Variation in the Virulence and Aggressiveness of Phytophthora infestans Isolates Collected from Different Potato Fields of Thailand

Abstract

One hundred and forty isolates of Phytophthora infestans collected from naturally late blight infected potato plants of different regions in Chiang Mai and Tak provinces were separately assessed for their pathogenicity, virulence and aggressiveness as the objectives, on detached leaflets and tuber discs of potato (Atlantic and Spunta cultivars) and detached leaflets of tomato (Delta and Seeda cultivars).  Pathogenicity, virulence and aggressiveness scored as infection frequency, disease severity assessed on 1-5 grade scale and sporulation intensity, respectively, were evaluated on leaflets of four cultivars of both hosts and on tuber discs of two cultivars of potato in a Completely Randomized Design (CRD) with 6 replications.  All 140 of the collected isolates were pathogenic to both hosts, indicating that extreme host specification has not occurred. The potato isolates were more virulent on potato than tomato but less aggressive on potato than tomato. In the experiments of potato tuber disc, the results showed stronger virulence and aggressiveness on Atlantic cultivar than on Spunta cultivar. The variation in virulence of P. infestans observed in the study indicated that the population of P. infestans in Thailand comprises various isolates with low to high virulence potentials as measured by disease severity assessment on detached leaflets of potato Atlantic cultivar and tomato Seeda cultivar.