Pesticide Resistance Strategies and Tactics for Management (1986) / Chapter Skim
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Response of Plant Pathogens to Fungicides
Response of Plant Pathogens to Fungicides
Pages 245-256
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From page 245... ...
The rate of pathogen response depends on a complex interaction between the exposure of the pathogen to the fungicide, the biology of the pathogen, and the environment. An example of this interaction is the response of the barley mildew pathogen Erysiphe graminis f.
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From page 246... ...
Fungicides are used extensively to remove this limitation so as to achieve the "ideal" of a diseasefree crop. Initially at least, fungicides remove these restraints consistently and reliably because the recommended dose rates are determined from field trials with adequate pathogen inoculum applied to the currently most susceptible commercial varieties.
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From page 247... ...
The increased potential for the crop provided by the fungicide is often so dramatic initially that some manufacturers suggest that breeders need no longer breed for host resistance. Any decrease in attention to inherent host resistance, however, is almost certain to exacerbate and accelerate selection of fungicide resistance, simply because pathogen survival is made easier.
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From page 248... ...
An effective sexual stage allows for more rapid formation of novel combinations of appropriate characters through recombination, which may increase the fitness of the resistant pathogen genotypes. With no sexual stage, linkage disequilibrium between resistance and other characters is likely to persist, which may limit or delay adaptation of the pathogen to the treated host population.
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From page 249... ...
The numbers of colonies that incubated on seedlings with different doses of the fungicide increased annually relative to the numbers on untreated seedlings. The early surveys could not always detect isolates with fungicide resistance in the small populations on treated crops; by 1984, however, such isolates were detected easily on untreated crops.
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From page 250... ...
During the early stages of the overall increase in resistance, the more resistant forms of the pathogen were less pathogenic on the range of host varieties in common use at the time (Table 11. In subsequent seasons, however, pathogenicity of the sensitive fraction remained constant, but the resistant fraction gradually increased to the same level.
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From page 251... ...
This small proportion will fluctuate from season to season; a real deterioration in fungicide performance will be signalled by a continuing increase in instances of poor control. For example, with triazoles and the control of barley mildew, following the increase in frequency of resistant forms in eastern England, performance of triazoles both in disease control and in yield benefit rapidly declined (Table 21.
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From page 252... ...
Adding a nonsystemic material may only temporarily reduce the absolute population size of the pathogen, while the systemic material will be more persistent so that after the initial combined action of the fungicides, the pathogen population will be exposed uniformly to the systemic compound on all plants and thus selected for resistance. A more effective system, analogous to the use of variety mixtures (Wolfe, 1981)
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From page 253... ...
From 1980 through 1984 four barley varieties with different resistance genes and the four mixtures of three varieties that can be made from them were grown in field trials at the Plant Breeding Institute, Cambridge, England (Wolfe et al., 1984b; Wolfe et al., 19851. Over the trial series the mixtures outyielded the pure stands by 7 percent (P < 0.0011.
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From page 254... ...
Based on this research variety mixtures are now grown commercially in the United Kingdom and Denmark, with generally favorable reports from the farmers involved. A much larger scale of development is being undertaken in the German Democratic Republic, particularly because of the high cost of fungicides in eastern Europe.
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From page 255... ...
Moreover, there was considerably less infection on these mixtures than on untreated mixtures; they were only slightly more infected than the mixtures that received the conventional fungicide treatment. Comparing the 1/3 treatments of the mixture with the conventional treatment of the pure stands, the mixture yields were higher, significantly so for the triazole treatments, and infection levels were the same.
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From page 256... ...
. Dynamics of the response of barley mildew to the use of sterol synthesis inhibitors.
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