Inoculum density-dependent, additive effectiveness of fungicides or biofungicides with cultivar resistance for integrated management of Cercospora leaf spot on beet greens

Abstract

Cercospora leaf spot, caused primarily by Cercospora beticola, is common on red beet wherever this vegetable crop is grown. The effectiveness of a conventional (propiconazole rotated with azoxystrobin) and an organic biofungicide (Bacillus amyloliquefaciens mixed with copper octanoate) program were tested on four cultivars of beet that differed in susceptibility. Three inoculum densities were established by inoculating a 1-m portion of one row, allowing conidia to splash to the adjacent row, and not inoculating the remainer of the plot. Foliar severity was reduced by both fungicide programs compared to the water-treated control, but the conventional fungicides were more effective in the high inoculum area than the organic fungicides. The conventional fungicides also increased the fresh weight of healthy leaves compared to the water control, but the organic fungicides did not differ from the other two treatments. All cultivars except the most resistant one, Bull's Blood, had greater healthy weights in 2022 when the inoculum density was lower than in 2021. Cultivars sorted in the same order of susceptibility regardless of fungicide program. In all treatments, mean weight of diseased individual leaves was greater than healthy leaves. Partial cultivar resistance and fungicides or biofungicides had additive effects for integrated management of Cercospora leaf spot on red beet.