Early blight caused by Alternaria solani and brown spot caused by A. alternata are two common foliar diseases of potato, with early blight typically predominating in incidence and severity. Renewed interest in these two diseases has arisen as a result of notable differences in fungicide resistance profiles of the pathogens and inconsistent outcomes of disease management tactics. The pathogens share similar disease cycles, but they differ in the shape and size of their conidia. A. solani has a host range that includes just the Solanaceae, whereas A. alternata has a broad host range spanning numerous plant families. Such differences may result in differences in dispersal of the pathogens and subsequently influence epidemiology and management outcomes. In the commercial potato fields investigated in this study, the aerial conidial concentrations of A. solani and A. alternata differed significantly, with those of A. alternata conidia being higher in number and more variable than those of A. solani. The aerial conidial concentration of A. solani almost always significantly decreased with height (0 to 3 m above the canopy), whereas such a decrease was only observed for 4 of 12 days for A. alternata. The atmospheric concentrations of A. alternata were higher than those of A. solani at both upwind and downwind edges of the field (P < 0.0001), indicating more potential for long-distance dispersal. A higher aerial concentration of conidia at the downwind than the upwind location was observed for A. solani (P < 0.05), whereas overall no such effect was observed for A. alternata. This indicated that the potato fields investigated were likely the source of A. solani conidia, but they may not be the sole source of A. alternata. Results are consistent with inoculum of A. solani coming primarily from within the potato crop, whereas that of A. alternata may be generated from diverse plant sources across the landscape.
In a comparison of different methods for estimating Verticillium dahliae in soil, 14 soil samples were analyzed in a blinded fashion by 13 research groups in seven countries, using their preferred methods. One group analyzed only four samples. Twelve soil samples were naturally infested, and two had known numbers of microsclerotia of V. dahliae added to them. In addition, a control was included to determine whether transport had an effect on the results. Results differed considerably among the research groups. There was a 118-fold difference between the groups with the lowest and highest mean estimates. Results of the other groups were evenly distributed between these extremes. In general, methods based on plating dry soil samples gave higher numbers of V. dahliae than did plating of an aqueous soil suspension. Recovery of V. dahliae from samples with added microsclerotia varied from 0 to 59%. Most of the variability within each analysis was at the petri dish level. The results indicate the necessity to check the performance of detection assays regularly by comparing recoveries with other laboratories, using a common set of soil samples. We conclude that wet plating assays are less accurate than dry plating assays.
Mean total leaf area consumption of potato foliage by Peridroma saucia (Hübner) instars four to six was 158.4 ± 18.7 cm2. Mean number of days for development for instars four to six was 16.2 ± 1.5 days. Larval instars three to six showed significantly different preferences for different parts of the potato plant. Leaf measurements of 'Russet Burbank' and 'Superior' cultivar potatoes were taken during four and five growing seasons, respectively. Leaf-area change over the growing season was predicted using the Weibull model. Information on larval consumption, feeding site preference, and plant leaf area was used in conjunction with defoliation-yield loss information to calculate economic injury level for variegated cutworm on potatoes.
