Citrus black spot is a fungal disease caused by Phyllosticta citricarpa(previously known as Guignardia citricarpa). This Ascomycete fungus affects citrus plants throughout subtropical climates, causing a reduction in both fruit quantity and quality.
Symptoms include both fruit and leaf lesions, the latter being critical to inter-tree dispersal. Strict regulation and management is necessary to control this disease since there are currently not many citrus varieties that are resistant.
Fungus Phyllosticta citricarpa is a plant pathogen, some strains of which cause a leaf condition called black spot on citrus plants. As a result, such strains are subject to phytosanitary legislation in the European Union and the United States.
Metabolite secreted by P. citricarpa have an inhibitory growth effect on some endophytic bacterial species, and stimulatory growth effect on others.
An isolate of P. citricarpa was found to produce the medically important compound, taxol under certain growth conditions.
Origin Citrus Black Spot was first found in Sydney, Australia, in 1895Citrus Black Spot Pest Profile, California Department of Food and Agriculture, https://www.cdfa.ca.gov/citrus/pests_diseases/cbs.html, (May 4, 2023)Citrus Black Spot (Guignardia citricarpa) discovered in Florida, Florida Department of Agriculture and Consumer Services, Division of Plant Industry, https://crec.ifas.ufl.edu/media/crecifasufledu/extension/plant-pathology-/black-spot/citrus-black-spot-pest-alert.pdf, (May 4, 2023) and then appeared in South Africa along the coast of Natal in 1929. It can be found in many countries around the world. These countries include: Argentina, Australia, Brazil, China, Ghana, Mozambique, Philippines, South Africa, Sub-Saharan Africa, Taiwan, The United States and Uruguay. Whether or not Citrus Black Spot is present in Japan and New Zealand is controversial. In both countries the fungus was thought to have been found, but after further testing it was identified as the non-pathogenic strain, Phyllosticta capitalensis, rather than the pathogenic strain Phyllosticta citricarpa which causes Citrus Black Spot. The disease was first reported in North America during March 2010, in Collier and Hendry counties of southern Florida. The range of the disease in North America has remained limited to Southern Florida at present. Florida has taken measures to try to control this disease, however, it is expected to rapidly spread to other areas over the next few years.
Hosts Phyllosticta citricarpa infects mostly citrus plants. However, this fungus is also seen to infect other plants such as golden apple, mango and guava as well. There are some plants that are more susceptible to the pathogen than others. Lemon and late maturing citrus, such as Valencia orange, are the most susceptible hosts.
Moderately susceptible hosts are Hamlin sweet oranges, tangerine/ mandarin type fruit, and grapefruit.
Though there are some hosts that are more susceptible to Citrus Black Spot than others, any citrus plant that is nutritionally stressed increases its chances of infection.
Fruit symptoms and signs Symptoms of Citrus black spot vary widely and have different names to best describe the symptoms. Hard spot lesions
Hard spots are the most common lesions. They are small, round, and sunken. The average diameter of hard spot lesions ranges from 3–10 mm (.12-.4 in). They have dark red to chocolate brown margins and often have pycnidia in the gray-colored centers. A green halo may be present around the lesion.
False melanose lesions False melanose symptoms can appear on unripe fruit early in the season. They are characterized by many small, tan to chocolate brown, slightly raised lesions. The lesions are much smaller than the hard spot variety with an average diameter of less than 1 mm (.04 in). They are difficult to observe later in the season. Unlike hard spot lesions, no pycnidia (asexual fruiting bodies) are present.
Cracked spot lesions
These lesions occur on both unripe and ripe fruit. They are large, slightly raised, dark brown spots. Cracked spot lesions do not contain pycnidia. They create raised cracks on the fruit surface that can be difficult to see later in the season. Studies have suggested a possible interaction between cracked spot lesions and rust mite colonization.EPPO. 2009. Guignardia citricarpa. OEPP/EPPO Bulletin 39:318-327, cited in Tara Holtz USDA-APHIS-PPQ-CPHST-PERAL Risk assessment of Citrus spp. fruit as a pathway for the introduction of Guignardia citricarpa Kiely, the organism that causes Citrus Black Spot disease. 2010.
Freckle spot lesions Freckle spot lesions are the early form of virulent spot lesions. They are small, reddish, irregularly shaped, and contain many pycnidia. These lesions are most noticeable during the end of the season, as they are found on mature fruit and during post-harvest storage.
Virulent spot lesions The mature stage of freckle spot lesions, they are found on mature fruit and during post-harvest storage. They are similar in appearance to the freckle spot lesions, but under high humidity can cover the entire fruit. Because of this direct damage to the fruit, this type of lesion can be economically devastating.
Leaf symptoms Leaf symptoms are generally observed on highly susceptible citrus varieties, such as lemons, as well in poorly managed orchards. The symptoms generally develop from latent infections after the leaves have died. Red-brown, pin-point dots may develop into larger, circular necrotic lesions with gray centers and red or brown margins.Wager, V. A. 1952. The black spot disease of citrus in South Africa. Citrus Grower 227:5-12, cited in Tara Holtz USDA-APHIS-PPQ-CPHST-PERAL Risk assessment of Citrus spp. fruit as a pathway for the introduction of Guignardia citricarpa Kiely, the organism that causes Citrus Black Spot disease. 2010.
