Metalaxyl resistance, mating type and pathogenicity of Phytophthora infestans in Uganda

A total of 81 isolates of Phytophthora infestans (Mont.) de Bary were recov ered from late blight infected samples collected from different areas of Ug anda in 1998/1999. They were analyzed for their resistance to metalaxyl fun gicide, mating types, and cross infection between potato and tomato hosts. Sensitivity to metalaxyl was determined by growing isolates on 10% VS mediu m amended with 0, 5 and 100 mug/ml metalaxyl. Overall 44.4% of the isolates tested were resistant to metalaxyl, 23.5% were intermediate and 31.2% were sensitive. Mating type was determined on 80 isolates using an Al (1724) is olate and by growing the same isolates in pure culture (selfing). Fifty per cent of the isolates produced oospores by matings and selfings, 22.5% by ma ting only and 10% by selfing only; 18% did not produce oospores in the two tests. Twenty seven of the P, infestans isolates from potato-infected tomat o. The majority of these isolates were highly resistant to metalaxyl and pr oduced oospores by either mating or selling or both. (C) 2001 Elsevier Scie nce Ltd. All rights reserved.