Russeting and Russet Scab of Prune, an Environmentally Induced Fruit Disorder: Symptomatology, Induction, and Control. Themis J. Michailides, Assistant Research Plant Pathologist, Department of Plant Pathology, University of California, Berkeley/Kearney Agricultural Center, Parlier 93648. Plant Dis. 75:1114-1123. Accepted for publication 20 May 1991. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/PD-75-1114.

Typical symptoms of russeting of immature fruit of French prune (Prunus domestica) originate as shiny, lacy areas on the stylar end of fruit and become brown and scabby (russet scab) on ripe fruit. These areas have a thin (1.9–3.8 ?m) cuticle, lack epicuticular wax, and bear abnormal stomata. In contrast, healthy areas have a thick (8–12 ?m) cuticle, abundant epicuticular wax, and normal stomata. Russeting and russet scab have been most severe when rainfall has occurred during and 1 wk after full-bloom stage. Russeting symptoms were induced by 1) hand-spraying entire trees with distilled water until runoff at 6 p.m., midnight, and 6 a.m. for four consecutive days, 2) spraying shoots bearing blossoms in full-bloom stage at 6 p.m. and 6 a.m. and keeping the shoots in plastic bags covered with paper bags for 3–4 days, and 3) intermittent misting (10–20 s every 10 min) with distilled water for 28 hr during full bloom using a mist generator and environmental monitoring system (a misting system operated automatically by a 21X Campbell datalogger). In all experiments, water applied on blossoms at full-bloom stage induced russeting and increased its severity on immature prune fruit. The severity of russeting symptoms on immature fruit correlated positively (r > 0.90) with the incidence of russet scab on dehydrated ripe fruits. Fungicides, such as captan, captafol, dichlone, folpet, ziram, and chlorothalonil, applied at full bloom, reduced the severity of russeting and the percentage of fruit with russet scab even though fungi were not isolated from the scab areas. However, a single captafol application on prune trees 2 wk after full bloom failed to control russet scab.

Keyword(s): jacket scab, lacy scab, scab year.