Alternaria brown spot

Brown spot is a fungal pathogen of beans, Cucurbits and potatoes.

Symptoms
Alternaria brown spot attacks young fruit, leaves and twigs, producing brown-to-black lesions surrounded by a yellow halo. The halo is caused by a fungal toxin which rapidly kills citrus tissue. Leaf lesions are generally circular but will often have a tail, following the leaf vein which gives the lesions an eye-spot appearance. The necrosis extends along the veins as the toxin spreads in vascular tissues. On young leaves, lesions can appear as early as 36-48 hours after infection. Lesions enlarge as leaves mature and can vary in size from 1-10mm (0.04-0.4in) and will be larger if the infection occurred earlier in the season. Cultivar susceptibility is also a factor that determines ultimate lesion size. A less susceptible cultivar will have smaller lesions. If brown spot is severe, the leaves may drop and entire shoots can wilt and die.

Severe fruit infections, especially shortly after petal fall, result in the drop of young fruitlets. Remaining fruit can have lesions that vary in size from dots to large pock marks on the peel. Young lesions form a corky protuberance that is able to be dislodged as the fruit matures, leaving a light tan pock mark. On occasion, A. alternata is able to penetrate the citrus rind and cause localized necrosis, but this is relatively rare. Fruit are susceptible to Alternaria brown spot for 4 months after petal fall. Even when the fruit are no longer susceptible, some fruit may fall as the result of earlier infections, especially if they are near the fruit stem.

Treatment
Fungicides are the primary means of controlling Alternaria brown spot.