e v e l p e r m i t s u t i l i z a t i o n o f genes and t h e i r b i o l o g i c a l l y a c t i v e p r o d u c t s i n b i o l o g i c a l c o n t r o l t a c t i c s i n ways q u i t e remote from t h e i r n a t u r a l occurrence. Genes a n d t h e i r b i o l o g i c a l l y a c t i v e p r o d u c t s c a n be i d e n t i f i e d a n d i s o l a t e d i n one l i v i n g e n t i t y a n d t h e n u s e d to t r a n s f o r m and enhance b i o l o g i c a l c o n t r o l p o t e n t i a l o f a n o t h e r more s u i t a b l e s p e c i e s . T h e r e i s o p t i m i s m t h a t t h e many new techniques i n biotechnological research will be the key to development o f b i o l o g i c a l c o n t r o l as the p r i m a r y p e s t c o n t r o l method i n t h e U n i t e d S t a t e s (1). New k n o w l e d g e a b o u t p o t e n t i a l b i o p e s t i c i d e s i n r e q u i r e d t o make t h i s b i o c o n t r o l t a c t i c more g e n e r a l l y a v a i l a b l e . C a n d i d a t e s may b e e x o t i c o r i n d i g e n o u s m i c r o o r g a n i s m s , s p e c i f i c n a t u r a l enemies of p a r t i c u l a r introduced or indigenous pest species. These n a t u r a l e n e m i e s n o r m a l l y o c c u r a t i n s i g n i f i c a n t l e v e l s b u t may b e i n c r e a s e d i n f e r m e n t a t i o n o r r e a r i n g f a c i l i t i e s , a p p l i e d as s p r a y s , and i n t e g r a t e d i n t o p e s t management s y s t e m s . Those n o t r e a c h i n g t h e i r natural pest hosts d i e and d e t e r i o r a t e i n the n a t u r a l cycle. Biological pesticides are considered the most nearly perfect pesticides. No e n v i r o n m e n t a l p e r t u r b a t i o n s , b e y o n d r e m o v a l o f t h e p e s t h o s t , have been d e t e c t e d n o r w o u l d be e x p e c t e d from temporary e l e v a t i o n o f t h e n a t u r a l enemy p o p u l a t i o n . B i o l o g i c a l p e s t i c i d e r e s e a r c h and development i s most e f f i c i e n t a n d p r o d u c t i v e when c o n d u c t e d b y teams o f s c i e n t i s t s : a research s p e c i a l i s t on the t a r g e t p e s t , a r e s e a r c h s p e c i a l i s t on the n a t u r a l enemy, a n d a r e s e a r c h s p e c i a l i s t on f e r m e n t a t i o n o r r e a r i n g t h e



