Chapter 11
Interactions of Pathogens Surfaces
on Plant Leaf
C. M . Kenerley and J . H. Andrews 1
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Department of Plant Pathology and Microbiology, Texas A & M University, College Station, TX 77843 Department of Plant Pathology, University of Wisconsin—Madison, Madison, WI 53706
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Leaves of terrestrial plants support a diverse assemblage of microbes, predominantly filamentous fungi, yeasts, and bacteria. The composition of this microbial community changes continuously as species enter by immigration and exit by death or emigration. Population densities fluctuate over several orders of magnitude and are influenced by many variables, including climatic and seasonal factors, host species, air spora, and leaf position within the canopy. Nonpathogens in the community buffer against infection, mainly by competing directly with pathogens for nutrients (exploitation competition) or by indirectly inhibiting pathogen growth (interference competition). These interactions are strongly influenced by the environment. As such they differ not only between greenhouse and field, but can be expected to vary in different microbial habitats. The development of successful bioherbicides will require a thorough understanding of the ecology of potential agents and their interactions with other phylloplane associated microbes within the phylloplane environment. This understanding will provide the base for defining the task expected of the bioherbicide and the combination of attributes necessary to accomplish the task. Interactions among nonpathogenic and pathogenic microorganisms on the leaf surface (phylloplane or phyllosphere) are dynamic and influence the outcome of i n f e c t i o n b y p l a n t p a t h o g e n s , w h e t h e r t h e p a t h o g e n p o p u l a t i o n is u n m a n i p u l a t e d or is d e l i b e r a t e l y i n c r e a s e d , as is the case w i t h m i c r o b i a l herbicides. T h e physical and chemical characteristics of the plant surface set the stage f o r this microbial community by presenting morphological features a n d e x u d i n g c o m p o u n d s that e n h a n c e o r i n h i b i t l e a f c o l o n i z a t i o n b y pathogens and the phylloplane microbes. F i n a l l y , the environment acts on a l l the m i c r o o r g a n i s m s , as w e l l as o n the p l a n t . S u c c e s s f u l b i o c o n t r o l o f pathogens or weeds necessitates a broad u n d e r s t a n d i n g o f the p h y l l o p l a n e m i c r o b i a l c o m m u n i t y a n d how it interacts w i t h the leaf a n d m i c r o c l i m a t e . T h e issue of biocontrol of weeds by microbial herbicides is really the same as b i o c o n t r o l o f f o l i a r pathogens, only v i e w e d f r o m the p e r s p e c t i v e o f increasing rather than decreasing disease development. 0097-6156V90A)439-O192$07.50A) © 1990 American Chemical Society
In Microbes and Microbial Products as Herbicides; Hoagland, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
11.
KENERLEY A ANDREWS
Interactions of Pathogens on Plant Leaf Surfaces 193
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Physical and Chemical Characteristics of Plant Surfaces Anatomy. T h e anatomical and morphological features of a leaf that surfacedwelling or epiphytic microbes encounter include the roughness or pattern of the s u r f a c e as d e t e r m i n e d b y the t o p o g r a p h y o f the e p i d e r m a l c e l l s , s u p e r f i c i a l wax a n d c u t i c u l a r layers, various cavities (stomata), a n d projections (trichomes). These features are continually changing as a result of leaf growth, a g i n g , m i c r o b i a l a c t i v i t y , weathering, plant n u t r i t i o n , a n d cultivation practices. In c o m b i n a t i o n w i t h the c u t i c l e , the e p i c u t i c u l a r w a x e s r e d u c e transpiration a n d control gas exchange ( F i g u r e 1). Waxes, w h i c h consist of complex mixtures of various l o n g - c h a i n a l i p h a t i c compounds and t r i t e r p e n o i d s (1-5), also r e d u c e s u r f a c e w e t t a b i l i t y a n d p o s s i b l y o f f e r a c h e m i c a l b a r r i e r to i n f e c t i o n by p l a n t pathogens (3,6,7) ( F i g u r e 2). T h e morphological appearance of surface waxes varies among plants a n d may be a m o r p h o u s - , t u b e - , r i b b o n - , or p l a t e - s h a p e d . T h e s e patterns are caused p r i m a r i l y b y the d i f f e r e n c e s i n the c h e m i c a l n a t u r e , c o