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Colonization of Potato Plants after Aerial Infection by Colletotrichum coccodes, Causal Agent of Potato Black Dot

Colonization of potato plants by Colletotrichum coccodes after artificial inoculation of stems was modeled in relation to the severity of chlorosis and necrosis on foliage using logistic regression. C. coccodes internally colonized the stems up to 24 cm above the inoculation court (mean of 11.3 ± 7.7 cm and 14.1 ± 9.3 cm in 2003 and 2004, respectively), and completely colonized the stem below the inoculation court including the roots (mean of 13.2 ± 3.1 cm and 12 ± 2.3 cm in 2003 and 2004, respectively). C. coccodes grew significantly faster from the inoculation court toward the roots, stolons, and tubers than toward the plant apex. Internal stem colonization was restricted before senescence but rapidly increased as the plants started to senescence, suggesting that physiological events associated with plant senescence activated fungal growth and plant colonization. The results showed no relationship between C. coccodes growth and plant colonization and the development of chlorosis and necrosis severity of leaves. The severity of chlorosis and necrosis on foliage in this study was the outcome of natural plant senescence, and was not caused by the fungal colonization. Therefore, chlorosis and necrosis severity is an inaccurate tool to diagnose infection in potato by C. coccodes and does not indicate the presence or amount of C. coccodes in potato plants. Disease assessment for C. coccodes in potato plants should not rely on severity of chlorosis and necrosis. Black dot assessment should be carried out by isolation and quantification of the fungus from stem and root tissues or by polymerase chain reaction techniques before plant senescence.