Metabolites of Fungal Pathogens and Plant Resistance - ACS

Biotechnology for Crop Protection Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SANTA BARBARA on 08/05/18. For personal use only.

Chapter 9

Metabolites of Fungal Pathogens and Plant Resistance J. David Miller and Roy Greenhalgh Plant Research Centre, Agriculture Canada, Ottawa, Ontario K1A 0C6, Canada The long term breeding of crops for yield and quality has substantially reduced their natural resistance to pests and pathogens, resulting in a dependency on the use of pesticides for the protection of crops. Biotechnology offers alternative strategies for crop protection. An approach thought to offer the best long term potential is one that improves disease resistance of crops. It is known that the production of phytotoxins by pathogenic bacteria and fungi are important determinants of disease and that an appreciable measure of resistance to a pathogen can be related to the ability of the plant to deal with phytotoxins. This general approach involving the characterization of the secondary metabolites produced by certain fungal pathogens and studying their phytotoxicity has been applied to the problem associated with the infection of cereals by Fusarium graminearum. A variety of secondary metabolites produced by this fungus have been isolated and their phytotoxicity determined with respect to a number of wheat cultivars. The results indicated a correlation between tolerance of cultivars to these secondary metabolites and the disease resistance. This work demonstrates the utility of this approach in terms of diagnosing resistant cultivars in vitro and determining the mechanisms involved.

This a r t i c l e w i l l discuss the disease of wheat and corn (Fusarium head b l i g h t , maize ear rot) resulting from Fusarium graminearum i n f e c t i o n . Recent discoveries have been made that are leading us to the i d e n t i f i c a t i o n of disease resistant germplasm in plants by the study of metabolites of t h i s fungus and t h e i r effects on plant c e l l s and vice versa. The integration of the d i s c i p l i n e s of organic and analytical chemistry involving high resolution spectroscopic techniques with fermentation mycology and plant biotechnology has been very productive and lends i t s e l f to the 0097-6156/88/0379-0117$06.00/0 Published 1988 American Chemical Society

118

BIOTECHNOLOGY FOR CROP PROTECTION

development of disease resistant crop v a r i e t i e s . In the past, the application of such sophisticated chemical techniques to plant disease-host interactions has rarely been done because plant pathologists and chemists have tended to ignore each others' efforts. In many cases, the t r a d i t i o n a l chemical solutions to crop protection problems ( e . g . fungicides) have now become less acceptable to a number of groups in society and hence s c i e n t i s t s responsible for developing resistance in plants may f i n d i t useful to apply t h e i r s k i l l s and resources in new ways. Different strains of Fusarium graminearum are known to produce 30 to 40 secondary metabolites from d i f f e r e n t biogenic o r i g i n s , including the trichothecenes deoxynivalenol, dihydroxycalonectrin and n i v a l e n o l , the sesquiterpenes sambucinol and culmorin (all mevalonate derived), zearalenone (polyketide derived), butenolide (amino acid derived), and fusarin C (unknown pathway). Most of these secondary metabolites are mycotoxins, some of which are found in wheat or corn, and are harmful to human and animal health ( 1 , 2 ) . Fusarium disease and i t s associated toxins represent a worldwide problem, but e s p e c i a l l y in countries with north-temperate climates. In recent years, a large e f f o r t has been made to i s o l a t e and identify mycotoxins i . e . toxins harmful to humans and domestic animals (3,4,5) and to determine t h e i r effects p r i n c i p a l l y on animal health (6). This work concludes that infections by F. graminearum must be minimized in order to produce crops thaTTare s u f f i c i e n t l y free of mycotoxins. Although several corn and wheat breeding programs exist in North America, China and other countries that are designed to produce very resistant c u l t i v a r s or hybrids, however, to date none have succeeded. FUNGAL PATHOGENS AND PLANT RESISTANCE Wild plants, including ancestors of today's agricultural crops, generally possess good resistance to both pests and pathogens. It i s generally assumed that pathogens evolved from a state of compatibility with the host rather than by acquiring incompatibility de novo. The mutation rate that drives t h i s change resulting in a virulence condition, has been examined for a few species of fungal pathogens and i t has been found to be modest. Plants have two types of genes that govern t h e i r responses to pathogens: (a) resistance genes and (b) so-called s u s c e p t i b i l i t y genes. Resistance genes are i d e n t i f i e d using the virulence genes in the pathogen. S u s c e p t i b i l i t y genes are thought to be linked t o , or are, useful genes for the health of the plant. Plant breeders r e a l i z e that resistance genes should be sought in the geographic o r i g i n of the plant ("Vavilov's Rule"). Intensive breeding for y i e l d normally increases the incidence of s u s c e p t i b i l i t y genes (7). Another "biological fact of l i f e " i s that resistance genes require metabolic energy in terms of the synthesis of various gene products, the plant y i e l d can be affected negatively. S i m i l a r l y , the lack of s u s c e p t i b i l i t y genes in wild plants contributes to ' y i e l d ' reduction. Evidence for these generalizations has come from intensive studies of a few host/pathogen combinations, especially the so-called biotrophic diseases ( e . g . rusts) so that whatever models have been developed are largely based on a s p e c i f i c

