DNA-FINGERPRINTING UNCOVERS A NEW SEXUALLY REPRODUCING POPULATION OF PHYTOPHTHORA-INFESTANS IN THE NETHERLANDS

The oomycetous fungus Phytophthora infestans (Mont.) de Bary, which ca uses late blight disease in potatoes, is heterothallic with two known mating types, A1 and A2. From 1845 until 1980 only A1 mating type isol ates were found in Europe. In 1980, the A2 mating type appeared permit ting sexual reproduction. Here we show that virulence properties and D NA fingerprint patterns of isolates collected in the Netherlands befor e and after the appearance of A2 mating type isolates are different. B efore 1980, eight different races were found in which virulence factor s 1, 2, 3, 4 and 10 were most common. After 1980, new virulence factor s (i.e. 5, 6, 7, 8 and 11) showed up and the diversity for virulence i ncreased tremendously: 73 different races were detected among 253 isol ates analyzed. DNA fingerprint analyses of isolates collected before 1 980 revealed that, for at least two decades, only one RG-57 fingerprin t genotype was present in Europe. Among 179 isolates collected after 1 980 134 distinct RG-57 fingerprint genotypes were identified. The dram atic increase in genetic diversity strongly suggests that the P. infes tans population in the Netherlands is now propagating sexually. The ch ange from asexual to sexual reproduction, and the resulting increased adaptability and ability to survive outside the host, may interfere dr astically with the regular disease control methods.