The Gardener's Book of Pests and Diseases by Roland Fox

Author:Roland Fox
Language: eng
Format: epub
Publisher: Pavilion Books Company Limited
Published: 2015-01-13T16:00:00+00:00

Powdery mildew on apple shoot.

Rose powdery mildew

Sphaerotheca pannosa is extremely common and widespread worldwide on Rosa spp. and some other members of the Rosaceae. Mildew develops most extensively on leaves and young shoots, but these become resistant on maturity. Petals, sepals and receptacles of flower buds may also be invaded. Infected buds frequently remain closed. Infected leaves sometimes become yellow or purple. Roses in glasshouses or dry sunny positions, such as against walls, are particularly susceptible to the disease and may be killed. Elsewhere the consequences of the main initial spring infection by windblown conidia are not normally seen before May. Two biologically distinct strains of Sphaerotheca pannosa (Wallr.) have been reported – one on rose, another on peach. In mild seasons, hyphae overwinter as whitish felt-like mats on leaves, calyx, fruits and shoots, especially around the thorns, or in dormant buds. Few cultivars listed as resistant are consistently reliable everywhere. Some varieties formerly considered very resistant are now often susceptible in some gardens. Cultural methods, such as pruning out disease, limiting the use of nitrogenous fertilisers and using mulches to provide extra moisture on dry sites, may need to be followed by a regime of approved fungicide sprays, often reapplied fortnightly.

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