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Fungal foes in your garden No 20
FUNGAL FOES IN YOUR GARDEN No 20
Fig. 1. A heavily m ildewed rose plant (on left) is usually far weaker than its healthy fungicide treated counterpart. (Photograph reproduced by kind permission of leI Agrocbemicals).
Rose
Powdery Mildew The Disease: 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 after amino compounds decrease 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 wind-blown conidia are not normally seen before May .
The Pathogen: Two biologically distinct strains of Sphaerotheca pannoss (W allr.) Lev. , have been reported, one on rose , another on peach. In mild seasons, thick patches of off-white densely interwoven hyphae overwinter as whitish felt-like mats on leaves, calyx, fruits and shoots , especially around the thorns, or in dormant buds. As they emerge the infected shoots produce numerous ellipsoidal hyaline conidia in chains on long conidiophores. The conidia are adapted for dissemination over short distances and germinate best at 21-27°C; above 36°C they are inhibited. Cleistothecia are very occasionally found embedded within the dense mycelial felt on stems. Control: Few cultivars listed as resistant are consistently reliable everywhere. Some varieties formerly considered very resistant are now often susceptible in some gardens . Since cultivar resistance is seldom wholly reliable , 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. To avoid repeated sprays, many professional commercial growers paint their heating pipes with sulphur to gently fumigate the roses in their glasshouses and plastic tunnels . 79
Fig. 1. A heavily m ildewed rose plant (on left) is usually far weaker than its healthy fungicide treated counterpart. (Photograph reproduced by kind permission of leI Agrocbemicals).
Rose
Powdery Mildew The Disease: 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 after amino compounds decrease 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 wind-blown conidia are not normally seen before May .
The Pathogen: Two biologically distinct strains of Sphaerotheca pannoss (W allr.) Lev. , have been reported, one on rose , another on peach. In mild seasons, thick patches of off-white densely interwoven hyphae overwinter as whitish felt-like mats on leaves, calyx, fruits and shoots , especially around the thorns, or in dormant buds. As they emerge the infected shoots produce numerous ellipsoidal hyaline conidia in chains on long conidiophores. The conidia are adapted for dissemination over short distances and germinate best at 21-27°C; above 36°C they are inhibited. Cleistothecia are very occasionally found embedded within the dense mycelial felt on stems. Control: Few cultivars listed as resistant are consistently reliable everywhere. Some varieties formerly considered very resistant are now often susceptible in some gardens . Since cultivar resistance is seldom wholly reliable , 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. To avoid repeated sprays, many professional commercial growers paint their heating pipes with sulphur to gently fumigate the roses in their glasshouses and plastic tunnels . 79