For juicy berries all summer long, the blackberry plant, a relative of the raspberry, can be cultivated in the backyard. Proper spacing and a little care will produce an abundance of dark, juicy fruit. However, blackberry plants are susceptible to several types of fungus that cause discoloration of the leaves and canes, impede harvests and even kill the plant.
Botrytis blight, or gray mold, develops as masses of silver-gray or black spores on dead or dying fruit or leaves. Botrytis can affect leaves, stems, flowers and fruit and is prevalent during extended periods of wet or very humid weather. The best way to manage this disease it to remove blighted leaves and fruit and place in a paper bag headed to the garbage or to be burned. Be careful not to handle plants while wet.
Cane blight can appear on new canes late in the growing season. Dark brown cankers form on canes with a wound, pruning cut or stub. Growth on the infected canes is weak and wilted with very little fruit. This fungus produces black spores that spread easily on the wind and the wings of insects to neighboring plants. Cane blight favors wet conditions, so to prevent infections, space plants properly to maintain good air circulation to allow plants to dry between rains and waterings.
Anthracnose appears as dark circles on canes, leaves and/or berries. As the spots grow, the centers may become gray with the edges remaining dark. The spots may also grow together, killing canes and deforming or rotting fruit. Anthracnose can be found on many other common garden plants and spreads easily with rain, wind and splashing irrigations. Be sure to space plants properly, remove all infected material and do not handle wet infected plants.
A common fungus on blackberry plants is leaf spot, which appears as dark red spots with white centers. Leaf spot severely weakens the plant. For effective management of this disease, infected leaves, canes and fruit must be removed and burned.
A blackberry plant with rust may show symptoms of leaf spot on the topside of its leaves. However, the underside of the leaves will be covered with yellowish-orange masses of spores. Rust is systemic and cutting off infected leaves, canes or fruit will not remove the disease from the plant. Rust is also easily spread on the wind to neighboring plants. Although rust does not kill its host plant, it does weaken it and renders it useless for fruit production. The only effective solution for rust is to remove and burn all infected plants.