School of Biological Sciences, University of Wales, Bangor, Gwynedd LL57 2UW UK

Background and objectives Phytophthora infestans is an oomycete pathogen that causes late-blight disease of potato and tomato. Late blight populations in England and Wales, as elsewhere in the world, have changed following the introduction of new genotypes in the mid 1970s. A single clonal lineage of Al mating type (showing a characteristic RG-57 fingerprint pattern and designated US-1 [1]) has been displaced by isolates showing a variety of different fingerprint patterns, and which may be either Al or A2 mating type. Increased aggressiveness on potato leaves, or increased insensitivity towards fungicides, may have been factors contributing to this displacement [2].

Where Al and A2 isolates occur together there is a possibility that oospores may be formed; this was not possible in the years prior to the introduction of the new isolates, when the pathogen was confined to the asexual part of the life cycle and could reproduce only by sporangia and zoospores. In an attempt to determine the likelihood of oospores being formed, and to assess the possible impact of oospore-derived isolates, populations throughout England and Wales have been sampled from both commercial and noncommercial (garden & allotment) sites for the years 1994 to 1997. These isolates have been characterized on the basis of mating type and RG-57 fingerprint pattern, and compared with isolates collected in earlier years (1978 onwards) and stored in liquid nitrogen.

Results and conclusions
The first UK A2 mating type was detected in 1981 but in subsequent years it has failed to increase to the level that might be expected in a freely sexually reproducing population. Between 1993 and 1997, the percentage of either A2 or self fertile isolates varied between 1 and 4%, with the remainder of the isolates being Al mating type (the number of isolates sampled varied from 144 in 1994 to 1384 in 1997, a year when late-blight was widespread). RG-57 fingerprinting revealed, as expected, that many of the oldest isolates tested were of the ubiquitous 'old' genotype, US-1 (found in stored isolates collected in 1978, 1981 and 1982). However, as early as 1982, different genotypes were found alongside US-1 (in isolates from the same field). In recent years (1994-1996) most of the isolates tested showed one of a few common fingerprint patterns. One of these patterns was associated with A2 mating type, the remainder were all Al. Unique patterns occurred for only nine out of 325 isolates tested (1 978-1996), and six of these nine 'unique patterns' were A2. The predominance of a few fingerprint patterns of single mating type, along with the highly unbalanced ratio of Al to A2 mating types, suggests that asexual sporangia and zoospores constitute the major source of inoculum for blight epiphytotics. The large number of sporangia produced in the course of a growing season, along with the 'bottleneck' which occurs over winter, would be expected to result in a few common 'clones' in any one growing season.

Oospores might provide an alternative route for overwintering to that in blighted tubers. However, the rarity of A2 isolates indicates that the opportunity for sexual reproduction / oospore formation is limited. Out of 110 commercial sites sampled in 1997, 106 were Al only and only four yielded either A2 or self fertile isolates. For noncommercial sites the opportunity for oospore formation would have been greater, i.e. six out of 43 sites yielded A2 or self-fertile isolates (five of these six sites were tomato crops). The importance of oospores as a source of inoculum within or between seasons remains to be proven, but at present seems unlikely except perhaps at a few localized sites. The present diversity in late-blight populations may be due to asexual reproduction of genotypes introduced in ware tubers imported from Mexico in 1976.

References 1. Goodwin SB, Cohen BA, Fry WE, 1994. Proceedings of the National Academy of Sciences USA 91, 11591-11595.
2. Day JP, Shattock RC, 1997. European Journal of Plant Pathology 103, 379-391.