The Pathogenesis of Plant Diseases - ACS Symposium Series (ACS

4 The Pathogenesis of Plant Diseases The Effect of Modern Fungicides

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on November 5, 2015 | http://pubs.acs.org Publication Date: April 22, 1986 | doi: 10.1021/bk-1986-0304.ch004

Hans Scheinpflug Bayer A G , Agrochemical Division, Research and Development, Biological Research, D5090 Leverkusen, Federal Republic of Germany

Most of the older conventional fungicides such as the copper com pounds, the dithiocarbamates, and the halogenalkylmercapto-imids (e.g. Captan) can be classified as broad-spectrum protectant biocides. In general, these fungicides are not absorbed or translocated by the plant and inhibit spore germination of fungal pathogens by contact a c t i v i t y . In order to prevent infection, they must be present on the surface of the plant at or before appearance of the pathogen and are suited for prophylactic use only, which requires a uniform film on the plant surface. These characteristics limit the possibility of successful disease control, and in many cases exclude it. In contrast to these technically old-fashioned chemicals, compounds have been developed which differ in their chemical struc ture and mode of action. In addition to inhibiting spore germina tion, they interfere with different processes during the host coloni zation. These chemicals are absorbed and often translocated inter nally throughout the host plant. Translocated chemicals can have an inner-therapeutic action, stopping infection during and after the incubation period; in other words, they may act curatively or eradicatively. Histological and cytological studies were conducted to investigate the influence of several of these fungicides on various stages of fungal plant pathogens. Over the last few years, a number of papers have described the influence of the azole fungicides on fungal growth and the important points of attack in the host-pathogen-system. In this paper several of these results will be presented. Fungicide Mediated Morphological and Ultrastructural Changes of Fungi Most of the work on the mode of action of the azole fungicides has been conducted with Ustilago species. In early experiments by Buchenauer (2), i t was evident that triadimefon caused the sporidia of Ustilago avenae to lose their capacity for normal c e l l division. In nutrient solutions containing the fungicides, the sporidia, which normally separate after division, remained bound in mycelial-like clusters. 0097-6156/86/0304-0073$06.00/0 © 1986 American Chemical Society

In Fungicide Chemistry; Green, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.


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A f t e r treatment of IK avenae w i t h t r i a d i m e f o n , n u a r i m o l or o t h e r compounds w i t h the same mode of a c t i o n , s e v e r a l d i s t i n c t m o r p h o l o g i c a l changes c o u l d be seen a t the u l t r a s t r u c t u r a l l e v e l . This mainly c o n s i s t e d o f t h i c k e n i n g o f the p e r i p h e r a l c e l l w a l l and of i n c o m p l e t e f o r m a t i o n o f s e p t a , r e s u l t i n g i n a l a c k o f c e l l d i v i s i o n of a m u l t i branched mycelium. Further morphological and u l t r a s t r u c t u r a l changes c o u l d be determined u s i n g t h i n s e c t i o n i n g and f r e e z e e t c h i n g : a) C o n s i d e r a b l e v a c u o l i z a t i o n and a c c u m u l a t i o n o f l i p i d b o d i e s b) D e g e n e r a t i o n o f i n t r a c e l l u l a r o r g a n e l l e s and membranes, e s p e c i a l l y the e n d o p l a s m a t i c r e t i c u l u m c) D e f o r m a t i o n o f the plasmalemma d) I r r e g u l a r a g g r e g a t i o n o f intramembrane p a r t i c l e s e) R e g u l a r , h e x a g o n a l c l u s t e r i n g o f intramembrane p a r t i c l e s Richmond (12) made s i m i l a r o b s e r v a t i o n s w i t h B o t r y t i s a l l i i . P r i n g (11) s t u d i e d Uromyces v i c i a fabae i n f e c t i n g l e a v e s of b r o a d beans and P u c e i n i a r e c o n d i t a i n f e c t i n g wheat. R e s u l t s i n these t e s t s conducted i n the i n t a c t h o s t - p a t h o g e n system c o r r e l a t e d w e l l w i t h the changes observed i n n u t r i e n t s o l u t i o n . I n f l u e n c e of T r i a d i m e f o n , T r i a d i m e n o l genesis of Several Fungal Diseases

