Phytophthora infestans from blighted potato tubers imported from Egypt and on sale to the public in North Wales were all of the A2 mating type which was thought, until recently, to be confined to Mexico, Zygotes from Welsh and Egyptian matings germinated readily; progeny were variable. The threat posed by a general spread of the A2 mating type is discussed.

In a survey of 1864 isolates derived from samples of Phytophthora infestans obtained from potato and tomato collected in England and Wales between 1985 and 1988, the A2 mating type was identified in approximately 10% of samples. There was no evidence for an overall increase in the frequency of the A2 mating type over this period. In agar culture, A2 isolates exhibited a distinct lumpy‐colony morphology. Lumps were made up of intensely dichotomously branched hyphae. The incidence of resistance to metalaxyl among samples increased from 41 to 60% between 1986 and 1988. Some of the A2 isolates were resistant to metalaxyl and like other A2 isolates gave rise to progeny from in vitro matings with Al isolates. Except in one case, Al and A2 isolates were uniformly homozygous at an isoenzyme locus for glucosephosphate isomerase but isolates exhibited some variation at a peptidase locus. The source of the A2 isolates could not be established from these data.

Following the discovery of oospores of Phytophthora infestans in Mexico {Niederhauser, 1956) it was generally accepted that the Al mating (compatibility) type was widely distributed, bul that the A2 mating type was confined to Mexico (Smoot et al.. 1958). A2 isolates, however, have smce heen discovered in Switzerland (Hohl & Iselin, 1984). in tubers imported into the UK from Egypt (Shaw et al,. 1985) and in Scotland (Malcoimson, 1985). We have therefore searched for A2 mating types among isolates of P. infestans in our culture collection and in samples of late-biight from affected crops of potato and tomato collected by the Agricultural Development and Advisory Service (ADAS) and ourselves m England and Wales in 1985.British and foreign isolates of P. infestans coWected between 1978 and 1984 and held m hqutd nitrogen storage or under mineral oil on rye A agar (Caten & Jinks. 1968) were transferred to 10% V8 agar (100 ml V8 juice, 1 gCaCOs, 12 g agar, 900 ml H2O). P. (w/V.^fam was isolated from samples of leaf and tuber tissue by placing small pieces of infected tissue onto rye A agar containing 100 /(g/ml vancomycin, IOO /.(g/ml pimariein and 50 /ig/ml rifamycin.The mating types of the isolates were determined by inoculating plates of V8 agar centrally with a mycelial strip (30 x 5 mm) of the unknown isolate and placing inocula (5x5 mm) of a known Al (isolates 83/6 and 82/38/29 from Wales) and A2 (isolate El 4c from Egypt) (Shaw et al.. 1985) mating type on opposite sides and at a distance of 30 mm from the central strip. Cultures were incubated at 18'C for 5_10 days and the presence or absence of cospores recorded at the hyphal interface between isolates and A I and A2 tester strains. The formation of oospores between an isolate and either the AI or A2 tester strain indicated the presence of either an A2 or Al mating type, respectively. To exclude the possibility that oospore fonnation in mating type tests was the result of mating between tester strains, the mating type of all isolates forming oospores with AI was confirmed as A2 in pairings with single Al or A2 testers on two occasions. In all cases the A2 mating types were confirmed. Occasionally oospores developed between an isolate and both AI and A2 tester strains. These particular isolates produeed large numbers of oospores on their own as well as adpressed growth in pure culture and were identified as self-fertile. Table 1 shows the incidence of the A2 mating type in our culture collection made in recent years in North Wales. In a small number of isolates from 1978, no A2 types were detected. A2 isolates were first recorded from collections made in 1981 and from those of suhsequent years except i 984. although from that year a single tomato isolate was identified as self-fertile. Of the 24 A2 isolates identified from 1982, 17 came from one farm and the rest from four other locations on the Isle of Anglesey. The four self-fertiie isolates identified from that year oceurred at two separate localities in North Wales.During I 985, samples of blighted foliage...

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