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United States Department of Agriculture

Agricultural Research Service


item Mizubuti, Eduardo - CORNELL UNIVERSITY
item Goodwin, Stephen
item Fry, William - CORNELL UNIVERSITY

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 20, 1997
Publication Date: N/A

Interpretive Summary: Fungicide-resistant strains of the potato late blight pathogen, Phytophthora infestans, were introduced into the United States and Canada during the early 1990s, probably in infected tomato fruits from northwestern Mexico. The new strains spread rapidly and caused extensive losses to United States potato and tomato growers. Part of the reason for their rapid increase is that the new strains were resistant to the systemi fungicide metalaxyl, which previously had provided almost complete disease suppression. However, two other potential explanations for the increase of new strains are that they were: a) less sensitive to protectant fungicides; and/or b) more aggressive in the absence of fungicide. The purpose of this paper was to test these alternative hypotheses. Ten isolates each from the most commonly detected new and old strains were tested for sensitivity to two common protectant fungicides. No significant differences were detected dbetween strains for sensitivity to protectant fungicides so this does not explain the rapid increase of the new types. However, significant differences were detected between the old (US-1) strain and two new strains (US-7 and US-8) for fitness components on detached potato leaflets. In general, old types took longer to incite disease, caused smaller lesions and produced fewer spores than new strains. Computer simulations revealed that the new strains will require up to 25% more protectant fungicide to achieve the same level of control compared to the previously dominant clonal lineage. This analysis explains the predominance of the new strains, and reveals that they can be controlled with protectant fungicides, but at an increased cost to United States potato and tomato growers.

Technical Abstract: Since 1991, dramatic changes have occurred in the genetic composition of populations of Phytophthora infestans in the United States. Recently introduced clonal lineages (US-7, US-8) are more common now than the previously dominant lineage (US-1). To help determine why these changes occurred, four clonal lineages that were common during the early 1990s were eevaluated for sensitivity to the protectant fungicides mancozeb and chlorothalonil using amended-agar assays. There were no significant (P = 0.05) differences among clonal lineages for sensitivity to mancozeb or chlorothalonil, so differences in sensitivity to protectant fungicides cannot explain the rapid increase in frequency of the new lineages. Three components of pathogenic fitness (latent period, lesion area and sporulation after 96 hr) were tested for the three clonal lineages that were detected most commonly during 1994 (US-1, US-7, US-8) by inoculating detached leaflets of the susceptible potato cultivar Norchip. There were significant differences between the US-1 and -8 clonal lineages for lesion area and sporulation, and between US-1 and -7 for latent period. The US-6 clonal lineage was excluded from the pathogenic fitness experiments because it was not collected during 1994. Compared to US-7 and -8, US-1 had the longest latent period, and the smallest lesions with the least sporulation. Incorporation of the differences between US-1 and -8 in computer simulation experiments revealed that significantly more protectant fungicide (e.g., 25%) would be required to suppress epidemics caused by the US-8 clonal lineage compared to US-1. These differences in pathogenic fitness components probably contribute to the general predominance of the "new" clonal lineages (especially US-8) relative to the "old" US-1 lineage.

Last Modified: 11/25/2015
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