9.

MILLER AND GREENHALGH

Metabolites of Fungal Pathogens

119

type of fungal host disease i n t e r a c t i o n . Thus, i t i s not known whether a l l the ' r u l e s ' about fungal disease of crop plants which were derived from the 'gene for gene' hypothesis (8) always apply. With fungal diseases, i t i s clear that many genes govern the a b i l i t y of a given s t r a i n to attack a plant. Vanderplank (7), from a breeding perspective noted, that "It i s a common assumption in the l i t e r a t u r e to assume the existence of any two a l l e l e s for avirulence/virulence. A genotype of a pathogen i s , on t h i s assumption, either virulent for a p a r t i c u l a r resistance gene or i t i s not. This assumption i s wrong. Within a pathogen, virulence i s determined by multiple a l l e l e s having considerable v a r i a t i o n " . From a biochemical/plant pathology perspective, de Wit (9) stated that "Instances where resistances can be attributed solely to a single defense mechanism are very few; several responses may occur simultaneously or consecutively. Undoubtedly, many genes must be involved in the successful development of a parasite in i t s host and the relationships with host gene functions could be complex." Yoder and Turgeon (10) i l l u s t r a t e several examples where multiple pathogenecity factors are being analyzed from a molecular biology perspective. From a plant breeding point of view, efficacious resistance genes regulating biochemical events which control the attack of a given pathogen are the most desired. Many, but not a l l aspects of the host pathogen interaction are amenable to further chemical study. These include: (A) e l i c i t o r s systems wherein molecules given off by the fungus induce the plant to produce new compounds or enzyme systems that r e s t r i c t further growth of the pathogen e . g . phytoalexins, l i g n i f i c a t i o n , chitinases (9); chemistry i s needed to purify and characterize the fungal products and the resulting plant products; (B) phytoalexin production by virulent strains of pathogens and degradation by resistant plants are known to occur in a variety of host/pathogen interactions (10,11). New research i s needed to i d e n t i f y phytoalexins and t h e i r degradation products e s p e c i a l l y where the responsible pathogen genes are known (12). In both A and B, only the easy-to-identify molecules (by modern standards) have been adequately studied to date. (C) Phytotoxins. PHYTOTOXINS REVISITED The subject of phytotoxins i s a contentious one in plant pathology. Only a generation ago, few s c i e n t i s t s believed that phytotoxins played a role in pathogenesis. The discovery of a number of host s p e c i f i c phytotoxins has brought general acceptance to the notion that they are important in disease development but that non-specific phytotoxins are usually considered of secondary importance. Almost a l l the data we have on phytotoxins are from a few fungi and plant species. Several of the known phytotoxins are r e l a t i v e l y complex and thus have proved to be d i f f i c u l t molecules to characterize. These problems among others, which are highlighted below, arguably conspire to devalue the importance of phytotoxins in plant diseases as well as the biochemical degradation of such compounds by resistant plant types. For the purposes of t h i s discussion, i t i s useful to raise a few issues covered in Yoder's valuable review on toxins in pathogenesis (13).

