Potato tubers (cv. Alpha) were harvested at various time intervals after planting, or removed from storage at various times after harvest, and inoculated with different sporangial concentrations of three metalaxyl-sensitive (MS) and three metalaxyl-resistant (MR) isolates ofPhytophthora infestans. Disease parameters recorded were: (a) percentage of infected tubers; (b) percentage of blighted tuber surface area; and (c) depth of lesions. Blight was negatively correlated with tuber age both in the field and in storage. Susceptibility of tubers declined with age in the field but increased temporarily in storage.MR isolates did not differ significantly from MS isolates in percent of infected tubers but produced significantly larger and deeper lesions in tubers. Whereas inoculum concentration had a significant effect on the percent of infected tubers and of infected tuber surface area, it did not affect significantly the depth of the lesions. The results indicate that the MR isolates tested are stronger tuber blighters than the MS isolates.

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.

EPPO BulletinVolume 22, Issue 2 p. 311-314 4. Determination of the sensitivity of Phytophthora infestans to phenylamides: a quantitative tuber disc method Y. COHEN, Y. COHEN Department of Life Sciences, Bar-Ilan University, 52900 Ramat-Gan (Israel)Search for more papers by this authorD. KADISH, D. KADISH Department of Life Sciences, Bar-Ilan University, 52900 Ramat-Gan (Israel)Search for more papers by this author Y. COHEN, Y. COHEN Department of Life Sciences, Bar-Ilan University, 52900 Ramat-Gan (Israel)Search for more papers by this authorD. KADISH, D. KADISH Department of Life Sciences, Bar-Ilan University, 52900 Ramat-Gan (Israel)Search for more papers by this author First published: June 1992 https://doi.org/10.1111/j.1365-2338.1992.tb00501.xCitations: 2AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Citing Literature Volume22, Issue2June 1992Pages 311-314 RelatedInformation

(...) Sporulation in leaflets and tuber disks was poorly correlated, while infection frequency was correlated (r=0.56) with sporulation in leaflets. The MR group of isolates had a significantly higher compound fitness index compared with the MS group of isolates in foliage of the potato cultivars tested. This higher fitness, however, did not render the foliage resistance of some cultivars ineffective

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. 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 of MR sporangia compared with MS sporangia results from faster indirect germination

Each of six pairs of metalaxyl‐sensitive (MS) and metalaxyl‐resistant (MR) field isolates of Phytophthora infestans was inoculated as a mixture of 90% MS sporangia with 10% MR sporangia onto fungicide‐free potato plants (cv. Alpha) in six walk‐in plastic tunnels in each of 2 years. In both experiments, late blight killed the crops in 26‐31 days. In all tunnels, the frequency of MR sporangia increased during the logarithmic phase of the epidemic to 70‐85% of the population. Later, however, the frequency of M R sporangia decreased to 7‐38% in six tunnels but continued to increase to 94‐100% in the other six. The increased frequency of MR during the logarithmic phase was attributed to the shorter latent period and larger lesion size of MR compared to MS isolates and to the unlimited availability of uninfected host tissue. The later decrease in the frequency of MR in six tunnels was attributed to the lower sporulation capacity and the shorter infectious period of MR compared to MS isolates, and to the limitation of uninfected host tissue. The data showed that the population dynamics of P. infestans was dependent not only on fitness components of isolates, but also on the availability of uninfected host tissue.

Seventeen metalaxyl-sensitive and 21 metalaxyl-resistant isolates ofPhytophthora infestans collected from blighted potato fields during the years 1983–1988 were tested for mating type on rye seed agar medium. All isolates except one (MS3, collected in 1986 at Sufa, in the western Negev) were found to belong to the A2 mating type. A2 isolates produced oospores within 2 weeks when cultured together with isolate 163 (A1 from the U.S.A.) or with the A1 isolate MS3 from Israel. When cultured singly, A2 isolates produced some oospores within 4–8 weeks. Blighted potato tubers harvested from potato crops artificially inoculated with a mixture of A1+A2 sporangia were found to contain some oospores. No oospores were detected in blighted tubers harvested from A2 + A2 inoculated crops. It was concluded that the A2 mating type ofP. infestans has occurred in Israel since 1983 or even earlier. The rare occurrence of the A1 mating type was unexpected and indicated that sexual reproduction of the fungus in the country might be limited.

Three metalaxyl‐sensitive (MS, wild type) and three metalaxyl‐resistant (MR) field isolates of Phytophthora infestans were compared, in the absence of metalaxyl, for non‐competitive and competitive fitness on potato leaf and tuber tissues. When inoculated singly onto intact plants MR isolates produced larger lesions in leaflets than MS isolates, but no significant differences were recorded in infection frequency or sporulation capacity. When mixtures of MS and MR isolates were inoculated onto intact plants, all MR isolates exhibited a strong competitive ability: their proportion increased in the sporangial populations from 10 to 100% after eight to 10 sporulation cycles. In contrast, when mixtures were inoculated onto detached leaflets or tuber slices in a moisture‐saturated atmosphere, only MR2 was a strong competitor; MR1 was a weak and MR3 a moderate competitor. The results showed that in intact plants all MR isolates were able to compete successfully with their MS partner isolates, because of the larger lesions they produced and the unlimited availability of host tissue. However, in detached tissues only MR2 was able to compete successfully with MS2, because of its higher infection and sporulation capabilities compared to MS2. The results explain the severe MR‐induced late blight epidemics in potato crops in Israel.