<p/>In the Netherlands, the source of inoculum of the late blight fungus on tomatoes is the late blight fungus on potato crops. In regions of Europe mentioned, where tomatoes are grown in the open, <em>P. infestans</em> on tomatoes is the main source of inoculum. Especially in Bulgaria and Hungary, the effect of year-round cropping of tomatoes on prevalence and severity of the disease is obvious.<p/>In field and laboratory experiments, the pathogenicity to tomatoes of <em>P. infestans</em> originating from potatoes can be increased by serial passages through tomato foliage. In Europe, no other hosts than potatoes and tomatoes played a role in the tomato late blight epidemidogy. <em>Phacelia tanacetifolia</em> is added to the host list of the fungus. For the purpose of genetics classification, two laboratory methods, probit - log dosage analysis and components analysis, and one field method have been developed to assess the resistance of tomatoes against <em>P.</em><em>infestans.</em><p/>In particular the components infection ratio, lesion extension and sporulation intensity were studied. In the field method, the 'apparent infection rate' r was used. For breeding purposes, a simplified field method was indicated. A comparison of the three methods was made.<p/>The resistance of tomatoes increased considerably until at least 8 weeks after seeding. Partial resistance of W.Va 700 against the 1-t-pathotype of <em>P.</em><em>infestans</em> is based on a single gene, here named Ph <sub><font size="-1">2</font></sub> .
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