120


1. HOST SPECIFICITY. Phytotoxin s p e c i f i c i t y for susceptible plants suggests a role in disease. Yoder argues that host s p e c i f i c i t y i s not proof of t h e i r involvement in pathogenesis because a number of man-made chemicals as well as culture f i l t r a t e s from saprophytic fungi also have apparent host s p e c i f i c i t y . Caution must be used here, in that as fungal metabolites share biogenic o r i g i n s , the number of biochemical lesions possible in a plant i s l i m i t e d . 2. PHYTOTOXINS IN INFECTED PLANTS . Yoder notes that the presence or absence of toxins i s i n s u f f i c i e n t evidence to determine i f a fungal metabolite i s a phytotoxin or not since " I n a b i l i t y to detect toxins in tissues may be due to t h e i r inactivation by host t i s s u e , or t h e i r presence at concentrations below l i m i t s of detection". An example of the l a t t e r are the AM toxins I and II which induce necrosis in susceptible "Indo" apples at O.lppb (14,15). These points need to be considered in l i g h t of recent information on the physiology of toxin production by fungi in vivo (discussed below) and the power of analytical techniques. 3. INDUCTION OF TYPICAL DISEASE SYMPTOMS. Yoder notes that the production of visual or physiological (e.g. conductivity or respiration changes) symptoms are not r e l i a b l e indicators of a compound's involvement in pathogenesis. It would be surprising i f t h i s were not so because fungi use multiple strategies to invade plant t i s s u e . The application of a pure phytotoxin to a plant tissue cannot be considered evidence for involvement in pathogenesis, especially with respect to visual symptomology. S i m i l a r l y , the absence of a plant tissue response to a putative phytotoxin i s not necessarily evidence that a phytotoxin i s not involved. It i s necessary to examine both susceptible and resistant germplasm in order to discover the resistance genes. Yoder (13) makes the additional comment that "virulence may not always be correlated with the quantity of toxin produced in culture" even when the toxin i s known to play a role in pathogenesis. It i s important to note that the production of either primary or secondary metabolites in v i t r o i s a complicated problem as w i l l be discussed l a t e r . Research to optimize metabolite production in v i t r o has seldom been carried out for phytotoxins. This renders any positive correlation between in v i t r o toxin production and virulence of a strain purely f o r t u i t o u s . A poor correlation may simply indicate that suboptimal conditions for the production of the putative phytotoxins were used. Reduced to the simplest case, fungal toxins are either primary or secondary metabolites, and a l l phytotoxins f a l l into one of these two categories in a physiological sense. Normally, wild type i s o l a t e s of fungi do not produce large quantities of primary metabolites because - by d e f i n i t i o n - these compounds are required for the normal growth of the fungus. As a result almost a l l non-volatile toxins excreted by filamentous fungi are secondary metabolites. As noted e a r l i e r , there are few studies that suggest the type of experiments necessary to determine whether a phytotoxin i s a true secondary metabolite. Some phytotoxins are d e f i n i t e l y secondary metabolites but i t i s arguable that most are.

9.



121

The s t r i c t d e f i n i t i o n of a fungal secondary metabolite i s a compound that i s produced as a result of a nutrient l i m i t a t i o n . This means that a fungal c e l l , possessing the requisite genes, w i l l excrete a secondary metabolite when i t has grown, matured and i s subjected to a l i m i t a t i o n of a c r i t i c a l nutrient. In cultures grown in s t i r r e d j a r fermentors, fungi have (A), f i r s t a rapid growth phase during which a synchronous population of c e l l s are exposed to optimal nutrient and physical/chemical conditions, resulting in an increase in dry weight and the assembly of the necessary enzymes to produce one or more of the secondary metabolites controlled by the c e l l ' s genetic information followed by (B), exhaustion of some nutrient by the growth of the c e l l s , which i s required for primary metabolism. At t h i s point the c e l l s stop increasing in dry weight. During t h i s time, when the c e l l s are deprived of essential nutrients, e . g . nitrogen, the pools of primary metabolites such as acetate, mevalonolactone and/or amino acids accumulate. The c e l l ' s enzymes involved in secondary metabolism u t i l i z e these primary metabolites and the biosynthesis of the secondary metabolite commences and f i n a l l y , at stage (C), the c e l l s die (16, 17, 18). The above steps in secondary metabolism have been known since the mid-1940's. It has also been known for a long time that certain fungal secondary metabolites end up in crop residues ( e . g . Fusarium mycotoxins) although i t has been d i f f i c u l t to correlate data from s t i r r e d j a r fermentors studies with that from crop studies e . g . corn ears. Recently, however, Hale and Eaton (19) demonstrated that the c h a r a c t e r i s t i c c a v i t i e s of wood soft rot fungi (mostly molds) are explained by the fact that filamentous fungi do not grow in a continuous fashion in s o l i d substrates. Fungal growth was demonstrated to be o s c i l l a t o r y in nature and there are three stages of growth. A f i n e hyphal extension f i r s t grows out, growth stops, then the hyphae fatten and the cavity around the mycelium widens, during which time the nutrients around the hyphae become exhausted. The production of certain wood-degrading enzymes are known to be 'secondary' processes induced by nitrogen l i m i t a t i o n (e.g. 20). Since hyphal growth i s o s c i l l a t o r y in solids such as wood, t h i s i s comparable with our current understanding of the induction of 'secondary' biochemistries such as the production of certain degradative enzymes. This growth pattern of fungi and the production of secondary metabolites in s o l i d substrate appears to be correlated with the data from fermentor studies. In the 19th century, mycologists reported that growth and metabolic a c t i v i t y occured primarily in a few terminal c e l l s of a hyphae (21). In molds such as Fusarium, nuclei occur most frequently in the terminal few c e l l s . The production of the secondary metabolite zearalenone by F. graminearum in a s o l i d fermentation has been shown to occur in localized segments of the mycelia which then stops (22) and the zearalenone metabolite diffuses into the substrate. In our opinion, t h i s information reconciles the in vivo and in v i t r o data concerning the production of secondary metabolites by filamentous fungi and suggests that a very serious look be taken at

