Verticillium wilt of chrysanthemum: suppression of fungal colonization of leaves and prevention of wilt symptom development by foliar applications of benomyl

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Abstract
This report describes the use of a systemic fungicide in a study of the relation between fungal colonization of a plant and symptom expression. The systemic fungicide benomyl (methyl 1-(butylcarbamoyl)-2-benzimidazolecarbamate) was used to selectively suppress development of Verticillium dahliae in leaves of chrysanthemum plants infected via the roots.Benomyl sprayed or painted onto foliage at weekly intervals as a 20 mg/liter solution in 0.25% aqueous Tween 20 restricted development of V. dahliae in leaves and prevented wilt symptom development. Numbers of propagules of V. dahliae recovered from stems of symptomless benomyl-treated plants were similar to those recovered from stems of untreated inoculated plants expressing wilt symptoms. Material toxic to Verticillium was detected in leaves but not in stems of plants receiving foliar paint applications of benomyl. Benomyl applied repeatedly to a single leaf on an inoculated plant suppressed fungal growth and symptom development in that leaf while untreated leaves wilted and became necrotic in the usual way. The observations support the hypothesis that symptom expression in leaves of chrysanthemum plants infected with Verticillium is largely due to fungal activity within infected leaves rather than fungal activity in the roots or stem.