m p o s i t i o n a n d distribution patterns (2,4,8). In the case of johnsongrass (Sorghum halepense L.), the leaves at emergence are covered with a smooth amorphous wax, but plates of crystalline wax form on the amorphous wax w i t h i n 1 or 2 days (9). A f t e r 3 or 4 weeks f r o m emergence, a smooth layer o f coalescence wax is d e p o s i t e d over the wax plates (9). T h e thickness also v a r i e s w i t h p l a n t species, age o f the p l a n t , s u r f a c e o f the l e a f assayed, a n d e n v i r o n m e n t a l conditions, especially light. Wax thickness and ornamentation have recently been correlated with contact angles of droplets of spray formulations applied to the adaxial (upper) surface (1,2,4,5). A n i n h i b i t o r y effect (generally a measure o f spore g e r m i n a t i o n a n d germ tube g r o w t h ) o f l e a f w a x e x t r a c t e d f r o m s e v e r a l p l a n t s (e.g. c h r y s a n t h e m u m , Chrysanthemum morifolium L . ; sugar beet, Beta vulgaris L . ; apple, Malus pumila M i l l . ) has been demonstrated against such pathogens as Botrytis cinerea a n d Podosphaera leucotricha (7,10,11). C o n i d i a o f B. cinerea were completely inhibited f r o m germinating when applied with nutrients i n aqueous droplets to cavities c o n t a i n i n g d r i e d deposits o f wax m a t e r i a l at concentrations o f 0.05 to 0.25 m g / c m ^ o f c a v i t y surface (7). H o w e v e r , too few s t u d i e s h a v e been c o n d u c t e d to assess the s i g n i f i c a n c e o f these compounds i n relation to their influence on microbial colonization of leaves. Wax chemistry, as it determines wax structure, affects the wettability of the leaf surface. Surface wetting is important i n f o l i a r uptake a n d spread of c h e m i c a l s a n d n u t r i e n t s a n d is u n d o u b t e d l y i n v o l v e d i n d i s t r i b u t i o n o f microbes on leaf surfaces. Chemicals are generally deposited either u n i f o r m l y over the entire droplet area or as an annulus at the periphery of the d r o p l e t . A n n u l a r p a t t e r n s o f d e p o s i t i o n w i l l d e c r e a s e the a r e a o f interface between the leaf and the droplet and create chemical concentration gradients which do not result i f the compound is u n i f o r m l y distributed. T h i s effect on pesticide uptake a n d d i s t r i b u t i o n has been examined w i t h some c o m p o u n d s a n d f o u n d to v a r y w i t h species a n d the c h e m i c a l (4). T h e i n f l u e n c e o f wax on n u t r i e n t c o n c e n t r a t i o n g r a d i e n t s a n d d e p o s i t i o n o f m i c r o b e s i n aersols or r a i n f a l l , a n d t h e i r subsequent d i s t r i b u t i o n on l e a f surfaces, has not been investigated. Beneath the e p i c u t i c u l a r wax layer is the cuticle w h i c h bounds the e p i d e r m a l c e l l s a n d l i n e s the s u b - s t o m a t a l c a v i t i e s . S t r u c t u r a l l y , it is noncellular and often multi-layered, comprising an inner region which merges with cellulose fibrils of the epidermal cell wall (cuticular layer, f i b r i l l a r i n o r g a n i z a t i o n ) (2). T h e c h e m i c a l c o m p o n e n t of the c u t i c l e p r o p e r is an
In Microbes and Microbial Products as Herbicides; Hoagland, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 18, 2015 | http://pubs.acs.org Publication Date: September 25, 1990 | doi: 10.1021/bk-1990-0439.ch011
194
MICROBES AND MICROBIAL PRODUCTS AS HERBICIDES
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Fig 1. The outer covering of land plants. W wax rodlets; L lamellae of cutin and wax; R r e t i c u l a t e region of cutin and wax with c e l l u l o s e f i b r i l s ; P, pectin; CW c e l l wall of alternating layers of c e l l u l o s e f i b e r s and layers of hemicellulose plus pectin; PL, plasmalemma; E ectodesma. (Reproduced with permis sion from Ref. 2. Copyright 1983 Edward Arnold.) #
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In Microbes and Microbial Products as Herbicides; Hoagland, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
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11.
KENERLEY & ANDREWS
Interactions ofPathogens on Plant Leaf Surfaces
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Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 18, 2015 | http://pubs.acs.org Publication Date: September 25, 1990 | doi: 10.1021/bk-1990-0439.ch011
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fatty acids
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