Initially, lesions occur on young leaves as small brown to black spots that develop prominent yellow halos, often near the margin. Lesions expand into irregular or circular necrotic areas covering large areas of the leaf. Necrosis and chlorosis may spread along the veins. Lesions are flat and visible on both sides on the leaf. Older lesions have a brittle paper-like texture in their middle. Immature fruits display slightly sunken dark spots with yellow halos. On more mature fruit, lesions can vary from small specks to large pockmarks. The fruit skin forms a barrier of corky tissue that seems to have erupted from the surface. If the corky tissue falls off, craters or pockmarks become visible. Premature fruit drop is common.
The symptoms are caused by the fungus Alternaria alternata. It is spread by airborne spores by either wind or water splash. Rain events or sudden changes in relative humidity favor the production and release of spores from the fungal structures located on twigs, leaf, or fruit spots. Alternaria brown spot is often spread among groves on nursery stocks transported by humans. On young leaves, symptoms first appear between 36 and 48 hours after infection. Fruits remain susceptible for up to 4 months after petal fall.
Organic fungicides based on copper oxichloride show good results against Alternaria brown spot. In case you know of something else, that might help to fight this disease, please get in touch with us. We are looking forward to hearing from you.
Always consider an integrated approach with preventive measures together with biological treatments if available. Fungicides based on iprodione, chlorothalonil and azoxystrobin provide good control of Alternaria brown spot. Products based on propiconazole and thiophanate methyl have also been proved highly effective. It is important to follow the specified concentrations and to use fungicides with different mode of actions to prevent the development of resistance.