Potato haulm resistance to Phytophthora infestans

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Abstract
SUMMARY: The slower increase of blight (Phytophthora infestans) (Mont.) de Bary in field plots of Majestic (MJ) and Arran Viking (AV) than of Up‐to‐Date (UD) and King Edward (KE) potatoes seems more closely related to the production of fewer spores than to greater resistance of leaves to infection.Sporing was decreased, not by differences in generation time, but because fewer spores were produced per lesion on MJ and AV, especially when lesions were small (young). Infected cells of AV died more rapidly than cells of UD and KE and so left a smaller area of sporing tissue in young lesions. Although the fungus advanced at much the same rate in leaves of the four varieties, sporing annuli were narrower on older (larger) lesions on AV unless high relative humidity persisted continuously. Sporangiophores and sporangia were formed more rapidly on UD than AV when non‐sporing lesions were transferred to high humidity. The ecoclimate of a crop of UD did not increase the sporulation on infected leaves of AV exposed within it, but sporulation was slightly decreased when infected UD leaves were exposed in a crop of AV. Generally KE behaved like UD, with MJ intermediate between these and AV.The lower leaf‐surface was more susceptible to infection than the upper. Differences in susceptibility of lamina between varieties were inconsistent. MJ and AV were more resistant than UD and KE when inoculated in leaf axils.