18. TEMPLETON


327

c l a s s o f o r g a n i s m t o be d e v e l o p e d . A s p e c i a l i s t knowledgeable of the t a r g e t p e s t , i t s l i f e h i s t o r y and b e h a v i o r i s most l i k e l y t o d i s c o v e r n a t u r a l enemies w i t h p o t e n t i a l , u n d e r s t a n d the p e s t ' s most v u l n e r a b l e g r o w t h s t a g e s a n d k n o w how t o e v a l u a t e f i e l d e f f i c a c y o f a n a t u r a l enemy f o r i t s p o t e n t i a l a s a b i o l o g i c a l p e s t i c i d e . A r e s e a r c h s p e c i a l i s t k n o w l e d g e a b l e a b o u t t h e n a t u r a l enemy i s a b l e to assess the s t r e n g t h s and weaknesses of a p o t e n t i a l b i o l o g i c a l pesticide i n c o n t r o l l e d environments and p r e c i s e l y define the environmental optima and l i m i t s to guide e f f e c t i v e f i e l d assessment of an agent. A specialist knowledgeable about large-scale p r o d u c t i o n of v a r i o u s c l a s s e s of organisms i s able to determine the n u t r i t i o n a l and p h y s i c a l requirements f o r l a r g e - s c a l e p r o d u c t i o n of a n a g e n t a n d means f o r f o r m u l a t i n g i t i n t o a c o n s i s t e n t , stable preparation and assess economic feasibility of production, f o r m u l a t i o n and storage on a commercial s c a l e . The successful c o l l a b o r a t i o n o f s u c h a team - - c r i t i c a l a s s e s s m e n t o f a n a g e n t s ' p o t e n t i a l from p e r s p e c t i v e s of three s c i e n t i f i c d i s c i p l i n e s - - p e r m i t s r a p i d and e f f e c t i v e technology t r a n s f e r to p r i v a t e e n t e r p r i s e f o r production scale-up, EPA r e g i s t r a t i o n , f o r m u l a t i o n , packaging, s h i p p i n g , and m a r k e t i n g o f a b i o l o g i c a l p e s t i c i d e . A g e n t s deemed t o be i m p r a c t i c a l b y s u c h a c o l l a b o r a t i n g t e a m , ineffective because of biotic or other constraints, become c a n d i d a t e s f o r s t r a i n improvement. G e n e t i c enhancement o f c a n d i d a t e potential may b e by either classic methods or new genetic transformation techniques. A few b i o p e s t i c i d e s have b e e n c o m m e r c i a l l y s u c c e s s f u l and s e r v e as models f o r development o f o t h e r s ( 1 5 , 1 6 ) . The d i v e r s i t y o f natural systems, however, provides a wealth of potential biopesticides and requires adjustment of researchable goals appropriate for particular classes of candidate agents, either f u n g i , b a c t e r i a , v i r u s e s , or nematodes. The b i o s y s t e m a t i c s o f a l l t h e s e c l a s s e s and t h e p e s t s t h e y a t t a c k need t o be researched. R e g i o n a l , n a t i o n a l and i n t e r n a t i o n a l s u r v e y s need t o be c o n d u c t e d to develop or update catalogues and c o l l e c t i o n s of e x i s t i n g n a t u r a l enemies and the p r e c i s e h o s t a s s o c i a t i o n and geographic range o f e a c h , a p p a r e n t v a r i a b i l i t y o r s t a b i l i t y o f h o s t o r n a t u r a l enemy, and p a r t i c u l a r c l i m a t i c or o t h e r c o n s t r a i n t s t h a t appear o p e r a t i v e under n a t u r a l c o n d i t i o n s . Biosystematic research thus provides a g e n t s a n d some i n s i g h t i n t o t h e i r p o t e n t i a l f o r s p e c i a l i s t s who e v a l u a t e them i n c o n t r o l l e d c o n d i t i o n s . Specialists of each c l a s s use the c o l l e c t i o n f o r strain selection and s t r a i n improvement research. Supplies of the p r o s p e c t i v e a g e n t s must be e v a l u a t e d i n l a b o r a t o r y c u l t u r e ; t h e n epidemiological parameters must be researched in controlled environments of growth chambers and greenhouses. Promising c a n d i d a t e s move t o f i e l d p l o t t e s t s i n c o l l a b o r a t i o n w i t h s c i e n t i s t s s p e c i a l i z i n g i n the t a r g e t p e s t s , p r e f e r a b l y those b i o s y s t e m a t i s t s responsible for o r i g i n a l c o l l e c t i o n of the p o t e n t i a l agent. Other c a n d i d a t e s are s u b j e c t s f o r s t r a i n improvement r e s e a r c h . A g e n t s t h a t a r e f o u n d t o be c o n s t r a i n e d b y f a s t i d i o u s c u l t u r e requirements, too s t r i c t e p i d e m i o l o g i c a l parameters, or inadequate v i r u l e n c e o r t h a t a r e t o o l a b i l e must be t h e s u b j e c t of more n u t r i t i o n a l , g e n e t i c o r p h y s i o l o g i c a l r e s e a r c h t o enhance strain potential. Understanding the molecular b a s i s f o r these s o r t s of


328


i n a d e q u a c i e s a n d t h e genes t h a t c o n t r o l t h e s e mechanisms presents o p p o r t u n i t i e s f o r t r a n s f o r m i n g t h e organism g e n e t i c a l l y t o overcome d e f i c i e n c i e s a n d a c h i e v e s u c c e s s f u l b i o l o g i c a l p e s t i c i d e s w i t h many m o r e a g e n t s t h a n i s now p o s s i b l e .


