S14 - Session P5 - Environmental effects on the sporulation of Corynespora cassiicola in cucumber leaf spots

Authors: Yui Sonoke *, Toshiyuki Usami

In Japan, the target leaf spot caused by Corynespora cassiicola has recently become a serious greenhouse disease in cucumber cultivation. Conidia formed on diseased plants from the previous crop often survive long lengths of time in greenhouses, causing primary disease occurrences during the next growing season. Leaf spots producing numerous conidia disperse the disease. In a previous study, we elucidated the optimum temperatures and necessary leaf wetness durations defining the infectious behavior of the pathogen. Here, the relation between the physical conditions and the pathogen sporulation in leaf spots was studied. The results showed that conidia formation mainly occurred at 20-30°C. The optimum temperature for sporulation was around 30°C. On the other hand, minimal conidia formation was observed under 15°C and above 35°C. We previously reported that the optimal temperature for conidial germination, hyphal growth, and infection to the host plant is 25-30°C. The result of this study indicated that conidial formation occurs within a similar temperature range. In contrast, few conidia formed at 35°C, whereas this pathogen grows and germinates well at this temperature. Profuse conidia formation on leaf spots happened when water was sprayed on leaves, indicating that free water induces sporulation. The role of relative humidity (RH) was also investigated in sporulation. RH was manipulated using saturated salt solutions. The results showed that significant sporulation on leaf spots was induced under 100% RH even when no free water was present. However, the number of conidia decreased significantly under conditions of lower RH. These findings are useful in evaluating the risk of disease occurrence. This research was supported by grants from the Project of the NARO Bio-oriented Technology Research Advancement Institution (Research Program on Development of Innovative Technology) in Japan.