Citrus Black Spot Pest Profile

Citrus black spot (CBS) is a plant disease caused by the fungus Phyllosticta citricarpa. This name describes the imperfect stage; it is also known by its perfect stage, Guignardia citricarpa. Both forms produce infective spores. Symptoms are primarily on the fruit, with unsightly dark lesions formed on the rind, which reduces the fresh market crop value. All commercial citrus varieties are susceptible, but late-maturing cultivars and lemons are most vulnerable.

CBS was originally found on citrus in Australia in 1895. International spread is assumed to have taken place mainly through the movement of infected plants for planting, rather than with infected fruit. By the late 1970s it was established in Southern Africa and described as an “epidemic and a major crop destroyer.” In 2010, the United States Department of Agriculture (USDA) Animal and Plant Health Inspection Service (APHIS) confirmed the first CBS detection in the United States in Collier County, Florida, in a commercial orange grove. Unable to eradicate the disease, today the USDA maintains a Federal domestic quarantine against six infested counties in Florida https://www.fdacs.gov/content/download/10168/file/DA-2012-09federalorder.pdf.

In the United States, CBS is established in southern Florida. The disease is not known to be in widespread in the Caribbean or Mexico, although it has been intercepted in fruit from Cuba. It does occur in many tropical citrus producing countries in Asia, Africa, Australia, and parts of South America including Brazil.

Symptoms on mature leaves and petioles are circular red to red-brown spots, visible on both leaf surfaces. Small spots ( <3 mm), darken with a brown to black ring. Leaf lesions can have yellow halos. CBS produces similar symptoms to those caused by other pathogens, and mechanical or insect damage. Diagnosis should not be made on field symptoms alone. Symptoms on fruit have multiple descriptions including black spot, hard spot, shot hole spot, false melanose, speckled blotch, freckle spot, and virulent spot.

Most symptoms do not appear for several months after infection, usually not until the fruit begins to ripen. Spores form in fruit lesions, leaves, and twigs. The spores are spread by wind driven rain and rain splash and can reinfect fruit and leaves. The Phyllosticta form can produce multiple cycles of infective conidiospores. The disease is favored by warm, humid weather with frequent rainfall during the summer. In order to infect, the fungus requires a long wetting period of 24–48 hours.

Lesions on Leaves

Lesions on Fruit

Lesions on fruit are diverse and grouped by symptom names.

Hosts for CBS include all commercial citrus varieties. Late-maturing varieties and lemons are among the most vulnerable to infection. Fruit are susceptible to infection for six months following fruit set.

CBS is an economically important fungal disease found in tropical areas with hot, humid, wet climates. The necrotic lesions caused by the pathogen render fruit unacceptable for fresh markets. Severe infection causes premature fruit drop which reduces yield of fruit for processing. The disease may be managed through combined efforts of regulation and chemical fungicides. CBS is both a domestic and international quarantine pest. California maintains a state exterior quarantine against citrus pests that includes CBS. (http://pi.cdfa.ca.gov/pqm/manual/pdf/301.pdf).