Metalaxyl resistance and control of cucumber downy mildew with oomycete-fungicides
1982
Pappas, Ch.A. (Benaki Phytopathological Inst., Kifisia-Athens (Greece))
Widespread distribution of Metalaxyl resistant strains of Pseudoperonospora cubensis occurred in cucumber greenhouses all over the country. Resistant isolates infected cucumber leaves floated on solutions containing 250 mg Metalaxyl/ml, whereas sensitive isolates were inhibited at concentrations of 10 mg/ml. Protective sprays with Cymoxanil, Fosetyl-Al and Metalaxyl gave no better control than Mancozeb against Psendoperonospora cubensis on potted plants. Metalaxyl was effective only against sensitive isolates and its residual efficacy did not exceed 3 days. The residual activity of Mancozeb and Fosetyl-Al lasted up to 15 days. None of the fungicides protected new growth from infection when applied to lower leaves, although soil applications of Metalaxyl and Cymoxanil gave sufficient systemic protection. Fosetyl-Al showed better curative effect than Metalaxyl or Cymoxanil and inhibited symptom development when sprayed up to 24 h after inoculation. In polyethylene tunnel trials Cymoxanil, Fosetyl-Al and Mancozeb significantly decreased infection when they applied as protectants, whereas Metalaxyl controlled only the sensitive biotypes. Mixtures of each of the three specific fungicides with Mancozeb gave better control than the fungicides alone. Disease upward spread was suppressed by weekly sprays started with the appearance of the first symptoms. However, both plant growth and yield were severely affected when the first application was delayed
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