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I n v e s t i g a t i o n s w i t h s e v e r a l host-pathogen-systems have shown t h a t the a z o l e f u n g i c i d e s can have t h r e e e f f e c t s : a) A d i r e c t i n f l u e n c e on the pathogen by i n t e r f e r i n g w i t h i t s s t e r o l s y n t h e s i s system b) An i n d i r e c t i n f l u e n c e on the pathogen by the r e a c t i o n of the h o s t p l a n t c) An e p i d e m i o l o g i c a l e f f e c t by r e d u c i n g the inoculum p o t e n tial I n f l u e n c e on the P a t h o g e n e s i s of E r y s i p h e g r a m i n i s on B a r l e y and Wheat. I n a d d i t i o n t o c o n i d i o s p o r e s , w h i c h are formed i n response t o weather c o n d i t i o n s i n s h o r t e r o r l o n g e r g e n e r a t i o n c y c l e s , c l e i s t o t h e c i a a l s o bear s p o r e s w h i c h are formed as a r e s u l t of a s e x u a l p r o c e s s and d e v e l o p b e f o r e the r i p e n i n g of the c e r e a l s . A f t e r a p r o t e c t i v e treatment w i t h t r i a d i m e f o n , i n c o n c e n t r a t i o n s r a n g i n g from 0.001% - 0.025 % a . i . , and a f t e r a seed treatment w i t h 7.5 g - 27.5 g a i / 1 0 0 kg s e e d , i t was found t h a t spore g e r m i n a t i o n , as w e l l as development o f a p p r e s s o r i a of E. g r a m i n i s f . sp. h o r d e i were not a f f e c t e d (13). C e r t a i n changes i n appearance o f the a p p r e s s o r i a were n o t e d ; however, the f u n c t i o n o f the a p p r e s s o r i a was not i n f l u e n c e d ( 7 ) . Treatment d i d not a f f e c t the f o r m a t i o n of the i n f e c t i o n peg. Addi t i o n a l l y , t h e r e was no i n f l u e n c e o f the treatment on the f o r m a t i o n of c e l l w a l l a p p o s i t i o n s , termed p a p i l l a e , w h i c h are produced by the h o s t p l a n t i n response t o the fungus beneath the s i t e o f p e n e t r a t i o n . The f i r s t n o t a b l e d i f f e r e n c e between t r e a t e d and untreated o c c u r r e d when the fungus forms p r i m a r y h a u s t o r i a . At the b e g i n n i n g of the normal f o r m a t i o n of h a u s t o r i a , w h i c h t a k e s p l a c e j u s t one day a f t e r i n o c u l a t i o n , the e x t r a h a u s t o r i a l membrane shows a l i g h t t h i c k e n i n g and d i s t e n s i o n . L a t e r on h a u s t o r i a w h i c h d e v e l o p i n u n t r e a t e d



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p l a n t s a r e c h a r a c t e r i z e d by w e l l formed " f i n g e r s " ( F i g u r e 1 ) . The e f f e c t o f t r i a d i m e f o n as a s p r a y a p p l i c a t i o n and o f t r i a d i m e n o l a s a seed d r e s s i n g were b a s i c a l l y t h e same, i n h i b i t i n g t h e t y p i c a l forma t i o n o f these s t r u c t u r e s o r they underwent abnormal development ( F i g u r e s 2, 3, 4) ( 1 3 ) . On t h e f o u r t h and f i f t h day a f t e r i n o c u l a t i o n o f t h e t r e a t e d p l a n t s , s t a i n a b l e d e p o s i t s form on t h e h a u s t o r i a . These c o n s i s t m a i n l y o f p o l y s a c c h a r i d e s and a r e p r o b a b l y formed by t h e h o s t p l a n t ( F i g u r e 4 ) . The m a t e r i a l i s t h e same as t h a t i n t h e p a p i l l a e formed by t h e p l a n t i n response t o t h e p e n e t r a t i o n i n f e c t i o n peg. T h i s e n c a p s u l a t i o n c a n be completed 4 t o 8 days a f t e r i n o c u l a t i o n and i n d u c e s t h e fungus t o s t o p development and b l o c k s t h e f o r m a t i o n o f secondary h a u s t o r i a ( F i g u r e 5) ( 1 3 ) . I t i s o f s p e c i a l i n t e r e s t t h a t t h e type o f e n c a p s u l a t i o n seen i n b a r l e y v a r i e t i e s w i t h h i g h r e s i s t a n c e a g a i n s t powdery mildew i s t h e same a s t h a t seen i n t h e a z o l e f u n g i c i d e t r e a t e d p l a n t s . This s u g g e s t s t h a t t h e f u n g i c i d e i n d u c e s some k i n d o f r e s i s t a n c e r e a c t i o n i n the h o s t p l a n t . E n c a p s u l a t i o n o f h a u s t o r i a c o u l d a l s o be seen a f t e r c u r a t i v e f u n g i c i d e t r e a t m e n t . P r i m a r y h a u s t o r i a u s u a l l y were n o t encased b u t a l l h a u s t o r i a produced d u r i n g and a f t e r t h e t r e a t m e n t were d i s t i n c t l y e n c a p s u l a t e d , c a u s i n g a complete i n h i b i t i o n o f f u n g a l growth and b l o c k i n g spore f o r m a t i o n . T r i a d i m e f o n a t 250 ppm a i was sprayed on young mildew p u s t u l e s . A f t e r 24, 48 and 72 h o u r s , spores from t r e a t e d p u s t u l e s were h a r v e s t e d and t e s t e d f o r t h e i r c a p a c i t y t o germinate and cause i n f e c t i o n ( 7 ) . I t was found, t h a t t h e number o f g e r m i n a t i n g s p o r e s w h i c h caused i n f e c t i o n d e c r e a s e d w i t h i n c r e a s i n g time (Table I ) . The change i n appearance o f t h e mildew p u s t u l e s a f t e r t r e a t m e n t i s shown i n F i g u r e s 6 and 7 ( 7 ) .

