Cultivar susceptibility and fungicide control of black dot root rot
Meyer, Jack Robert
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Black Dot (Colletotrichum coccodes) is an important potato disease worldwide causing reported yield losses in the 10-30% range. It is involved in the early dying disease complex along with Verticillium dahliae or V. albo atrum, and root lesion nematodes. Besides early dying, black dot also causes silvery blemishes on the tuber surface that resemble those of silver scurf which results in reduced value in fresh markets. The purpose of this work was to evaluate North American cultivars for black dot tolerance and to evaluate fungicides for their efficacy in controlling this disease. To evaluate cultivar resistance, thirty-four commercial cultivars were evaluated in inoculated, greenhouse experiments for susceptibility to C. coccodes. Plant growth was then evaluated 60 days post inoculation. Inoculated plants were stunted and had reduced dry weight of 0% to 53.6% when compared to un-inoculated controls. Significant reductions in growth were observed for 25 of the 34 cultivars (P<0.05).In field studies one treatment, azxoystrobin (Quadris, Syngenta) in-furrow soil treatments applied at planting, followed by a foliar spray of chlorothalonil (Bravo, Sygenta) applied when plants were 25 cm tall, provided optimal control as measured by providing significant (P<0.05) reductions in percent early dying (2005) and yield increases (2006). Azoxystrobin infurrow plus chlorothalonil foliar treatment reduced early dying by 46.1% and increased yields by 5.4% compared to untreated plots. In 2005, all in-furrow fungicide treatments and foliar treatments provided statistically similar control of early dying. Yield differences were not statistically significant (P<0.05) when compared to the untreated with the exception of azoxystrobin in-furrow plus the same fungicide applied foliar. In 2006, all fungicide treatments significantly (P<0.05) increased yields above the untreated. No differences in colony forming units of C. coccodes /g stem tissue were noted as resulting from any fungicide treatments in the field in 2005, but in 2006 the azoxystrobin in-furrow plus chlorothalonil foliar treatment resulted in reduced cfu/g stem tissue (P<0.05) when compared to the untreated.