Citrus black spot, caused by the fungal pathogen Guignardia citricarpa (sexual stage) and Phyllosticta citricarpa (asexual stage), was first found in south Florida in March 2010. The initial find was contained to a 15-mile area centered in South Florida near Immokalee. By the first week of May, the disease had been found in another location about 14 miles northeast of the original find. It is expected to be found in additional areas when the new harvest season begins in the fall.
Around the world, black spot can be found in Argentina, Australia, Brazil, China, Ghana, Mozambique, the Philippines, South Africa, Sub-Saharan Africa, Taiwan and other regions of South America.
Black spot symptoms occurs in several forms called hard spot, cracked spot, false melanose and virulent spot, which are described below.
Hard spot is the most common and diagnostic symptom. The lesions are small, round, sunken, with gray centers and brick-red to chocolate-brown margins. Green halos are often seen around hard spot lesions. Fungal structures appear as slightly elevated black dots in the center of lesions. They appear as fruit begins to color where light exposure is greatest.
False melanose is observed as numerous small, slightly raised lesions that can be tan to dark brown. It may occur on green fruit and does not have pycnidia (fungal structures). False melanose may become hard spot later in the season.
Cracked spot has large, flat, dark brown lesions with raised cracks on their surface. It is thought to be caused by an interaction between the pathogen and rust mites. It occurs on green as well as mature fruit and can become hard spot later in the season.
Early virulent spot, also known as freckle spot, has small reddish irregularly shaped lesions. It occurs on mature fruit as well as post-harvest in storage. It can develop into either virulent spot or hard spot. Virulent spot is caused by the expansion and/or fusion of other lesions covering most of the fruit surface toward the end of the season. Many fungal structures can be found in these lesions.
Severely affected fruit can drop before harvest causing significant yield loss.
If you suspect you may have citrus black spot, please contact your local CHRP office for further diagnostic testing.
Leaf and stem symptoms are not as common as fruit symptoms, but can occur when there is insufficient disease control on any cultivar. They are most commonly found on lemons, a very susceptible species.
Lesions begin as small reddish brown lesions that are slightly raised. With age they become round sunken necrotic spots with gray centers and prominent margins that are brick-red to chocolate brown.
If you suspect you may have citrus black spot, please contact your local CHRP office for further diagnostic testing.
