The growth behaviour of several Alternaria species on leaf discs of host and nonhost plants was examined by u.v. microscopy. Specific patterns of development were found for the pathogens. Differences in development which correlated to host specificity, or which differentiated specialized pathogens from weak or opportunistic pathogens, were obvious only after attempted penetration. Thus, little variation was observed amongst different plants for individual Alternaria species in the rate of germination, in the extent of germ‐tube growth or in the frequency of appressorium production. However, plant responses to attempted penetration, which included the formation of callose‐containing papillae, callose deposition in the walls of attacked cells and their neighbours and cell necrosis, varied with specific pathogen–plant interactions. Callose deposition occurred at sites of both successful and unsuccessful penetrations and may, therefore, not be a determining factor in the plant–pathogen interactions examined. A biplot technique is used to illustrate the different degrees of host specificity apparent at pre‐ and post‐penetration stages of fungal development.

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SUMMARY When potato tubers with small amounts of silver scurf at lifting (less than 10 per cent of surface cover) were stored over periods of about five months in a dry condition, very little silver scurf developed irrespective of storage temperature. At high humidity, there was little disease after storage at 5°C and very little at temperatures below 4°C: moderate to severe infection (20 per cent or more of surface area covered) occurred at storage temperatures of about 6°C or above. Weight losses of tubers with severe silver scurf (50 per cent surface area affected) stored in a dry atmosphere for 27 weeks were greater than those of tubers with moderate (25 per cent area cover) and slight infection (5 per cent area cover), 8·5, 5·9 and 4·5 per cent respectively. However, tubers stored iti a dry atmosphere showed greater weight losses than tubers stored in a humid atmosphere, despite having less silver scurf infection. Low storage temperatures aggravated symptoms of skin spot. Slightly affected seed tubers gave rise to more silver scurf in the subsequent crop than tubers with over 75 per cent surface affected, and crops from light sandy soils showed more infection at lifting than crops from heavier soils. These differences persisted during storage, but were largely masked if storage conditions favoured disease development, indicating that in normal stocks there is always an adequate inoculum, at lifting to produce severe symptoms. Very small amounts of inoculum giving little silver scurf after storage, resulted from using seed from stem‐cutting material or by treating seed with benomyl.

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SummaryUsing the fluorescent lipid stain fluorol yellow 088 to detect suberin in tubers, the numbers of layers and the thickness of the suberized cells in the periderm and within the lenticels showed changes after the early stage of tuber formation. These changes as tubers matured were affected by cultivar and soil moisture conditions. Penetration of the surface tissues of tubers by the water soluble stain safranin O was confined to the lenticels. Although the zone of suberized cells in lenticels acted to some extent as a barrier to penetration, there was not a clear relationship between permeability and suberization. Stored tubers showed thicker suberin barriers in lenticels than freshly harvested tubers. When lenticels of stored tubers proliferated the suberin barrier was disrupted but a further thick suberin barrier formed after exposure to air for two weeks. The findings are discussed in relation to biological control of Erwinia by antagonistic bacteria.