Impact of weather parameters on Alternaria leaf spot of soybean incited by Alternaria alternata

Weather attributes play a crucial role in the infection process and spread of pathogen. Alternaria leaf spot incited by Alternaria alternata is most destructive disease of soybean appeared in southern and eastern parts of Rajasthan as well as India. The effect of various weather parameters along with different date of sowing on the development of Alternaria leaf spot in susceptible soybean cultivar RKS-24 was investigated during Kharif season 2018 and 2019. The various weather factors viz., temperature, relative humidity and rainfall under inoculated conditions and with staggered dates of sowing were taken to observe effect on disease progression and their effect on seed yield. The maximum increase in disease severity (57.82 and 58.22%) and AUDPC (389.45 and 394.42) recorded in crop sown on 18th June (inoculated on 8th July). Lowest disease severity (39.80 and 38.50%) and AUDPC (266.18 and 259.18) were observed during 39–43th standard meteorological week (September, 24–October, 28) in year 2018 and 2019, respectively. Maximum seed yield (1699 kg ha(−1)) was recorded in plants sown on 9th July, while, lowest seed yield was recorded in plants sown on 18th June with 1441.20 kg ha(−1). The trend of disease severity and AUDPC value decreased from early sowing to late sowing (18th June–9th July). Major reasons were fluctuations in temperature, rainfall and relative humidity. It was also observed that the soybean plants for Alternaria leaf spot disease in early sowing were predisposed and so farmers should be advised to practice delayed sowing of soybean crop.