Table I . P e r c e n t I n f e c t i o n o f B a r l e y by C o n i d i a from B a y l e t o n - T r e a t e d Powdery M i l d e w P u s t u l e s , i n R e l a t i o n t o Time o f A c t i o n .

Time o f A c t i o n (Hrs.) 24 48 72

P e r c e n t E. g r a m i n i s f . sp. h o r d e i I n f e c t i o n * Untreated** BAYLETON 94 56 91 27 88 4

*Means from 3 experiments u s i n g 10 p l a n t s p e r p o t w i t h r e p l i c a t i o n s . **Percent of l e a f area i n f e c t e d .

I n f l u e n c e on t h e P a t h o g e n e s i s o f V a r i o u s P u c c i n i a S p e c i e s o f S m a l l Grain Cereals. Wheat stem r u s t ( P u c c i n i a g r a m i n i s v a r . t r i t i c i ) i s an example o f a m a c r o c y c l i c r u s t fungus w i t h a l t e r n a t e h o s t s . The production o f s e v e r a l generations o f uredospores during the annual growth p e r i o d p r o v i d e s t h e b a s i s f o r an endemic appearance o f t h e brown r u s t ( P u c c i n i a r e c o n d i t a ) , as w e l l a s f o r t h e development o f an epidemic o f s t r i p e r u s t ( P u c c i n i a s t r i i f o r m i s ) . Kuck e t a l . (6)



FUNGICIDE CHEMISTRY

F i g u r e 1. H a u s t o r i a o f powdery mildew i n an u n t r e a t e d p l a n t . Note t h e presence of numerous w e l l - d e v e l o p e d " f i n g e r s " . (Repro duced w i t h p e r m i s s i o n from Ref. 13. C o p y r i g h t 1983 P f l a n z e n s c h u t z - N a c h r i c h t e n Bayer.)

F i g u r e 2. H a u s t o r i a of powdery mildew w i t h a b n o r m a l l y formed e x t r a h a u s t o r i a l membrane a f t e r p r o t e c t i v e a p p l i c a t i o n o f 0.005% B a y l e t o n (3 days p o s t - i n o c u l a t i o n ) . (Reproduced w i t h p e r m i s s i o n from Ref. 13. C o p y r i g h t 1983 P f l a n z e n s c h u t z - N a c h r i c h t e n B a y e r . )



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F i g u r e 3. Deformed h a u s t o r i u m of powdery mildew w i t h abnormally formed " f i n g e r s " a f t e r treatment w i t h 0.005% B a y l e t o n (6 days post-inoculation). (Reproduced w i t h p e r m i s s i o n from Ref. 1 3 . C o p y r i g h t 1983 P f l a n z e n s c h u t z - N a c h r i c h t e n B a y e r . )

F i g u r e 4. Encased h a u s t o r i u m o f powdery mildew a f t e r seed t r e a t ment w i t h Bayan 5 DS a t 0.015 g/10 g seed (5 days p o s t i n o c u l a t i o n , combined s t a i n i n g w i t h PAS and Coomassie B r i l l i a n t B l u e ) . (Reproduced w i t h p e r m i s s i o n from Ref. 13. C o p y r i g h t 1983 P f l a n z e n s c h u t z - N a c h r i c h t e n Bayer.)


