Role of Four Alternaria spp. Causing Leaf and Stem Blight of Sunflower in Thailand and Their Chemical Controls

Abstract

A symptom of sunflower leaf and stem blight was observed in the cultivated areas of Thailand throughout 1988-1989, primarily on dark flecks of blackened area of leaves. Alternaria spp. were isolated from infected plants of several sunflower cultivars/lines from 10 farms where samples were taken. Isolates were identified as A. helianthi or helminthosporium helianthi (10 farms). A. zinniae (5 farms), A. alternata (3 farms) and A. longissima (1 farm). After sunflower plants were artificially inoculated for pathogenicity test, it was found that all four Alternaria spp. were extremely virulent, causing completed leaf and stem blight. This is the first report of A. longissima, causing leaf spot or leaf blight of sunflower while the other 3 species, A. helianthi, A. zinniae and A. alternata have already been widely reported in the cultivated areas. Curvularia sp. and Pestalotia sp. were also observed to cause leaf spot of sunflower (2 farms). Comparison of seven fungicides applied individually or couplingly at 1000 ppm for management of sunflower blight caused by Alternaria spp. was studied. After receiving three applications (at 20-day intervals, beginning at V4 growth stage), noninoculated plants resulted in higher yields and lower disease severity than fungicide-treated inoculated (mixed inoculum) plants. Plants treated with each fungicide plus adjuvant averaged significantly higher yields than the nontreated controls. Iprodione coupling with mancozeb had the highest 100 seed weight, yield, and oil content with no statistical difference from imazalil and iprodione alone but it differed significantly from mancozeb, copperoxide, triforine, fentin acetate and benomyl. However, iprodione (Rovral) coupling with adjuvant (Triton CS-7) had higher yields but insignificantly differences to Rovral without Triton CS-7.