| Summary: | Field trials with two potato (Solanum tuberosum L.) cultivars Norland and Russet Burbank were conducted in 1983 and 1984 to evaluate the effect of early blight, caused by fungal pathogen Alternaria solani Sorauer, on yield under Manitoba conditions. To generate different disease epidemics various schedules of fungicide (mancozeb) application were used. In 1983 disease did not become epidemic; maximum early blight severities were under 2%. In 1984 a single mid-July fungal inoculation applied to only half of the plots increased disease severity of both cultivars. Average severity with respect to spray schedule ranged from O% for the initial ratings of both cultivars to 11.2% and 6O.6% for the final ratings of plots receiving zero fungicide applications (cultivars Russet Burbank and Norland, respectively). Trends toward reduced yield as early blight intensity increased were apparent, although significant only for cultivar Norland in 1984, where marketable tuber weight was increased as much as 19.3% in plots sprayed weekly compared to unsprayed plots. Disease assessment data and yield data were subjected to regression analysis in order to define models for estimating yield. Multiple point models using early blight severity or defoliation assessments as the independent variables provided the best yield prediction models for cultivar Norland, explaining over 60% of the variation in yield; for cultivar Russet Burbank multiple point models using defoliation assessments as the independent variables explained almost 50% of the variation in yield. Environmental conditions within the potato plant canopy (cultivar Russet Burbank) were monitored at two locations in Manitoba during 1982, 1983, and 1984 in order to compare early blight disease progression, through disease assessments and spore trapping, with ambient air temperature, relative humidity, duration of leaf wetness, and rainfall. Disease severity was more severe at Graysville in 1983 and 1984 than at Portage la Prairie; this was explained in part by drought stress (1983) and longer periods of leaf wetness (1984) at Graysville. Spore counts were similar at both locations in in all three years; numbers of spores trapped increased near the end of July each year, at which only a few initial lesions were visible on the crop.
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