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F i g u r e 5. E n c a p s u l a t i o n of an 8-day-old h a u s t o r i u m of powdery mildew a f t e r the p r o t e c t i v e treatment w i t h B a y l e t o n 25 WP ( s t a i n e d w i t h Coomassie B r i l l i a n t B l u e ) . (Reproduced w i t h p e r m i s s i o n from Ref. 13. C o p y r i g h t 1983 P f l a n z e n s c h u t z - N a c h r i c h t e n Bayer.)

F i g u r e 6. P u s t u l e of _E. g r a m i n i s f . sp. h o r d e i u n t r e a t e d (7 days post-inoculation). (Reproduced w i t h p e r m i s s i o n from R e f . 7. C o p y r i g h t 1979 P f l a n z e n s c h u t z - N a c h r i c h t e n Bayer.)



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worked w i t h stem r u s t ( P u c c i n i a g r a m i n i s v a r . t r i t i c i ) to s t u d y the i n f l u e n c e o f seed t r e a t m e n t w i t h t r i a d i m e n o l and p r o t e c t i v e and c u r a t i v e l e a f t r e a t m e n t w i t h t r i a d i m e f o n on the d i s e a s e development. N e i t h e r spore g e r m i n a t i o n nor f o r m a t i o n of a p p r e s s o r i a were s t r o n g l y a f f e c t e d by the d i f f e r e n t t r e a t m e n t s . Even the c o r r e c t l o c a t i o n of the a p p r e s s o r i a o v e r a stoma was not d i s t u r b e d ( F i g u r e 8 ) . D u r i n g the t y p i c a l i n f e c t i o n p r o c e s s , a s m a l l tube emerges from the a p p r e s s o r i u m w i t h i n 24 hours and, p a s s i n g through the s t o m a t a l a p e r t u r e , e n t e r s the s u b s t o m a t a l c a v i t y where the s u b s t o m a t a l v e s i c l e i s formed. I n t e r c e l l u l a r hyphae growing from the v e s i c l e produce mycelium w h i c h r a m i f i e s between h o s t c e l l s . Specialized terminal c e l l s of the i n t e r c e l l u l a r hyphae, the h a u s t o r i a mother c e l l s (HMC), produce a p e n e t r a t i o n peg t h a t e n t e r s the h o s t c e l l and g i v e s r i s e t o the h a u s t o r i u m . One day a f t e r t r e a t m e n t the number of h a u s t o r i a l mother c e l l s was not i n f l u e n c e d i n the t r i a d i m e n o l - and t r i a d i m e f o n - t r e a t e d p l a n t s when compared w i t h the u n t r e a t e d c h e c k s . Four days a f t e r i n o c u l a t i o n , 75% of the u n t r e a t e d m y c e l i a p r e s e n t i n the h o s t t i s s u e formed more than f i v e h a u s t o r i a l mother c e l l s compared t o 20-30% i n t r e a t e d tissue. I n the time between 6 and 14 days a f t e r i n o c u l a t i o n , the u n t r e a t e d mycelium had grown so i n t e n s e l y t h a t i t was i m p o s s i b l e to count the HMC and s p o r u l a t i o n had begun. Development of m y c e l i a , w h i c h formed more than 5 h a u s t o r i a l mother c e l l s , slowed down to a c o n s i d e r a b l e e x t e n t between day 6 and day 14 i n the t r e a t e d p l a n t s . W i t h p r o l o n g e d i n c u b a t i o n o f t r e a t e d p l a n t s an i n c r e a s i n g number of n e c r o t i c h o s t c e l l s c o u l d be v i s u a l i z e d i n e p i d e r m a l and m e s o p h y l l cells. I n o n l y a few c a s e s , the i n f e c t i o n c y c l e proceeded to form s p o r o g e n i c c e l l s ( 6 ) . N o r m a l l y , h a u s t o r i a do not s t a i n b r i g h t b l u e u s i n g f l u o r e s c e n t dye; however, Kuck e t a l . (5) m o d i f i e d the p r o c e dure to a l l o w o b s e r v a t i o n o f h a u s t o r i a . Host c e l l s a u t o f l u o r e s c e a b r i g h t y e l l o w when they become n e c r o t i c . F i g u r e 9 shows a g e r m i n a t i n g spore on a t r e a t e d l e a f s u r f a c e , an a p p r e s s o r i u m p o s i t i o n e d above a stoma and two e p i d e r m a l c e l l s w h i c h are a t t a c k e d by the fungus. Their c h a r a c t e r i s t i c yellow s t a i n i n d i c a t e s n e c r o t i c c e l l s c h a r a c t e r i s t i c of a h y p e r s e n s i t i v e r e a c t i o n . The next f i g u r e ( F i g u r e 10) demonstrates t h a t the i n t e r c e l l u l a r mycelium i n the t r e a t e d t i s s u e advanced to the m e s o p h y l l c e l l s where i t e l i c i t e d a h y p e r s e n s i t i v e response w i t h i n c o l o n i z e d , as w e l l as i n adjacent c e l l s . The f u n g i c i d e treatment i n d u c e d i n the s u s c e p t i b l e wheat v a r i e t y a r e a c t i o n ( h y p e r s e n s i t i v i t y ) w h i c h resembled t h a t of a h i g h l y r e s i s t a n t h o s t p l a n t a f t e r r u s t a t t a c k , as d e s c r i b e d e a r l i e r f o r powdery mildew and b a r l e y . Among the r u s t f u n g i , t h i s o b s e r v a t i o n was not o n l y r e s t r i c t e d to stem r u s t (P. g r a m i n i s f . sp. t r i t i c i ) . P a u l (9) c o u l d show a c o n s i d e r a b l e d i f f e r e n c e between t r e a t e d and u n t r e a t e d p l a n t s i n the s t a g e o f s u b s t o m a t a l v e s i c l e f o r m a t i o n of brown r u s t (P. r e c o n d i t a ) and, more pronounced, d u r i n g the development of i n t e r c e l l u l a r myce l i u m and h a u s t o r i a . A l s o , when i n f e c t e d wheat p l a n t s were t r e a t e d w i t h t r i a d i m e f o n p r i o r to the opening of the r u s t p u s t u l e s , these p u s t u l e s f a i l e d t o open. F u r t h e r s p r e a d i n g of the fungus i n the l e a v e s was stopped ( 1 4 ) . I n f l u e n c e of B i t e r t a n o l on the P a t h o g e n e s i s of V e n t u r i a i n a e q u a l i s (Apple Scab). Apple scab i s the most s e r i o u s d i s e a s e of a p p l e s and



