Effect of metalaxyl on formation and germination of oospores of Phytophthora infestans

Abstract

Oospores of Phytophthora infestans were produced in potato leaf discs floating on metalaxyl solution (100 mg mL -1 a.i.) and inoculated with all combinations of two metalaxyl-sensitive and two -resistant parental isolates. Numbers of oospores produced varied between different matings, depending on parents, in the absence of the fungicide and when metalaxyl was added 0, 7, 14 and 21 days after inoculation. Oospores were not produced when metalaxyl was added at the time of inoculation (0 days) when either one or both parents were sensitive to metalaxyl. In two of three such matings further oospore formation was arrested when metalaxyl was added either 7 or 14 days after inoculation. Oospores extracted from leaf discs 14, 21 and 28 days after inoculation were assessed for germination on water agar after 21 days. Germination of oospores from water control treatments varied between 6 and 30% depending on the cross. Germination was significantly reduced in oospores of metalaxyl-sensitive parents extracted 28 days after inoculation of leaf discs treated with metalaxyl 0, 7 and 14 days after inoculation compared with the 21-day treatment. Minimal differences in germination were observed for oospores from the mating of resistant parents irrespective of metalaxyl treatment, although germination was generally low, not exceeding 8·5%.

In vitro somatic growth and reproduction of phenylamide‐resistant and ‐sensitive isolates of Phytophthora erythroseptica from infected potato tubers in Idaho

“…To the authors' knowledge this is the first report of fungicide resistance developed by any Peronosporales pathogen that increases the sexual reproductive capabilities of that pathogen above that of sensitive phenotypes. A previous study observing sexual fitness of MR and MS isolates of P. infestans observed that the lowest oospore production was obtained when crossing two MR isolates, while the highest oospore production was obtained when crossing two MS isolates (Hanson & Shattock, 1998). However, there may be a different response in oospore production between MR isolates that are homothallic (P. erythroseptica) verses those that are heterothallic (P. infestans).…”

Formation of oospores of Phytophthora infestans in cultivars of potato with different levels of race‐nonspecific resistance

“…Where A1 and A2 mating type isolates of P. infestans occur together in host tissue, oospores may form in a wide range of cultivars. Currently the A2 mating type occurs infrequently (Shattock & Day, 1996; Day & Shattock, 1997) but ‘hot‐spots’ are known, especially in private gardens and allotments, where both mating types exist and where putative recombinants have been identified: Hanson & Shattock, 1998; Shattock & Day, 1996). In the presence of an equal ratio of both mating types, cultivars with very high resistance to late‐blight would appear to keep oospore formation to a minimum.…”

“…To assess the presence or absence of oospores within the tissue, five A1‐ and A2‐inoculated leaflets were cleared chemically by autoclaving in 80% ethanol at 115°C for 5 min and incubating for 30 min in NaOH at 80°C, then for a few minutes in NaClO (sodium hypochlorite, 4% available chlorine) at 60°C (Drenth et al ., 1994), and another five were transferred to deionized water agar to clear naturally over 24 days at 18°C. In addition, five leaflets inoculated with A1 and A2, five with only A1 and five with only A2 were homogenized using a glass blender, and mean numbers of oospores per leaflet were counted (Hanson & Shattock, 1998) with a haemocytometer.…”

“…Oospores of Phytophthora infestans are produced in plant tissues inoculated with, or naturally infected by interacting A1 and A2 mating type isolates (Niederhauser, 1956;Gallegly & Galindo, 1958;Grinberger et al, 1989;Mosa et al, 1991;Pittis & Shattock, 1994;Hanson & Shattock, 1998). Drenth et al (1995) compared oospore production in three cultivars of potato: Bintje (very susceptible), Irene (moderately susceptible) and Pimpernel, which has the highest racenonspecific resistance to late-blight of registered cultivars in the Netherlands.…”

“…Oospores of Phytophthora infestans are produced in plant tissues inoculated with, or naturally infected by interacting A1 and A2 mating type isolates (Niederhauser, 1956; Gallegly & Galindo, 1958; Grinberger et al ., 1989; Mosa et al ., 1991; Pittis & Shattock, 1994; Hanson & Shattock, 1998). Drenth et al .…”