Control of Bacterial Leaf Spot of Zinnia with Captan

Abstract

STRIDER, D. L. 1980. Control of bacterial leaf spot of zinnia with captan. Plant Disease 64:920922. In dense plantings in the greenhouse, bacterial leaf spot of zinnia caused by Xanthomonas nigromaculans f. sp. zinniae (= X. campestris) was reduced from 100 to 2% diseased plants when infected seeds were treated with captan. Control was best using a 30-min seed-soak in 360 g a.i. captan per liter of water containing the wetting agent Tween 80. Dry treatment with captan 50 WP was not as effective, but the percent diseased plants was reduced from 100 to 12.4. In plant beds outside the greenhouse, spread was reduced by weekly foliar applications of captan (4.8 g a.i. / L of water). Streptomycin was effective but moderately phytotoxic. In vitro sensitivity of Xanthomonas nigromaculans (Takimoto) Dowson f. sp. zinniae Hopkins and Dowson (= X. campestris (Pammel) Dowson [2]), the cause of bacterial spot of zinnia, to captan was recently reported (19). That captan has utility in control of bacterial spot of zinnia as a seed treatment and as a foliar spray was hypothesized because 250 Ag/ml a.i. captan in nutrient agar prevented growth of X. nigromaculans f. sp. zinniae (19) and because captan is commonly used at a much higher rate (1,230 Ag/ml a.i.) as a foliar spray for control of many fungal diseases (14). Since its development in the early 1940s, captan has been a useful and versatile fungicide (14), but its activity against X. nigromaculans f. sp. zinniae was surprising. However, a review of the literature revealed several reports on the efficacy of captan as a bactericide (4-9,11,12,15,16,22). In 1960 Diener and Carlton (7) reported that captan appeared to increase the effectiveness of dodine for bacterial spot control of peach caused by Xanthomonas pruni. Daines (4-6) also found that captan reduced the incidence of bacterial spot of peach. Shekharvat and Srivastava (15) reported that a 0.05% captan seed-soak reduced seedling infection of rice by Xanthomonas translucens f. sp. oryzae. A preinoculation spray of seedlings gave reduction of the disease equal to that given by a streptomycin-chlorotetracycline treat