122


phytotoxins in plant disease. Yoder (13) notes that the problem of detecting a putative phytotoxin in infected plant tissue can be due to l a b i l i t y , i n a c t i v a t i o n of the compound or insensitive analytical procedures for detecting low concentrations. A d d i t i o n a l l y , the argument i s made against a role of certain phytotoxins in plant disease i s that t h e i r presence i s merely coincidental with pathogenesis and i s not responsible for pathogenicity. The l i k e l i h o o d that phytotoxins are produced as hyphal extension occurs raises serious issues regarding t h e i r detection and coincidence. In other words, the l a b i l i t y , detectabi1ity and turnover of phytotoxins in host tissue are real p o s s i b i l i t i e s . Coincidence as an explanation for the presence of potent phytotoxins i s not tenable since secondary metabolites, i f they are produced at a l l , must be produced just after hyphal penetration of a new area of plant t i s s u e . FUSARIUM GRAMINEARUM METABOLITES IN CEREALS Our studies on phytotoxins and plant disease started in 1982 when a 15-acetoxydeoxynivalenol producing s t r a i n of F. graminearum was used to inoculate corn. One parameter of the study was to monitor the occurrence of various mycotoxins in the infected ears weekly u n t i l harvest. Two remarkable observations were made: (A) very l i t t l e 15-acetoxydeoxynivalenol was found, only high levels of deoxynivalenol and (B) when fungal growth stopped, there was a decline in the absolute amount of deoxynivalenol (23). Similar phenomena were observed in both experimental (24) and natural infections of wheat (25; A. Tiech, pers. com.). Analysis of experimentally infected spring cereals showed that susceptible wheat c u l t i v a r s have a fungal biomass (ergosterol) to deoxynivalenol r a t i o s of c a . 2:1; t h i s r a t i o was also observed in heavy natural infections of Fusarium head blight of susceptible c u l t i v a r s (26). By comparison, resistant c u l t i v a r s experimentally inoculated with F. graminearum gave much higher ergosterol to deoxynivalenol radios (27,28). These data imply that some c u l t i v a r s of wheat can modify Fusarium toxins, a suggestion also made by Yoshizawa in 1975. This speculation has since been confirmed for a number of Fusarium head blight resistant c u l t i v a r s while susceptible c u l t i v a r s tested do not show t h i s phenomena (29; M i l l e r , unpublished). Further experiments were i n i t i a t e d in which the effects of various metabolites produced by F. graminearum were evaluated using a c o l e o p t i l e tissue assay devisecTby Cutler (30). These experiments demonstrated that the trichothecenes deoxynivalenol (XII) and 3-acetoxydeoxynivalenol (XI) are very phytotoxic to wheat tissue (28; Table I ) . Trichothecenes have been reported to affect plant tissue in various ways, especially increasing e l e c t r o l y t e loss (31,32). Fusarium graminearum i s known to produce a large number of minor metabolites (Figure I) (3); some of these compounds such as dihydroxycalonectrin (VIII), butenolide (II) and sambucinol (XXII) were also tested using the c o l e o p t i l e bioassay but were found to be less phytotoxic than the deoxynivalenol. The c y c l i c depsipeptides enniatin A and B, produced by related Fusarium species, were also found to be phytotoxic (see 33). For a l l the