FUNGICIDE CHEMISTRY

F i g u r e 7. P u s t u l e o f _E. g r a m i n i s f . s p . h o r d e i t r e a t e d w i t h B a y l e t o n 4 days a f t e r and then photographed 7 days a f t e r i n o c u l a tion. (Reproduced w i t h p e r m i s s i o n from R e f . 7. C o p y r i g h t 1979 P f l a n z e n s c h u t z - N a c h r i c h t e n Bayer·)

F i g u r e 8. Germinated r u s t spore t h a t has developed an a p p r e s s o r i u m above a stoma. (Reproduced w i t h p e r m i s s i o n from Ref. 6. C o p y r i g h t 1982 P f l a n z e n s c h u t z - N a c h r i c h t e n B a y e r . )



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F i g u r e 9. T h r e e - d a y - o l d i n f e c t i o n i n wheat t r e a t e d w i t h B a y l e t o n — g e r m i n a t i n g r u s t spore w i t h an a p p r e s s o r i u m p o s i t i o n e d above a stoma and two epidermal c e l l s w i t h h y p e r s e n s i t i v e r e a c t i o n ( f l u o r e s c e n c e m i c r o s c o p y m a g n i f i c a t i o n X 190, photograph reduced 8 0 % ) . (Reproduced w i t h p e r m i s s i o n from R e f . 6. Copy r i g h t 1982 P f l a n z e n s c h u t z - N a c h r i c h t e n Bayer.)

F i g u r e 10. Seven-day-old r u s t i n f e c t i o n i n wheat t r e a t e d w i t h B a y l e t o n showing h y p e r s e n s i t i v e r e a c t i o n of m e s o p h y l l c e l l s (mag n i f i c a t i o n X 190, photograph reduced 8 0 % ) . (Reproduced w i t h p e r m i s s i o n from R e f . 6. C o p y r i g h t 1982 P f l a n z e n s c h u t z - N a c h r i c h t e n Bayer.)