Final

Comments

Biological control c a n be used to s u b s t a n t i a l l y reduce our dependence upon c h e m i c a l p e s t i c i d e s and achieve p e s t c o n t r o l w i t h o u t negative non-target effect. To a c h i e v e s i g n i f i c a n t t r a n s i t i o n t o b i o l o g i c a l p e s t c o n t r o l , however, w i l l r e q u i r e new k n o w l e d g e of p e s t s a n d t h e i r n a t u r a l enemies a n d r e a l i g n m e n t o f r e s e a r c h a n d development r e s p o n s i b i l i t i e s f o r p e s t c o n t r o l among p u b l i c a n d private sectors. The major r e s p o n s i b i l i t y f o r b i o l o g i c a l control development must be w i t h t h e p u b l i c s e c t o r r a t h e r t h a n w i t h t h e p r i v a t e s e c t o r , which has advanced chemical technology. Biological c o n t r o l a g e n t s a r e s p e c i f i c a n d , when u s e d i n t h e c l a s s i c a l o r I P M tactic, self-perpetuating, thus h a v i n g no p r o f i t p o t e n t i a l f o r private enterprise. When a g e n t s a r e u s e d a s b i o l o g i c a l p e s t i c i d e s , t h e i r m a r k e t p o t e n t i a l may b e l i m i t e d b y s t r i c t s p e c i f i c i t y t o a single pest that generally represents small markets. Thus t h e v e r y t r a i t t h a t makes b i o l o g i c a l c o n t r o l s o d e s i r a b l e f r o m e n v i r o n m e n t a l and human s a f e t y p e r s p e c t i v e s , s p e c i f i c i t y f o r t h e t a r g e t p e s t , i s the exact c h a r a c t e r i s t i c t h a t e l i m i n a t e s economic p o t e n t i a l o f biologicals for private enterprise. New b i o t e c h n o l o g i c a l techniques o p e n new o p p o r t u n i t i e s for g e n e t i c improvement o f b i o l o g i c a l c o n t r o l a g e n t s , e s p e c i a l l y i n t h e b i o - p e s t i c i d e t a c t i c , a n d p r o v i d e some i n c e n t i v e f o r p r i v a t e - s e c t o r Involvement i n biological control. An extensive, pluralistic, public-sector research effort i s required to f i n d andunderstand the b i o l o g y o f 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 , s e l e c t a n d Improve s t r a i n s f o r f u r t h e r development, a n d engage i n t e c h n o l o g y transfer with private enterprise when candidates with potential for commercialization are developed. The c h a l l e n g e i s t o s u b s t a n t i a l l y enhance p u b l i c - s e c t o r r e s e a r c h enough t o e n a b l e b i o l o g i c a l c o n t r o l t o become t h e p r i m a r y method o f p e s t c o n t r o l i n t h e U n i t e d S t a t e s .

Literature Cited 1. Report of the Research Briefing Panel on Biological Control in Managed Ecosystems, National Academy of Sciences, 1987. 2. Wilson,F. and Huffaker, C.B. In Theory and Practice of Biological Control; Huffaker, C. Β., Messenger P.S., Eds., Academic Press; New York, 1976; Chapter 1. 3. Barron, K.C. Are Pesticides Really Necessary?; Regnery Gateway, Inc. Book Publishers: Chicago, 1981. 4. Hallberg, E. R. Am. J. Alt. Agri. 1987, 2, 3-15. 5. Pimentel, D; Levitan, L. BioScience 1986 36 86-91. 6. Berry, C. R. Ark. Farm. Res. 1986. 35 (6), 9. 7. McWhorter, C. G.; Chandler, J.M. In Biological Control of Weeds with Plant Pathogens; Charudatton, R.; Walker, H.L. Eds: John Wiley & Sons; New York, 1982; Chapter 2. 8. Bennett, E.W.; Runstrow, E.S.; Wieland, F.A., Bul. Entomol. Soc. Am. 1983, 29 31.



18. TEMPLETON


329

9. Milner, A. Sierra. 1986, 71(428-32). 10. Mott, L . ; Snyder, K. The Amicus Journal 1988. Spring 20-29. 11. Cook, R. J., Baker, K.E., the Nature and Practice of Biological Control of Plant Pathogens: The American Phytopathological Society: St. Paul, Minnesota 1983; Chapter 1,2. 12. National Interdisciplinary Biological Control Conference. U.S. Department of Agriculture, U. S. Government Print Office; Washington, D. C. 13. Biological Agents for Pest Control, U.S. Department of Agriculture, U.S. Government Print Office: Washington, D.C., 1978. 14. Anon., Plant Dis. 1987, 71 1060. 15. Templeton, G. E . ; Heiny, D. In Biotechnology of Fungi for Improving Plant Growth; Whipps, J. M.; Lumsden, R. D., Eds.; British Mycological Society: Kew, Surrey, United Kingdom; Chapter 6. 16. Templeton, G. E . ; Heiny, D. In New Directions in Biological Control..Alternatives for Suppressing Agricultural Pests and Diseases; Baker, R.; Dunn, P., Eds.; UCLA Symposia on Molecular and Cellular Biology New Series Vol.112 ; A. R. Liss: New York 1989; pp. 279-286. 17. Turgeon, G.; Yoder, O.C. In Biotechnology: Applications and Research, Chermenisinoff, P.N.; Ovellette, R.P., Eds.; Technomic: Lancaster, PA, 1985; pp. 221. 18. O'Connell, R. J.; Bailey, J. Α.; Richmond, D. V. Physiological Plant Pathology 1985, 25 75-98. 19. Hoch, H. C.; Staples, R. C.; Whitehead, Β., Comeau, J., Wolf, E. D. Science 1987, 235 1659-62. RECEIVED

February 16,

1990


Weed Control with Pathogens - ACS Publications - American

Recommend Documents