Host range of Oidium lycopersici occurring in the Netherlands

Nine accessions of three cucurbit species, ten of eight legume species, thr ee of lettuce (Lactuca sativa) and 34 of 14 Solanaceae species were inocula ted with a Dutch isolate of the tomato powdery mildew fungus (Oidium lycope rsici) to determine its host range. Macroscopically, no fungal growth was v isible on sweet pepper (Capsicum annuum), lettuce, petunia (Petunia spp.) a nd most legume species (Lupinus albus, L. luteus, L. mutabilis, Phaseolus v ulgaris, Vicia faba, Vigna radiata, V. unguiculata). Trace infection was oc casionally observed on melon (Cucumis melo), cucumber (Cucumis sativus), co urgette (Cucurbita pepo), pea (Pisum sativum) and Solanum dulcamara. Eggpla nt (Solanum melongena), the cultivated potato (Solanum tuberosum) and three wild potato species (Solanum albicans, S. acaule and S. mochiquense) were more heavily infected in comparison with melon, cucumber, courgette, pea an d S. dulcamara, but the fungus could not be maintained on these hosts. All seven tobacco (Nicotiana tabacum) accessions were as susceptible to O. lyco persici as tomato (Lycopersicon esculentum cv Moneymaker), suggesting that tobacco is an alternative host. This host range of the tomato powdery milde w differs from that reported in some other countries, which also varied amo ng each other, suggesting that the causal agent of tomato powdery mildew in the Netherlands differ from that in those countries. Histological observat ions on 36 accessions showed that the defense to O. lycopersici was associa ted with a posthaustorial hypersensitive response.