Greasy spot

Greasy spot is a disease that's frequently found in tropical and semitropical regions where citrus cultivars are grown. The disease is caused by Mycosphaerella citri. Symptoms appear as yellow to dark brown to black lesions occurring first on the underside of mature citrus leaves (Fig. 1). As the lesions develop on the underside of the leaves, they become darker and a corresponding chlorotic spot will appear on the upper leaf surface. These yellow (chlorotic) spots occur when infected cells fail to develop chlorophyll. Lesions are more yellowish and diffuse on lemons and grapefruit and more raised and darker on tangerines. Affected leaves fall prematurely from the tree during the fall and winter resulting in reduced tree vigor and yield. Greasy spot also infects the fruit of grapefruit trees producing rind blotch.

Leaves of grapefruit, tangelos, and lemons are the most severely affected varieties but all commercial varieties are affected to some extent. Within the orange varieties, early and mid varieties (Hamlin and Pineapple) are more susceptible than late oranges (Valencia).

Historically, affected leaves were normally cultivated into the soil in normal weed control practices thereby reducing inoculum. With the switch to chemical weed control, leaves remain on the soil surface longer allowing for greater production of inoculum. Additionally, as groves were converted to low volume irrigation from overhead irrigation, the frequency of leaf litter wetting has increased allowing for a more rapid release of spores that subsequently infect citrus leaves. Prior to the implementation of low volume irrigation, the major infection period was in June or July. With the more rapid release of spores and earlier decomposition of leaf litter, the infection period had been moved forward into May when environmental conditions are less favorable. As the leaves decay, the production of spores is reduced.