9. MILLER AND GREENHALGH

to cx> ο

m

• *κ ***•* -κ* > r-»

en ^3- ο


ιο

ο ο ο

co

m οο m co

CM

co co

en cr» m co

m co

co m

ι—

* * * *

io

·— cr»

* * ** I O N O O

coNOcn

r — CO

CVJ m cxj · · • · i n co co ^ί· en CM co ——

r-

* * ** * * ** 00

LO

* * **

kOOCVJIs

^1" CM CO CM

Ι Ο •— P-* Cn

w o ^ « t

CM co en co

co cx» cr> r -

* * -te *

*

CM Ο

Ο

CO r -

Ο

* CM

m in

00

* * * * 4c

4c 4e 4c

co en m en

0000

CO CO CT»

^

CM

* * ** CM CM Ο

CO

co ^ - i n 10 1 1 1 1 O O O O

ΟΙΛΟΟ*

O N O N

ο r*. «a- ^ ·

ι— CO

r — CM Ο

LO CO CM CM

* * **

* * ** co en CM

ι ο f"-- ir> vo

I D CO t O CO

en i o

CM

ο

^

co^-inio 1 1 1 1 O O O O

co co r>»

«3-

* **

co i n «0 1 1 1 1 O O O O

Ο

1^ CO ι—

ο en η-»

CM

co ο

CM

m

ID

co

cr»

Γ

r—

co

1

J

9.


127


H

ρ

^m

—

ι

1 iB0

.

1 tee

•

1

i

1

140

i

1

120

100

•

1 B0

•

1

60

'

1

40

'

r20

PPM

Figure 2. C NMR spectrum of crude fungal extract showing the [presence of 3-acetyldeoxynivalenol and putative glycoside. Pure C labelled 3-acetoxydeoxynivalenol (20 mg) was added to 1L of a c e l l suspension of wheat (Su mei) in an N.B. S c i e n t i f i c Celligen fermentor for 14 days. The medium was f i l t e r e d and the f i l t r a t e extracted with ethyl acetate. The solvent was removed and the residues dissolved in CDC1 . 0

128


rather than evolve a whole series of host s p e c i f i c toxins. Plant types that can tolerate these potent i n h i b i t o r s of protein synthesis that are produced during fungal invasion should be expected to better r e s i s t the pathogen. New proteins can be synthesized as the fungus invades the plant that result in the synthesis of phytoalexins, chitinases, e t c . SUMMARY These experiments have i d e n t i f i e d a number of approaches involving the techniques of chemistry and biotechnology (tissue culture, molecular biology) that are helping to identify disease resistant germplasm. Rapid screening techniques based upon plant tissue or suspension culture response to the toxins from Fusarium are proving to be a useful way to v e r i f y the resistance of advanced breeding material (37). The enzymatic basis f o r the deoxynivalenol degradation phenomenon by c e l l culture i s also being explored. This approach should lead to the i d e n t i f i c a t i o n of the genetic basis of response, with a view to both the selection f o r and addition to the development agronomically of improved c u l t i v a r s (see 37). ACKNOWLEDGMENT The authors wish to thank Dr. B.A. Blackwell f o r the C-NMR spectral analysis, and Drs. George Fedak and John Simmonds f o r assistance with wheat tissue culture. 13

LITERATURE CITED 1.

Foster, B . C . ; Trenholm, H . L . ; Friend, D.W.; Thompson, B.K.; Hartin, K.E. Can. J . Anim. Sci. 1986, 66, 1149. 2. Hesseltine, C.W. In P.S. Steyn and R. Vleggar (Eds), Mycotoxins and Phycotoxins. Elsevier Science Publishers, Amsterdam. 1986, 1. 3. Greenhalgh, R.; Meier, R.M.; Blackwell, B.A.; Miller, J . D . ; Taylor, Α.; ApSimon, J.W.. J . Agric. Fd. Chem. 1984, 32, 1261. 4. Greenhalgh, R.; Levandier, D.; Adams, W.; Miller, J . D . ; Blackwell, B.A.; McAlees, Α.; Taylor, A. J . Agric. Fd. Chem. 1986. 34, 98. 5. Greenhalgh, R.; Meier, R.M.; Blackwell, B.A.; Miller, J . D . ; Taylor, Α.; ApSimon, J.W. J . Agric. Fd. Chem. 1986. 34, 115. 6. Trenholm, H . L . ; Hamilton, R.M.G.; Friend, D.W.; Thompson, B.K.; Hartin, K.W. J . Am. Vet. Med. Assoc. 1984. 185, 527. 7. Vanderplank, J.E. Disease resistance in plants. Academic Press, New York. 1984. 8. Flor, H.H. Ann. Rev. Phytopathol. 1971, 9, 275. 9. deWit, P.J.G.M. In J . Bailey (Ed), Biology and Molecular Biology of plant pathogen interactions. Springer-Verlag Berlin, 1986, 149. 10. Yoder, O.C., Turgeon, D.G. Gene manipulations in fungi. Academic Press, New York, 1985, 417. 11. Jost, J.D. Phytopathol. Z. 1965, 54, 338.

