Aerobiology, epidemiology and management strategies in apple scab: science and its applications

Abstract

Apple Scab, caused by Venturia inaequalis (Cke.) Wint. (anamorph Spilocea pomi Fr.), is considered to be one of the most important fungal diseases of apple, which occurs almost every year in severe epidemic form under favourable environmental conditions wherever apple is cultivated. The disease has two distinct phases in its life cycle i.e. the primary saprophytic stage on the fallen dead leaves and the secondary parasitic phase on the trees. The intensity of the disease varied with the environmental conditions, cultivar and orchard management practices. Yield losses during epidemic years in 1996, 2008 and 2013 in the Gangotri fruit valley of Dist. Uttarkashi went up to 84%. Among the environmental parameters, temperature, moisture and relative humidity have the highest influence on the epidemic development. The formations of pseudothecia and ascospore discharge were associated mainly with late autumn (October) infection. Urea @ 5% concentration had a significant effect on breaking of the life cycle of V. inaequalis in overwintered leaves. The pseudothecia formation takes 32–48 days more after sexual reproduction at temperature around 8–10 °C and this process completes between February and March every year. The maximum ascospores productivity was recorded at fruit development stage (pea size to walnut size) of apple with ascospore productivity of 1, 23,000/ml and thereafter declined gradually. The ascospore emission period was 64–78 days and mean numbers of cumulative degree days for 50 and 95% spore release were 456 and 960, respectively. Data accumulated over 17 years were analyzed for validation of Mills criteria to establish its relevance in rescheduling fungicide applications under monitored spray programme. On examination of the primary infection period from Gangotri valley, some differences were observed between our results and Mills table for ascospores infection. Potential ascospore dose is a useful tool for predicting the total amount of inoculums in an orchard and has been shown to effectively improve apple scab management. PAD values were 50 times higher in the poorly managed orchards than in the integrated orchards. However, integrated orchards were considered well-managed in the investigated years and showed that the epidemic risk was low and medium, while it was very high in the poorly managed orchards on the basis of PAD. Based on the long term studies carried out at different locations from 1993 to 2018, a computer based warning system was developed to forewarn the farmers for adoption of suitable management practices. The information generated are passed on to the growers by blowing a characteristic signaling, local news paper, SMS, telephonic communication, Govt. organization and through personal contacts or messages flashed 4–5 times through radio stations. In addition, growing season kisan mela, press releases and newsletters are issued via the extension workers of the University and Govt. department.