RESISTANCE TO PHENYLAMIDE FUNGICIDES:A Case Study with Phytophthora infestans Involving Mating Type and Race Structure

Abstract

Phenylamide-resistant isolates of Phytophthora infestans have gradually become an important part of populations in many countries. However, fungicide mixtures containing a phenylamide component are still an effective strategy for the control of late blight in potato and tomato. The proportion of phenylamide-resistant isolates fluctuates from year to year and within the season. Almost concurrent with the appearance of resistant isolates was the discovery of the A2 mating type of P. infestans in many European countries and in other parts of the world. However, no genetic correlation exists between resistance and mating type, and the proportion of A2 isolates in European populations remains small. Resistance to phenylamides became established in A1 populations before the appearance of A2 type. Resistant isolates express equal or greater fitness than sensitive isolates, but no correlation was detected between resistance and race structure. The continuous changes in P. infestans populations require careful adaptation of successful disease control programs.

Population studies on Phytophthora infestans on potatoes and tomatoes in southern Germany

“…Temporal fluctuations in the A1:A2 mating type frequencies as found in the present study between 1995 and 1999 have also been found in some other studies carried out in Europe (Gisi and Cohen 1996;Bouws and Finckh 2007;Lehtinen et al 2007). The reasons for the fluctuations are unexplained.…”

“…Therefore, the effect of mancozeb together with a residual effect of prior applications of fungicides may counteract the higher degree of insensitivity to metalaxyl within the population. During the 1990s metalaxyl resistance was associated with the A1 mating type in many European countries (Gisi and Cohen 1996;Day et al 2004;Lehtinen et al 2008). In the present study, no association between mating type and metalaxyl sensitivity was found.…”

“…Furthermore, they stated that phenylamide-resistant strains of P. infestans do not overwinter as effectively as sensitive strains. Gisi and Cohen (1996) found that at the end of the growing season, when host leaf area is severely limited at the end of the epidemic, the slower lesion expansion of the sensitive subpopulation together with its longer infectious period works in its favour. A further explanation for the apparent high effectiveness of metalaxyl applications in spite of the insensitivity found within the populations, might be that metalaxyl was applied together with another agent (mancozeb).…”

“…In Germany the first indications of the presence of both A1 and A2 mating types in P. infestans pathogen populations date back to 1980 (Daggett et al 1993). Analyses of these populations have yielded evidence suggesting an increase in complexity of virulence phenotypes and resistance to the systemic fungicide metalaxyl (Gisi and Cohen 1996). Sexual reproduction leads to the formation of sexual resting structures, termed oospores.…”

Optimizing Fungicide Applications for Plant Disease Management: Case Studies on Strawberry and Grape

“…One of examples would be benzimidazole resistance of apple scab pathogen (Venturia inequalis) where resistant isolates appeared only after two seasons of benzimidazole fungicide application (Shabi et al 1983;Staub 1991). In some cases, a gradual recovery of sensitivity can happen; however, as soon as an application of the compromised mode of action resumes, the resistance tends to come back quickly as in the example of potato late blight pathogen (Phytophthora infestans) to phenylamide fungicides (Gisi and Cohen 1996).…”