Citrus black spot

Scientific name

[Fungus] Anamorph - Phyllosticta citricarpa (McAlpine) van der Aa (syn. Phoma citricarpa McAlpine)

Teleomorph - Guignardia citricarpa Kiely

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.

Disease cycle

As in many diseases, timing is important for black spot to occur. Inoculuminoculum:
a.) the pathogen or its part that causes infection of the host b.) small amount of a microorganism that is transferred to a substrate or a culture medium in order to propagate it
in the leaf litter needs to be available during the period when the hosthost:
an organism that is infected with or fed upon by a pathogenic or parasitic organism
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 inoculuminoculum:
a.) the pathogen or its part that causes infection of the host b.) small amount of a microorganism that is transferred to a substrate or a culture medium in order to propagate it
, 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 inoculuminoculum:
a.) the pathogen or its part that causes infection of the host b.) small amount of a microorganism that is transferred to a substrate or a culture medium in order to propagate it
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.

Symptoms

Leaf - older lesions are small, round, sunken necroticnecrotic:
a cell, group of cells, or tissue that has suffered a rapid destruction and quick death; typically localized and may be caused by injury, radiation, chemicals, toxic substances, or microbiological interactions
spots with gray centers. The lesions are bordered with a dark brown ring. Young lesions are small, reddish, and slightly raised. A yellow halo can be associated with the lesions. Foliar lesions are most commonly seen on lemons. They are rarely seen in well managed groves. Similar lesions can be seen on twigs and pedicles.

Fruit - lesions on fruit are so diverse that lesions are grouped into several types: 1.) hard spot; 2.) false melanose/speckled blotch; 3.) freckle spot (early virulent spot); 4.) virulent spot; 5.) lacy spot; 6.) cracked spot.

Hard spot (1) is the most typical and diagnostic symptom of black spot as the fruit mature, often appearing around color change. The lesions are circular depressions with a diameter of 3-10 mm (0.12 - 0.4 inch). The brick red lesions have tan to grey centers with a distinct brown to black margin. Pycnidia are usually, but not always, present in the lesions. Pycnidia appear as small black dots and are visible with a hand lens but can be confused with acervuli of Colletotrichum spp. Preharvest, hard spot lesions tend to develop on the side of the fruit exposed to sunlight.

False melanose/speckled blotch (2) symptoms appear as numerous, small (< 1 mm (0.04 in) in diameter), raised, brown to black lesions. Most frequently, false melanose occurs on green fruit. The lesions can coalesce as the season progresses and do not contain pycnidia. False melanose/speckled blotch-type symptoms can develop into hard spot-type symptoms at the end of the season.

Freckle spot (early virulent spot) (3) symptoms are a sign of heavy infection. They occur on mature fruit, most often after harvest but can be seen when fruit is on the tree. The lesions are 1-3 mm (0.04 - 0.12 in) diameter depressions, potentially with pycnidia present. Lesions can be several colors, most often reddish with a dark red to brown border, but may be gray to tan, brownish, or have no color. Late in the season or during storage, freckle spot can develop into virulent spot or hard spot.

Virulent spot (4) lesions are sunken and irregular in shape and occur on heavily infected, mature fruit toward the end of the season. In high humidity, large numbers of pycnidia may develop. The lesions can turn brown to black with a leathery texture that eventually covers the entire fruit. Virulent spot may cause premature fruit drop and serious post harvest losses since the symptoms may extend into the fleshy part of the fruit.

Lacy spot-type (5) lesions are superficial, small, and yellow with dark yellow to brown centers and no defined margins. The fruit is still green when the lesions become apparent and can cover a large portion of the fruit surface. These lesions are considered to be a variation of false melanose. This term is not commonly used, and these symptoms have only been reported from South America.

Cracked spots (6) type lesions are superficial, slightly raised, variable in size, brown to black with cracked surface and irregular margins. Pycnidia are not present in the lesions. These lesions have often been associated with rust mite damage. Hard spot lesions can eventually form in the center of the lesions.

Regulatory information

The most current regulatory information can be found at:

http://www.aphis.usda.gov/plant_health/plant_pest_info/citrus/black_spot.shtml

Host range

Citrus black spot affects all species of cultivated citrus and their hybrids. With the exception of sour orange and its hybrids, all commercially grown citrus species and cultivars have been observed to be affected by the disease. Lemon is particularly susceptible, and it has been observed that when citrus black spot moves into an unaffected area it first appears on lemons with the exception of the epidemic in Florida. Grapefruit and Valencia orange are also highly susceptible.

Distribution

Citrus black spot is found in Argentina, Australia, Brazil, China, United States (Florida), Indonesia, Japan, Kenya, Mozambique, Nigeria, Philippines, South Africa Taiwan, Uruguay, Venezuela, and Zimbabwe. No reports have been made from Mediterranean countries.