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Sensitivity to metalaxyl, mefenoxam, and cymoxanil in Phytophthora andina isolates collected from tree tomato (Solanum betaceum) in Ecuador

Abstract:

In Ecuador, tree tomato (Solanum betaceum) production is continuous throughout the year. Blight caused by Phythophthora andina is usually controlled with frequent fungicide applications containing the active ingbkp_redients metalaxyl, mefenoxam, or cymoxanil. The constant presence of inoculum year-round, the high number of sprays per season, and a pathogen population constituted by a single clonal lineage, exert a high selection pressure for development of resistance to these fungicides. The objective of this study was to measure the sensitivity of P. andina from tree tomato to the active ingbkp_redients metalaxyl, mefenoxam, and cymoxanil. For that, 69 isolates were collected from tree tomato in five provinces of Ecuador and evaluated in vitro with standard techniques to classify them as resistant, intermediate, or sensitive to metalaxyl and mefenoxam. Thirty-one of these isolates were evaluated in vitro for their sensitivity to cymoxanil by estimating EC50 by probit analysis. Most isolates were sensitive to metalaxyl (44.9%) and mefenoxam (56.5%), and 29% of the isolates had intermediate sensitivity to both fungicides. The percentage of resistant isolates was higher with metalaxyl (26.1%) than with mefenoxam (14.5%). Isolates showed a wide range of sensitivity to cymoxanil with EC50 values between 0.016 and 208.288 μg/mL. Restriction of mycelial growth occurred even with the lowest concentration dosage (0.1 μg/mL). Most isolates (55.6%) had intermediate sensitivity to cymoxanil, 40.7% were sensitive, and only one (3.7%) was resistant. The resistant isolate was collected in Napo and is the first report of resistance to cymoxanil in Ecuador. The Ecuadorian population of P. andina attacking tree tomato shows some level of resistance to three of the fungicides used to control the disease, and therefore the alternating use of active ingbkp_redients is advisable to reduce the increase of resistance in the pathogen population.