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i s found i n a l l a r e a s o f the w o r l d where a p p l e s are p r e s e n t l y grown. I n most c a s e s , a w e l l - t i m e d c h e m i c a l spray program has t o be f o l lowed i n o r d e r t o c o n t r o l the d i s e a s e . There i s g r e a t i n t e r e s t i n d e v e l o p i n g compounds w i t h a c u r a t i v e e f f e c t , compounds w h i c h are a b l e t o c o n t r o l the d i s e a s e a f t e r the e s t a b l i s h m e n t of the pathogen. The d i s e a s e c y c l e of a p p l e scab ( V e n t u r i a i n a e q u a l i s ) s t a r t s i n s p r i n g w i t h a s c o s p o r e s , w h i c h are d i s c h a r g e d from a s c i , causing p r i m a r y i n f e c t i o n i n the new l e a v e s . D u r i n g the summer, s e v e r a l g e n e r a t i o n s o f a s e x u a l l y formed c o n i d i a are produced w h i c h may cause i n f e c t i o n i n the same way as a s c o s p o r e s do. Spore g e r m i n a t i o n as w e l l as a p p r e s s o r i u m f o r m a t i o n were u n a f f e c t e d by b i t e r t a n o l ( F i g u r e 1 1 ) ( 1 0 ) . The d i m e n s i o n o f the s u b c u t i c u l a r stroma produced by the fungus was s i g n i f i c a n t l y s m a l l e r i n b i t e r t a n o l treated leaves. Three days a f t e r i n o c u l a t i o n , numerous hyphae arose from the s u b c u t i c u l a r stroma and kept growing between the c u t i c u l e and the o u t e r c e l l w a l l o f the u n t r e a t e d e p i d e r m a l c e l l s (Figure 12)(1). The development of these hyphae was suppressed by the b i t e r t a n o l treatment ( F i g u r e 13), and seven days a f t e r i n o c u l a t i o n the stroma s t a r t e d t o t u r n brown and t o d i s i n t e g r a t e ( F i g u r e 14). At the same time the fungus i n the u n t r e a t e d l e a v e s began t o s p o r u l a t e ( 1 ) . I f l e a v e s are t r e a t e d w i t h b i t e r t a n o l by 72 t o 96 h o u r s a f t e r i n o c u l a t i o n , the pathogen has a l r e a d y formed a stroma and the subcu t i c u l a r mycelium has developed ( F i g u r e 1 5 ) ( 1 ) . Three days a f t e r treatment one can see t h a t the mycelium has stopped growing and the p r o d u c t i o n of s p o r e s i s suppressed ( F i g u r e 1 6 ) ( 1 0 ) . S i x days a f t e r the f u n g i c i d e t r e a t m e n t , the stroma t u r n s brown and s t a r t s t o d i s i n t e g r a t e ( F i g u r e 1 7 ) ( O , but the fungus has a l r e a d y s p o r u l a t e d on the u n t r e a t e d l e a v e s ( F i g u r e s 18, 1 9 ) ( 1 0 ) . A f u n g i c i d e treatment s h o r t l y b e f o r e or a t the b e g i n n i n g of c o n i d i a p r o d u c t i o n caused a sharp r e d u c t i o n of c o n i d i a f o r m a t i o n . T h i s c o u l d be demonstrated by c o u n t i n g c o n i d i a o f t r e a t e d and un t r e a t e d specimens. In a d d i t i o n t o t h i s , the t r e a t e d l e s i o n s t u r n e d brown and became n e c r o t i c because the fungus had stopped growing ( 1 ) . Summary A z o l e f u n g i c i d e s cause c o n s i d e r a b l e m o r p h o l o g i c a l and u l t r a s t r u c t u r a l changes of f u n g a l p l a n t pathogens. These e f f e c t s o c c u r not o n l y under i n v i t r o c o n d i t i o n s , but they can a l s o be observed i n the i n t a c t host-pathogen-system. These f u n g i c i d e s a c t as p r o t e c t a n t s and as c u r a t i v e agents when a p p l i e d a g a i n s t powdery mildew on c e r e a l s . F u n g a l spore g e r m i n a t i o n and f o r m a t i o n of a p p r e s s o r i a remain a l m o s t unchanged a f t e r t r e a t m e n t . These f u n g i c i d e s cause e n c a p s u l a t i o n of h a u s t o r i a . I n t h i s way, the uptake o f n u t r i e n t s by the fungus can be reduced o r s t o p p e d . Spore g e r m i n a t i o n and a p p r e s s o r i a f o r m a t i o n d u r i n g the p a t h o g e n e s i s of r u s t f u n g i i s a f f e c t e d t o a v e r y low e x t e n t . The i n t e r f e r e n c e w i t h h a u s t o r i a l f o r m a t i o n i s the most i m p o r t a n t effect. C o l o n i z e d c e l l s undergo a h y p e r s e n s i t i v i t y r e a c t i o n ; f i n a l l y they become n e c r o t i c . F u r t h e r development of the o b l i g a t e p a r a s i t e i s t h e r e f o r e stopped. Spore g e r m i n a t i o n of V e n t u r i a i n a e q u a l i s , the apple scab f u n g u s , i s a l s o u n a f f e c t e d by the s t u d i e d a z o l e f u n g i c i d e s . They p r e v e n t