Diagnosis To confirm a diagnosis of Citrus Black Spot, the pathogenic fungus must be isolated in culture. This can be difficult, as culturing of the fruit lesions takes up to 14 days with an efficacy of less than 10%. Diagnosis is also complicated by the morphological similarities of the fungal structures to the non-pathogenic strain Phyllosticta capitalensis (formerly referred to as Guignarida mangiferae).December 2015, DP 5: Phyllosticta citricarpa (McAlpine) Aa on fruit, International Plant Protection Convention, https://www.fao.org/3/ml511en/ml511en.pdf, (May 4, 2023) The two species can be distinguished by molecular testing using PCR.
Life cycle After overwintering in leaf debris, ascostroma begin producing ascospores. Ascospore formation is accelerated by sequential wetting and drying of the leaf litter.Kiely, T. B. 1948. Preliminary studies on Guignardia citricarpa n. sp.: The ascigerous stage of Phoma citricarpa McAlp. and its relation to black spot of citrus. Proceedings of the Linnaeus Society of New South Wales 73:249-292, cited in Tara Holtz USDA-APHIS-PPQ-CPHST-PERAL Risk assessment of Citrus spp. fruit as a pathway for the introduction of Guignardia citricarpa Kiely, the organism that causes Citrus Black Spot disease. 2010.Kotzé, J. M. 1981. Epidemiology and control of citrus black spot in South Africa. Plant Disease 65:945-950, cited in Tara Holtz USDA-APHIS-PPQ-CPHST-PERAL Risk assessment of Citrus spp. fruit as a pathway for the introduction of Guignardia citricarpa Kiely, the organism that causes Citrus Black Spot disease. 2010. However, very wet conditions inhibit ascospore development due to leaf decomposition and competition from saprophytes.Lee, Y. S., and C. S. Huang. 1973. Effect of climatic factors on the development and discharge of ascospores of the citrus black spot fungus. Journal of Taiwan Agriculture Research 22:135-144, cited in Tara Holtz USDA-APHIS-PPQ-CPHST-PERAL Risk assessment of Citrus spp. fruit as a pathway for the introduction of Guignardia citricarpa Kiely, the organism that causes Citrus Black Spot disease. 2010.CABI. 2006. Guignardia citricarpa. Crop Protection Compendium, CD-ROM. CAB International, Commonwealth Agricultural Bureau International (CABI) Wallingford, UK, cited in Tara Holtz USDA-APHIS-PPQ-CPHST-PERAL Risk assessment of Citrus spp. fruit as a pathway for the introduction of Guignardia citricarpa Kiely, the organism that causes Citrus Black Spot disease. 2010. The ascospores are ejected from the fruiting bodies of the fungus during rainfall or irrigation and then dispersed by wind and water.Kotzé, J. M. 1963. Studies on the black spot disease of citrus caused by Guignardia citricarpa Kiely with particular reference to its epiphytology and control at Lebata. Doctoral Thesis, University of Pretoria, Pretoria, South Africa. January, 1963, cited in Tara Holtz USDA-APHIS-PPQ-CPHST-PERAL Risk assessment of Citrus spp. fruit as a pathway for the introduction of Guignardia citricarpa Kiely, the organism that causes Citrus Black Spot disease. 2010. Upon landing on susceptible tissue, the ascospores germinate and form appresoria after a 24-48 hour wetting period.Citrus Black Spot Pest Profile, California Department of Food and Agriculture, https://www.cdfa.ca.gov/citrus/pests_diseases/cbs.html, (May 4, 2023) (Different tissues are susceptible to infection at different times. Leaves are susceptible up to 10 months of age.Truter, M., P. M. Labuschagne, J. M. Kotzé, L. Meyer, and L. Korsten. 2007. Failure of Phyllosticta citricarpa pycnidiospores to infect Eureka lemon leaf litter. Australasian Plant Pathology 36:87-93, cited in Tara Holtz USDA-APHIS-PPQ-CPHST-PERAL Risk assessment of Citrus spp. fruit as a pathway for the introduction of Guignardia citricarpa Kiely, the organism that causes Citrus Black Spot disease. 2010. Fruits are susceptible during the 4–5 months after fruitset,Kotzé, J. M. 2000. Black spot. Pages 23-25 in J. O. Whiteside, S. M. Garnsey, and L. W. Timmer, (eds.). Compendium of Citrus Diseases. American Phytopathological Society, St Paul, MN, USA. though trees less than ten years old are only susceptible up to 3 months .Kiely, T. B. 1969. Black spot of citrus. Agricultural Gazette of New South Wales 80(12):658- 662.) Soon after, the infection peg along with mycelia colonize the area between the cuticle and epidermal wall. The symptoms are not visible right away because the infection remains latent, or dormant, until the fruit is mature.