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Influence of Interrupted Dew Periods, Relative Humidity, and Light on Disease Severity and Latent Infections Caused by Cercospora kikuchii on Soybean. W. Schuh. Assistant professor, Department of Plant Pathology, The Pennsylvania State University, University Park 16802. Phytopathology 83:109-113. Accepted for publication 16 September 1992. Copyright 1993 The American Phytopathological Society. doi:10.1094/Phyto-83-109.

Germination of Cercospora kikuchii conidia, at 25 C, with relative humidities of 100% wet, 100% dry, and 99%, was influenced by light-period regime. Germination was significantly lower at 24 h of light (35.6%) compared to 24 h of dark, to, sequentially, 12 h of light/12 h of dark, and to, sequentially, 12 h of dark/12 h of light. Identical light treatments (25 C and 24 h of leaf wetness) had a similar, significant effect (P = 0.05) on disease severity, based on pairwise comparisons. When ranked, 12 h of dark/12 h of light was most conducive to infection, followed in descending order by 24 h of dark, 12 h of light/12 h of dark, and 24 h of light. Interruption of 24-h leaf-wetness periods for 4, 8, 12, and 16 h after an initial wetness period of 12 h had a significant influence on disease severity and on the number of latent infections. The effect depended on the relative humidity during the interruption. High-relative humidity (>95%) resulted in higher disease severity and number of latent lesions with increases in interruption periods, whereas low-relative humidity (~55%) resulted in lower values. The relationship between duration of leaf-wetness interruption and disease severity versus number of latent infections was statistically significant (P = 0.01) under both relative humidities. Additionally, there was evidence that the posttreatment relative-humidity conditions influenced the number of latent infections.