Colletotrichum-leaf blight
Host plants
A wide variety of host plants; in the ornamental sector, these include anemone (Anemone spp.), azalea (Azalea indica), cyclamen (Cyclamen persicum giganteum), spindle (Euonymus spp.), ivy (Hedera helix), dogwood (Cornus spp.), lavatera (Lavatera spp.), lupin (Lupinus spp.), buttercup (Ranunculus spp.), rhododendron (Rhododendron spp.), tulip (Tulipa spp.) and fatsia (Fatsia spp.); fruits include strawberry (Fragaria x ananassa), blueberries (Vaccinium angustifolium) and (Vaccinium myrtilles) and blackcurrant (Ribes nigrum); vegetables include cayenne pepper (Capsicum frutescens), common bean (Phaseolus vulgaris), bell pepper (Capsicum annuum), runner bean (Phaseolus vulgaris spp. nanus), Spanish pepper (Capsicum annuum), green bean (Phaseolus vulgaris spp. vulgaris), spinach (Spinacia oleracea), Mung bean (Phaseolus aureus), tomato (Lycopersicon esculentum), broad bean (Vicia faba), onion (Allium cepa) and potato (Solanum tuberosum).
Symptoms
Colletotrichum leaf blight is one of the most widespread fungal diseases in both tropical and moderate climate zones. The signs that are visible on the leaf can vary depending on the host plant. In some host plants, the symptoms first appear on the leaf as tiny watery spots. In others, they can appear as expanding necrotic and chlorotic blotches that are circular to irregularly shaped. The shape of the spot also depends on the host plant. On some, a frog-eye spot can develop. On others, the spots can be angular in shape due to their confinement between leaf veins. The lesions are yellow, tan, brown or black in colour, often with a bright-yellow chlorotic halo. These spots grow quickly and feel papery to the touch. At this stage, the fungus fills the acervuli, where it produces its spores. Infected leaves sometimes drop. Flowers can also be affected.
Favourable conditions
A high relative humidity and free water are conducive to disease development. The disease may therefore occur in areas with high rainfall or frequent overhead irrigation, especially if the plant does not get a chance to dry between showers. In some species, the disease can also occur after a heatwave. The disease is least likely to occur when humidity levels are low and the parts of the plant above the soil are regularly able to dry.
Dispersion/survival
All species of Colletotrichum fungus produce a vast number of rod-shaped spores (conidia). The spores are formed in the acervuli, the large, papery necrotic lesions on the leaf. Spores in the acervuli on fallen leaves survive less favourable conditions and later, during more favourable (moist) conditions, burst open, dispersing large numbers of spores. Spores are dispersed by drops of water (rain and/or irrigation).
