Language English Horsechestnut leaf blotch Photo: Espace pour la vie (Pascale Maynard) Horsechestnut leaf blotch Photo: Espace pour la vie (Pascale Maynard) OngletsDescriptionSummaryThis fungal disease commonly attacks various chestnut species, especially during wet springs and summers. Infected young leaves first show small yellowish blotches. In the summer, the blotches become larger and turn rusty brown, giving the leaves a burnt appearance. A serious infection causes the leaves to dry out and drop prematurely. However, because serious damage tends to appear late in the growing season, when the next year’s buds have already formed, the disease usually has little impact on the trees’ health. The damage is largely esthetic. Signs and symptomsThe first symptoms appear on young leaves in the spring, as small, yellowish, irregularly shaped blotches. Over the summer, the blotches become larger and turn rusty brown. They are usually bordered by a yellow band. The lesions may coalesce to form large burnt-looking patches. The fungus fruiting bodies, which resemble tiny black specks, are visible on the spots (mainly on the underside of the leaves). A serious infection causes the leaves to curl, dry up and drop prematurely. Petioles and young fruit are sometimes infected. The disease rarely threatens trees’ survival, because severe damage usually appears late in the season. Several years of severe defoliation will weaken a tree, however, and make it susceptible to other diseases, insect pests and environmental stress. The symptoms of horsechestnut leaf blotch are sometimes confused with physiological injury to a tree’s foliage caused by abiotic stress (drought, transplant shock, compacted soil, overly small planting hole, over-fertilization, high soil salinity, mineral deficiency, etc.). Abiotic stress does not cause black fruiting bodies to appear on affected tissues, however. In addition, the symptoms are often confined to the leaf edges and the tissue between the veins. Latin name (genus)Phyllosticta paviae (syn. Guignardia aesculi)Host plantsVarious chestnut species, including common horsechestnut (Aesculus hippocastanum), Ohio buckeye (Aesculus glabra) and red horsechestnut (Aesculs x carnea). Name of host plants Horsechestnut Horsechestnut Development cycleDescription and life cycleThe fungus that causes horsechestnut leaf blotch is a member of the Ascomycetes, spore-sac fungi. Fungi in this class produce two types of spores: ascospores (sexual) and conidia (asexual). The fungus overwinters in the sexual stage in fallen leaves that were infected the previous year. Early in the spring, they release ascospores, which are carried by the wind to the young horsechestnut leaves. In damp weather, the ascospores germinate and cause primary infections. Infected tissues develop yellowish blotches that later turn rusty brown. The fruiting bodies (pycnidia) of the asexual stage of the fungus, visible on the lesions, produce conidia, which are spread when it rains. They are responsible for secondary infections during the summer. Prevention and controlFavourable conditionsRainy springs and summers are conducive to the disease. IdentificationWatch for symptoms to appear in the spring. Continue to monitor trees during the growing season. PreventionMaintain adequate spacing between plants. Keep trees healthy by watering them during dry spells and fertilizing them properly. Avoid wetting the foliage so as not to promote the germination and dispersal of spores. Applying organic mulch at the foot of the trees helps keep the soil moist. Prune overly dense trees to allow air and light to reach the centre of the plant and allow the leaves to dry out faster. Physical controlRake up and dispose of infected leaves to prevent the disease from spreading. Never compost infected plant litter. Biological controlNone available. Chemical controlThe Montréal Botanical Garden does not recommend the use of pesticides to control this disease.