Effect of light duration on severity of black dot caused by Colletotrichum coccodes on potato

Abstract

The effect of light duration on potato black dot was examined in plants grown under controlled conditions and in plantlets in tissue culture. A short‐day (SD) photoperiod of 8 h light enhanced disease levels caused by Colletotrichum coccodes compared with a long‐day (LD) photoperiod of 16 h light. Four cultivars (Agria, Cara, Désirée and Nicola) exhibited higher disease levels under SD conditions. Fungal colonization rates, level of sclerotia on roots, and crown rot symptoms were significantly higher under SD conditions in all cultivars. Parameters related to plant development (height and shoot weight) were reduced in plants grown under SD conditions in all cultivars tested, regardless of inoculation with the fungus. Similar results were also obtained with tissue‐culture plantlets of cvs Cara, Désirée, Nicola and Alpha grown in an aseptic system under SD and LD conditions. Fungal colonization indices, level of sclerotia on roots, and crown rot symptoms on inoculated plantlets were significantly more severe in all cultivars tested when grown under SD conditions. Plant height and shoot weight of cvs Cara, Désirée and Nicola were significantly affected by both day length and inoculation with C. coccodes, with inoculated plantlets being shorter and smaller under SD conditions. These findings support the hypothesis that light duration plays an important role in C. coccodes infection, and may be one of the major factors responsible for the severe expression of black dot and yield losses observed in the Israeli autumn season, as may also occur in other potato‐production areas in the Mediterranean region.