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F i g u r e 11. G e r m i n a t i n g spore of V e n t u r i a i n a e q u a l i s on a b i t e r t a n o l - t r e a t e d l e a f . (Reproduced w i t h p e r m i s s i o n from Ref. 10. C o p y r i g h t 1983 P f l a n z e n s c h u t z - N a c h r i c h t e n Bayer.)

F i g u r e 12. Development o f t h e s u b c u t i c u l a r stroma and mycelium of V e n t u r i a i n a e q u a l i s i n an u n t r e a t e d l e a f 3 days p o s t i n o c u l a t i o n . (Reproduced w i t h p e r m i s s i o n from Réf. 1. C o p y r i g h t 1981 P f l a n z e n s c h u t z - N a c h r i c h t e n Bayer.)



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F i g u r e 13. Suppressed development of the apple scab fungus i n a l e a f t h a t was t r e a t e d w i t h b i t e r t a n o l b e f o r e i n o c u l a t i o n ( p h o t o graphed 3 days p o s t - i n o c u l a t i o n ) . (Reproduced w i t h p e r m i s s i o n from Réf. 1. C o p y r i g h t 1981 P f l a n z e n s c h u t z - N a c h r i c h t e n Bayer.)

F i g u r e 14. Stroma s t a r t i n g to t u r n brown and to d i s i n t e g r a t e a f t e r treatment w i t h b i t e r t a n o l (7 days p o s t - i n o c u l a t i o n ) . (Reproduced w i t h p e r m i s s i o n from Réf. 1. C o p y r i g h t 1981 P f l a n z e n s c h u t z - N a c h r i c h t e n Bayer.)



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F i g u r e 15. Stage o f c u r a t i v e treatment w i t h b i t e r t a n o l showing s u b c u t i c u l a r mycelium (3 days p o s t - i n o c u l a t i o n ) . (Reproduced w i t h p e r m i s s i o n from Réf. 1. C o p y r i g h t 1981 P f l a n z e n s c h u t z N a c h r i c h t e n Bayer.)

F i g u r e 16. Three days a f t e r the c u r a t i v e treatment w i t h b i t e r t a n o l , the c o n i d i a a r e unable t o p i e r c e the c u t i c u l e ( s c a n n i n g e l e c t r o n m i c r o s c o p y , m a g n i f i c a t i o n X 600, photograph reduced 80%). (Reproduced w i t h p e r m i s s i o n from R e f . 10. C o p y r i g h t 1983 P f l a n z e n s c h u t z - N a c h r i c h t e n Bayer.)



FUNGICIDE CHEMISTRY

F i g u r e 17. S i x days a f t e r the b i t e r t a n o l t r e a t m e n t , the stroma s t a r t t o t u r n brown and d i s i n t e g r a t e . (Reproduced w i t h p e r m i s s i o n from Réf. 1. C o p y r i g h t 1981 P f l a n z e n s c h u t z - N a c h r i c h t e n Bayer.)

F i g u r e 18. C o n i d i o p h o r e s and r i p e c o n i d i a on the l e a f s u r f a c e of the u n t r e a t e d c o n t r o l 10 days p o s t - i n o c u l a t i o n ( m a g n i f i c a t i o n X 600, photograph reduced 8 0 % ) . (Reproduced w i t h p e r m i s s i o n from R e f . 10. C o p y r i g h t 1983 P f l a n z e n s c h u t z - N a c h r i c h t e n Bayer.)



