Introduction
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 1895 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 citrus plants mostly. 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
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 lesions are characterized by many small, tan, 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 found on unripe fruit and are difficult to observe later in the season. Unlike hard spot lesions, no pycnidia 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.
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.