VIEW ARTICLE

Cytology and Histology

Viability of Venturia inaequalis in Chlorotic Flecks Resulting from Fungicide Application to Infected Malus Leaves. H. C. Hoch, Department of Plant Pathology, New York State Agricultural Experiment Station, Cornell University, Geneva, NY 14456; M. Szkolnik, Department of Plant Pathology, New York State Agricultural Experiment Station, Cornell University, Geneva, NY 14456. Phytopathology 69:456-462. Accepted for publication 5 November 1978. Copyright 1979 The American Phytopathological Society. DOI: 10.1094/Phyto-69-456.

Discrete, nonexpanding, and nonsporulating chlorotic flecks developed on apple leaves infected with Venturia inaequalis when treated with certain fungicides (benomyl, phenylmercuric acetate, dodine, and fenarimol) at time periods beyond those that completely prevent scab symptom development but at times prior to which they would have no control. The viability of the subcuticular V. inaequalis hyphae in fleck areas was determined on a nutrient medium. At 100 hr after inoculation, only fenarimol completely eradicated the pathogen. The effect of the fungicides on the pathogen at various periods after the treatments was studied ultrastructurally and found to correlate with the isolation techniques. Subthreshold treatments caused localized necrotic reactions, whereas fully effective treatments caused complete necrosis of V. inaequalis hyphae. Initial responses of the fungal cell organelles included swollen mitochondria and cysternae of the endoplasmic reticulum and the nuclear envelope.