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F i g u r e 19. R i p e c o n i d i a borne on t h e i r c o n i d i o p h o r e s ( m a g n i f i e ; t i o n X 3300, photograph reduced 8 0 % ) . (Reproduced w i t h permis s i o n from R e f . 10. C o p y r i g h t 1983 P f l a n z e n s c h u t z - N a c h r i c h t e n Bayer.)


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subcuticular growth of the fungus as well as conida formation. Since this effect is also achieved during the incubation period, these fungicides are suitable for curative use. Because of their systemic properties and their eradicative effect, these products are ideal as chemical control agents and especially suitable for integrated pest management. Literature Cited Downloaded by UNIV OF CALIFORNIA SAN DIEGO on November 5, 2015 | http://pubs.acs.org Publication Date: April 22, 1986 | doi: 10.1021/bk-1986-0304.ch004

1.

Brandes, W., and Paul, P. (1981): Studies on the Effect of Baycor on Apple Scab Pathogenesis, Pflanzenschutz-Nachrichten Bayer 34, 48 - 59. 2. Buchenauer, H. (1976): Mechanism of Action of Bayleton ( t r i a dimefon) in Ustilago avenae, Pflanzenschutz-Nachrichten Bayer 29, 281 - 302. 3. Hippe, S. (1982): Ultrastrukturelle Veranderungen in Sporidien von Ustilago avenae nach Behandlung mit systemischen Fungiziden, Dissertation Universitat Hohenheim. 4. Hippe, S. (1983): Morphology of Ustilago avenae after Treatment with Systemic Fungicides as Studied by Scanning Microscopy, Phytopath. Z. 106, 321 - 328. 5. Kuck, H. H . , Tiburzy, R., Hanssler, G., and Reisener, H. J . (1981): Visualization of Rust Haustoria in Wheat Leaves Using Fluorchromes, Physiological Plant Pathology 19, 439 - 441. 6. Kuck, Κ. H . , Scheinpflug, H . , Tiburzy, R., and Reisener, H. J . (1982): Fluorescence Microscopy Studies of the Effect of Bayleton and Baytan on Growth of Stem Rust in the Wheat Plant, Pflanzenschutz-Nacrichten Bayer 35, 209 - 228. 7. Paul, V . , and Scheinpflug, H. (1979): Studies of the Effect of Bayleton on Barley Mildew, Pflanzenschutz-Nachrichten Bayer 32, 80 - 89. 8. Paul, V. (1981): Biology of Venturia inaequalis (Cooke) Winter, the Pathogen of Apple Scab, Pflanzenschutz-Nachrichten Bayer 34, 60 - 74. 9. Paul, V. (1982): Studies on the Effect of Bayleton on Pathogensis of Brown Rust Wheat (Puccinia recondita f. sp. tritici), Pflanzenschutz-Nachrichten Bayer 35, 229 - 246. 10. Paul, V . , and Brandes, W. (1983): Scanning Electron Microscopy Studies on the Pathogens is of Apple Scab and i t s Control with Baycor, Pflanzenschutz-Nachrichten Bayer 36, 21 - 37. 11. Pring, R. J . (1984): Effects of Triadimefon on the Ultrastruc ture of Rust Fungi Infecting Leaves of Wheat and Broad Bean, Pesticide Biochemistry and Physiology 21, 127 - 137. 12. Richmond, D. V. (1984): Effects of Triadimefon on the Fine Structure of Germinating Conidia of Botrytis allii, Pesticide Biochemistry and Physiology 21, 74 - 83. 13. Smolka, S., and Wolf, G. (1983): Cytological Studies on the Mode of Action of Bayleton (triadimefon) and Baytan (triadimefon) on the Host-Parasite Complex - Erysiphe graminis f. sp. hordei, Pflanzenschutz-Nachrichten Bayer 36, 97 - 126. 14. Scheinpflug, Η., Paul, V . , and Kraus, P. (1978): Studies on the Mode of Action of Bayleton Against Cereal Diseases, Pflanzen schutz-Nachrichten Bayer 31, 101 - 115. RECEIVED

October

1, 1985


The Pathogenesis of Plant Diseases - ACS Symposium Series (ACS

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