OVERSEASONING OF METALAXYL-SENSITIVE AND METALAXYL-RESISTANT ISOLATES OF PHYTOPHTHORA-INFESTANS IN POTATO-TUBERS

The ability of Phytophthora infestans to overseason in infected potato tubers, cultivar Alpha, was studied. Recovery of infective sporangia from tubers 20 wk after inoculation with one of 10 metalaxyl-sensitive (MS) or with one of 10 metalaxyl-resistant (MR) isolates was significantly higher for the MS than for the MR isolates. Recovery tests with tubers infected with three pairs of mixed MS and MR isolates (1:1 sporangia) showed the predominance of MS isolates in two pairs and equal recovery of MS and MR isolates in a third pair. Lesions collected from the first late blight foci in commercial potato fields in 1987-1991 (autumn sowing) contained mostly MS isolates, although MR isolates predominated later in the season. The significantly higher recovery of MS isolates of P. infestans from potato tubers than of MR isolates explains the predominance of MS isolates in first foci of late blight in commercial potato fields in the autumn sowing in Israel.