SUMMARY
The late blight disease of potatoes is found in nearly all areas of the world where potatoes are grown. It is most destructive, however, in areas with frequent cool, moist weather. Late blight is also very destructive to tomatoes and some other members of the family Solanaceae. Late blight may kill the foliage and stems of potato (Solanum tuberosum L.) and tomato (Lycopersicum esculentum Mill) plants at any time during the growing season. Pathogens of the late blight of potatoes, P. infestans, exhibit two mating types which are called A1 and A2. These two mating types are physiologically differentiated by their ability to produce and respond to specific sexual compatibility substances. The genetic mechanism for differentiating the A1 and A2 types in P. infestans has been the subject of much speculation, particularly since the oomycetes are diploid and therefore mating- type alleles potentially operate in a heterozygous condition. This is quite unlike the common situation in ascomycetes and basidiomycetes, where the mating type is expressed in a haploid life age. Until the late 1980s, only one mating type (A1) was present in countries outside Mexico. In the last few years, however, both mating types (A1, A2) have become widely distributed in most countries. Strains of the new mating type (A2) are much more aggressive than those of the old one (A1) and quickly replace them. For a long time genetic studies on P. infestans and other oomycetes were hampered by the lack of morphological and biochemical mutans which have been very useful in unraveling the genetics of true fungi.. Recently, a new technique called amplified fragment length polymorpfism (AFLP) was developed by Vos et al. (1995). AFLP is a powerful, reliable, stable and rapid assay with potential application in genome mapping.