Effects of wetness duration, inoculum concentration and temperature on the development of Alternaria blight on transplants of sweet potato

Abstract

Alternaria blight (AB) of sweet potato caused by Alternaria bataticola, is a limiting factor for sweet potato industries in various countries. The disease has been recently reported in South Africa. A thorough literature search indicates that factors influencing the development of AB have not been studied adequately. In the current study wetness duration, inoculum concentration and temperature/wetness duration interaction were evaluated for their effects on the disease incidence and disease severity of AB of sweet potato. Cultivars 'W119', reported susceptible to AB, were inoculated with a spore suspension of A. bataticola (PPRI 11930) under glasshouse and plant growth chamber conditions. In the wetness duration experiment, at the constant temperature of 28°C, each increase of wetness duration significantly increased the mean disease incidence (MDI) and mean disease severity (MDS). Exposure for 48 h was the optimum condition of this experiment, as it resulted in the highest MDI and MDS. In the inoculum concentration experiment the MDI and MDS increased significantly as the inoculum concentration increased but there was no significant difference between 3×104 conidia mL-1 and the highest inoculum concentration of 5x104 conidia mL-1. In the interaction of temperature and wetness duration the optimal temperatures and wetness duration in this study were 20 and 25°C for 48 h, respectively. Exposure at 35°C irrespective of wetness duration period did not produce any AB symptoms as shown by the MDI and MDS.