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Detection of Helminthosporium solani and Colletotrichum coccodes in organically grown asymptomatic and symptomatic potatoes.
C. MATTUPALLI (1), R. K. Genger (1), A. O. Charkowski (1). (1) University of Wisconsin-Madison, Madison, WI, U.S.A.

Silver scurf, caused by the fungus <i>Helminthosporium solani</i>, and black dot, caused by <i>Colletotrichum coccodes</i>, are cosmetic diseases of potatoes (<i>Solanum tuberosum</i>) affecting processing and fresh market trade. An observational study was undertaken to assess the importance of asymptomatic tubers in perpetuation of the pathogen. Ten asymptomatic tubers and fifteen symptomatic tubers from ten different varieties were collected from three different organic farms. PCR or incubation assays were performed on asymptomatic tubers to detect <i>H. solani</i> and <i>C. coccodes</i>. 75% and 94% of the asymptomatic tubers tested were positive for silver scurf and black dot respectively. It is thus misleading to look only at the visual symptoms on the tuber for assessing the disease level of silver scurf or black dot. None of the ten cultivars studied were resistant to either silver scurf or black dot. PCR and incubation assays were comparable and can thus be used to determine which fungi are present since both the lack of symptoms at harvest and the similarity of the symptoms caused by these two fungi make rating tubers at harvest unreliable. Both pathogens were present together on the same tuber in 69% of the tubers assessed. The presence of one pathogen did not affect the presence or absence of the other pathogen. The presence of <i>H. solani</i> reduced black dot severity, but the presence of <i>C. coccodes</i> did not affect the severity of silver scurf.<p><p>Keywords: Fungus, Root-Tuber Crops, Potato

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