Infectivity to Potato, Sporangial Germination, and Respiration of Isolates ofPhytophthora infestansfrom Metalaxyl-Sensitive and Metalaxyl-Resistant Populations

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Abstract
Twenty field isolates of Phytophthora infestans from different locations in Israel were compared for infectivity on potato plants (cv. Alpha) in growth chambers. Lesions developed on significantly larger proportions of inoculated leaflets on plants inoculated with metalaxyl-resistant (MR) isolates and maintained for 3, 6, or 9 hr in a moisture-saturated atmosphere than on plants similarly inoculated with metalaxyl-sensitive (MS) isolates. No significant difference in infection frequency was recorded between MR and MS populations in plants kept for 20 hr in a moisture-saturated atmosphere. Germination tests in distilled water at 16 C revealed a significantly faster rate of zoospore liberation from MR than from MS sporangia at 60-150 min, but not at 240 min. Sporangia of the MR isolates consumed more than three times as much oxygen during zoosporogenesis as sporangia of the MS isolates. We propose that the greater infectivity or MR sporangia compared with MS sporangia results from faster indirect germination.