GENOTYPIC DIVERSITY OF PHYTOPHTHORA-INFESTANS IN THE NETHERLANDS REVEALED BY DNA POLYMORPHISMS

We used DNA fingerprinting to estimate genotypic diversity among 153 i solates of Phytophthora infestans collected from potato and tomato pla nts in 14 fields distributed over six regions in The Netherlands. The DNA fingerprint probe, RG57, hybridized to 21 fragments of genomic DNA , 16 of which were polymorphic. Thirty-five RG57 genotypes were identi fied among the 153 isolates. Half of the isolates had the most widely distributed genotype, which was found in 10 fields in five of the six regions sampled. However, 89% of the genotypes were detected in only o ne field, and 60% occurred only once. Two mitochondrial DNA types, des ignated A and B, were found. Type A occurred in 143 isolates and was f ound in all fields in every region. Type B, in contrast, was found in only 10 isolates, all collected in community gardens. Partitioning of the genotypic diversity into components with the Shannon diversity ind ex revealed that 52% of the diversity was associated with differences occurring within fields, 8% was due to differences among fields within regions, and 40% was accounted for by differences among regions. Geno typic differentiation was observed between isolates collected in commu nity gardens and in commercial potato fields. Canonical variate analys is grouped isolates from commercial potato fields together, regardless of the geographic distance between the fields. Isolates from communit y gardens differed among regions and differed from the isolates collec ted in commercial potato fields.