Effect of temperature, relative humidity and light on lesion length due to Alternariaporri in onion

Infection of Alternaria porri on onion was observed over a temperature range of 15 to 35°C, with maximum infection at 25°C. However, there was no infection at 40°C. The infection of onion plants was maximum when inoculated leaves were incubated under continuous light (24 h). Least infection was obtained when it was incubated under continuous darkness (24 h). The optimum relative humidity for the infection of onion by Alternaria porri was found to be 95 per cent, though disease development occurred over the range of 75 to 100% relative humidity.