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Fungal foes in your garden
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FUNGAL FOES IN YOUR GARDEN 19. RHODODENDRON GALL
The Disease: Most frequent and prevalent on Rhododendron and Vaccinium bushes in many north temperate regions of Europe, Asia and America with suitably mild moist climates or in artificial environments that pro- Fig. 1. Galls on a hardy azalea, showing white vide warm moist conditions. basidiospores on hymenium (Courtesy of Roger Hyam). Glasshouse azaleas are often regularly attacked. Such aerial infection may be localised and cause small irregular swellings on leaves, buds or flowers which later either become enclosed as leaf spots or hypertrophied. Leaves, flower-buds and shoot-tips may become malformed. Leaf concavities thicken, at first redden above a yellowish margin, then turn pinkish white and finally chalky-white as a covering of basidiospores develops on bud or leaf galls and hypertrophied leaf spots. However on some hosts infection may become systemic and cause shoot galls or even witches' brooms. By mid summer the affected leaves usually drop, flowering is reduced and general growth and vigour may suffer, resulting in a seriously weakened bush. Infected Rhododendron are usually unsaleable whereas infected Vaccinium plants produce only very small crops of berries. The Pathogen: The above symptoms are caused by a number of closely related species of Exobasidium Woron. of which the common one on Rhododendron cultivars is E. japonicum Shirai. This becomes obvious as the chalky-white hymenium develops bearing usually 4-spored basidia. Basidiospores are musiform, 1-3 septate when mature. On germination lunate to sub fusiform conidia are produced. Dissemination is by air-borne basidiospores and conidia, probably by rain splash, on hands, tools, clothing and possibly also by insects. Usually only young leaves become infected by direct penetration of the cuticle. Other species are found on wild Rhododendrons in the Alps, while ten or more narrowly host-specific species are known on Vacciniums (bilberry, cranberry etc). In some of these the mycelium spreads throughout the cortex and into the pith of infected stems and becomes systemic. Control: Many aspects of the pathology of these pathogens have not yet been fully investigated. Although a number of fungicides have been tried, the disease is most effectively treated by removing individual galls. This should preferably be done before they turn white and are able to infect healthy flowers and foliage. Cuttings should not be taken from plants that show symptoms. NB. Dr Bruce Ing would welcome material on Exobaridium spp. on named hosts
R T V Fox
FUNGAL FOES IN YOUR GARDEN 19. RHODODENDRON GALL
The Disease: Most frequent and prevalent on Rhododendron and Vaccinium bushes in many north temperate regions of Europe, Asia and America with suitably mild moist climates or in artificial environments that pro- Fig. 1. Galls on a hardy azalea, showing white vide warm moist conditions. basidiospores on hymenium (Courtesy of Roger Hyam). Glasshouse azaleas are often regularly attacked. Such aerial infection may be localised and cause small irregular swellings on leaves, buds or flowers which later either become enclosed as leaf spots or hypertrophied. Leaves, flower-buds and shoot-tips may become malformed. Leaf concavities thicken, at first redden above a yellowish margin, then turn pinkish white and finally chalky-white as a covering of basidiospores develops on bud or leaf galls and hypertrophied leaf spots. However on some hosts infection may become systemic and cause shoot galls or even witches' brooms. By mid summer the affected leaves usually drop, flowering is reduced and general growth and vigour may suffer, resulting in a seriously weakened bush. Infected Rhododendron are usually unsaleable whereas infected Vaccinium plants produce only very small crops of berries. The Pathogen: The above symptoms are caused by a number of closely related species of Exobasidium Woron. of which the common one on Rhododendron cultivars is E. japonicum Shirai. This becomes obvious as the chalky-white hymenium develops bearing usually 4-spored basidia. Basidiospores are musiform, 1-3 septate when mature. On germination lunate to sub fusiform conidia are produced. Dissemination is by air-borne basidiospores and conidia, probably by rain splash, on hands, tools, clothing and possibly also by insects. Usually only young leaves become infected by direct penetration of the cuticle. Other species are found on wild Rhododendrons in the Alps, while ten or more narrowly host-specific species are known on Vacciniums (bilberry, cranberry etc). In some of these the mycelium spreads throughout the cortex and into the pith of infected stems and becomes systemic. Control: Many aspects of the pathology of these pathogens have not yet been fully investigated. Although a number of fungicides have been tried, the disease is most effectively treated by removing individual galls. This should preferably be done before they turn white and are able to infect healthy flowers and foliage. Cuttings should not be taken from plants that show symptoms. NB. Dr Bruce Ing would welcome material on Exobaridium spp. on named hosts
R T V Fox