2010 Florida Citrus Pest Management Guide: Melanose
Control of melanose, caused by the fungus Diaporthe citri, is often necessary in mature groves where fruit is intended for the fresh market, particularly if recently killed twigs and wood are present as a result of freezes or other causes. It is not usually necessary in groves less than 10 years old or those where fruit is intended for processing. Grapefruit is very susceptible to melanose but all citrus may be damaged by the disease.
Inoculum of this fungus is spores produced by the asexual stage, Phomopsis citri. Diaportha citri is the sexual stage and is relatively unimportant for the disease. The asexual spores are produced in flask-shaped structures embedded in dead twigs in the tree canopy, especially those twigs that have died within the last several months. In addition to producing spores on melanose-affected twigs after they die, P. citri is able to colonize dead twigs that were originally healthy and produce inoculum on those twigs as well. Spores are produced during wet periods and dispersed to young, susceptible leaves and fruit by rain splash. No spores are produced on symptomatic leaves, fruit, or living twigs. At 75-80°F, 10-12 hours of leaf wetness are required for infection and at lower temperatures up to 18-24 hours may be needed.