9. MILLER AND GREENHALGH


129

12. Yoder, O.E.; Turgeon, B.G. In T.W.E. Timberlake (Ed), Molecular genetics of filamentous fungi. A.R. Liss, Inc. New York, 1985, 383. 13. Yoder, O.C. Ann. Rev. Phytopathol. 1980, 18, 103. 14. Veno, T.; Nakashima, T.; Hayashi, Y.; Fukami, H. Agri. Biol. Chem. 1975. 39, 1115. 15. Veno, T.; Nakashima, T.; Hayashi, Y.; Fukami, H. Agric. Biol. Chem. 1975. 39, 2081. 16. Anon. Protection against trichothecene mycotoxins, National Academy Press, Washington, D.C. 1983, 26. 17. BuLock, J.D. In J.E. Smith and D.A. Berry (Eds), The filamentous fungi, Vol. 1, Academic Press, New York, 1975, 33. 18. Miller, J.D.; Blackwell, B.A. Can. J. Bot., 1986, 64, 1. 19. Hale, M.D.; Eaton, R.A. Trans. Bro. Mycol. Soc. 1955, 84, 277. 20. Keyser, P.; Kirk, T.K.; Zeikus, J.G. J. Bact. 1978, 135, 790. 21. Burnett; J.H. Fundamentals of mycology, Edward Arnold, London, 1976, 61. 22. Morita, H.; Singh, J.; Fulcher, R.G. Can. J. Plant Pathol. 1984, 6, 179. 23. Miller, J.D.; Young, J.C.; Trenholm, H.L. Can. J. Bot. 1983, 61, 3080. 24. Miller, J.D.; Young, J.C. Can. J. Plant Pathol. 1985, 7, 132. 25. Scott, P.M.; Nelson, K.; Kanhere, S.R.; Karpinski, K.F.; Hayward, S.; Neish, G.A.; Teich, A.H. Appl. Environ. Microbiol. 1984, 48, 884. 26. Love, G.R.; Seitz, L.M. Cereal Chem. 1987, 62, 124. 27. Miller, J.D.; Young, J.C.; Sampson, D.R. Phytopathol. Z. 1985, 113, 354. 28. Wang, Y.Z.; Miller, J.D. J. Phytopathol. 1987, in press. 29. Miller, J.D.; Arnison, P.G. Can. J. Plant Pathol. 1986, 8, 147. 30. Cutler, H.G.; Jarvis, B.B. Environ. Expt. Bot. 1985, 25, 115. 31. Brian, P.W.; Dawkins, W.; Grave, J.F.; Aemming, H.G.; Lowe, D.; Norvis, G.L.F. J. Exp. Bot. 1961, 12, 1. 32. Jacobellis, N.S.; Bottalico, A. Phytopath. Medit. 1981, 20, 129. 33. Drysdale, R.B. In M.O. Moss and J.E. Smith (Eds), The applied mycology of Fusarium. Cambridge University Press. 1984, 95. 34. Kuti, J.O.; Ng, T.J.; Bean, G.A. 1987. Hort Science. 1987, 22, 635-637. 35. Kuti, J.O. Genetic and biochemical studies of resistance in muksmelon (Cucumis melo L.) to Myrothecium roridum Tode. ex. Fries and its trichothecene mycotoxin roridin E. Ph.D. thesis, University of Maryland, 1987. 36. Schroeder, H.W.; Christensen, J.J. Phytopathology, 1963, 53, 831. 37. Wang, Y.Z.; Miller, J.D. Proc. CIMMYT conference on tropical wheat production, Jan. 1987, Thailand, CIMMYT, Mexico. 38. Hall, R. Can. J. Plant Pathol. 1987, 9, 152. RECEIVED March 8, 1988

Metabolites of Fungal Pathogens and Plant Resistance - ACS

Recommend Documents