As in many diseases, timing is important for black spot to occur. Inoculum in the leaf litter needs to be available during the period when the host is susceptible and the environment is favorable for infection. Fruit are susceptible from fruit set until 5-6 months later, when they become age resistant.

Both the ascospores (sexual spores) and the conidia ( asexual spores) of G. citricarpa are able to infect susceptible tissues.

Ascospores are found in microscopic fungal structures embedded in the leaf litter. They are the most important source of inoculum, in some regions causing nearly all infections. Ascospores have never been found in fruit lesions or lesions on attached leaves. Spores are released when the leaf litter is wetted by heavy dew, rainfall, or irrigation and can be carried by air currents over long distances.

Dark brown or black pycnidia, structures that produce conidia, are formed on fruit, fruit pedicles and leaf lesions. They are also abundant on dead leaves. Conidia are not wind-borne, but may reach susceptible fruit by rain splash. These spores are not considered a significant source of inoculum in climates with dry summers; however, in climates with frequent summer rains, conidia play a larger role in the epidemic when there are multiple fruit ages present on trees simultaneously. Often late hanging fruit with lesions remain on the tree and spores can be washed onto young susceptible fruit.

Infections are latent until the fruit becomes fully grown or mature. At this point the fungus may grow further into the rind producing black spot symptoms months after infection, often near or after harvest. Symptom development is increased in high light intensity, intensifying temperatures, drought, and low tree vigor.

Other Common Names

Often abbreviated as CBS. This however may lead to confusion since citrus bacterial spot uses the same abbreviation. Many of the common names for black spot are based on observed symptom types but all refer to the same disease which can be confusing. The symptom based names are: hard spot ( shot-hole spot), false melanose (speckled blotch), freckle spot, lacy-spot, cracked spot and virulent (spreading or galloping) spot.

Serrano, D., Serrano, E., Dewdney, M., and Southwick, C. (2010). Citrus Diseases. USDA/APHIS/PPQ Center for Plant Health Science and Technology. [09-14]