Overview
An exotic citrus disease is a disease that does not occur in Florida, but can be found in other major citrus growing regions around the world. Huanglongbing (HLB), citrus canker, and citrus black spot were all at one time considered exotic citrus diseases. These diseases, especially HLB and canker, have profoundly changed the way we grow citrus in Florida and have been economically devastating to our industry.
Any new exotic disease, if introduced, may further increase production costs and decrease profitability for Florida growers. Exotic diseases will affect the viability of the industry and the varieties that could be profitably grown. Not only is the environment here in Florida favorable to these exotic diseases, but also there are established populations of the insect vectors already in the state for three of the diseases.
The disease triangle has three elements needed for disease infection-susceptible host, environment, and pathogen. A fourth element, the vector, is needed for disease spread by insects. All three or four elements must be present for a disease to occur and spread. In Florida, we have the susceptible host, citrus, and we have the favorable environment. If the pathogens of these exotic diseases enter Florida, the potential for a disease outbreak is imminent.
This webpage focuses on five exotic citrus diseases that are of economic importance in other parts of the world and can potentially become established in Florida: Citrus Leprosis, Citrus Tristeza Virus-Stem Pitting, Citrus Variegated Chlorosis, Pseudocercospora Fruit and Leaf Spot, and Sweet Orange Scab. This information can be used as a basis for creating response plans for each of the diseases, as well as educational tools for scouting and identification efforts.
If you suspect you may have any of these exotic diseases Report It! Contact your local Florida Department of Agriculture and Consumer Service's CHRP office for further diagnostic testing. Do not move infected plant material, fruit, or insects out of the area. Early detection is essential to protecting Florida citrus.
History
Citrus leprosis is a non-systemic viral disease that is currently found in Mexico, Brazil, and other countries in South and Central America. The causal agent is the Citrus leprosis virus and the virus is transmitted by the Tenuipalpidae family of flat mites, genus Brevipalpus. These mites are found throughout Florida on many plant hosts.
There are two main types of leprosis: C-type and N-type. The names refer to where in the cell the virus is located (see image below). The virus in the C-type leprosis cytoplasm of the cell and the virus in the N-type leprosis in the nucleus. The C-type Leprosis is more aggressive than the N-type. The symptoms of each type of virus vary subtly and are best identified via the appropriate molecular test. If leprosis were to be detected, identifying the specific virus type would help regulators and researchers better understand what mites and viruses were part of an outbreak. The C-type virus is the most widespread of the viruses and has spread from southern Mexico to rest of the citrus producing regions of Mexico between 2013 and 2019.
In the 1860’s, leprosis was found in Florida and resulted in reduced citrus production. By 1926, there was a decline in the incidence of leprosis, and by the 1950’s it was reported only on the east coast in small, isolated areas. It is unclear why leprosis disappeared from Florida. It is thought that the increased use of miticides and cold winter temperatures contributed to the severe decline in mite populations, resulting in reduced inoculum and transmission. Leprosis had not been reported in Florida since 1968.
Spread
Citrus leprosis is spread by several species of flat mites (Brevipalpus sp.). As the mite feeds on the leaf, stem, or fruit, the plant tissue is infected with the virus. The mite can transmit the virus in any active stage of its life, but the virus is not thought to be transovarial, which means it is not transmitted from the female to her offspring. For a mite to be infected with the virus, it must feed on infected plant material. Once the mite has the virus, the mite remains infectious for the remainder of its lifespan.
Fruit Symptoms
Sweet orange and mandarin varieties are most susceptible to leprosis, but it can also affect grapefruit and other varieties. The rind of the fruit is affected by lesions produced where flat mites feed. Interestingly, the greatest amount of virus are found in the fruit lesions. As the fruit matures, the lesions become more pronounced and can be circular or irregularly shaped. The lesions can be either single or become coalescent. The centers of the lesions may crack with high temperatures. Fruit infected with leprosis can change color early and become susceptible to rot. Premature fruit drop can occur from multiple lesions being present on the fruit, resulting in yield loss. Flowering is unaffected by leprosis.
Leaf and Stem Symptoms
Each lesion represents a feeding spot from a mite. The symptoms typically take 1-2 months to appear after inoculation (when the virus is spread to the plant by the mite). Leaf symptoms of leprosis start as yellow chlorotic spots and become brown in color. The lesions may also develop necrotic centers as they age. Leaf lesions are smooth to the touch. With severe infection, premature defoliation of the canopy can occur, which affects the health of the tree and impacts fruit production.
Stem and twig symptoms begin as chlorotic, shallow lesions that become dark brown to reddish in color. As the lesions age, they become corky and develop into scaling of the bark. The lesions can also cause girdling of the stem when they coalesce, resulting in stem and twig dieback. Tree sections and an entire tree can die if not managed.
Regulations and Management
USDA-APHIS has developed a recovery plan for if and when leprosis enters the United States. This recovery plan can be found below in the links section. Brevipalpus mites are present in Florida, which increases the potential of a leprosis outbreak. When replanting in the field, only use clean nursery stock from registered nurseries. Monitoring of groves and early detection are essential to reduce the spread of the disease and aid in eradication efforts. Educational resources and identification tools should be utilized by grove workers, managers, and other industry professionals to increase awareness and knowledge of this disease. Controlling weeds, movement of infected plant debris, and the movement of people in the grove can reduce the risk for leprosis. In affected areas, growers should prune out and burn diseased wood to reduce the amount of inoculum present and chemical control of mites is recommended to reduce populations.
How to Report a Suspected Find
As with all exotic diseases, it is important to be vigilant for potential symptoms. If you suspect you may have leprosis, please submit a sample to the Florida Department of Agriculture and Consumer Services.
History
The Citrus tristeza virus (CTV) has been a problem for growers in Florida since the 1960’s. There are 3 main strains of CTV which cause varying symptoms: quick decline, stem pitting, and seedling yellows. The strain that is found in Florida causes quick decline. This strain of CTV caused a major change in horticultural practices. Sour Orange was once a widely used rootstock and now is no longer a primary rootstock in Florida, because of its high susceptibility to the disease. Trees planted on Sour Orange were being lost to this disease at an alarming rate. The stem pitting (SP) strain of CTV, which is not currently found in Florida, is more severe and is caused by different isolates. CTV-SP is found in Asia, Australia, South Africa, Brazil, and Columbia. Limes and grapefruit are the most susceptible varieties, but sweet orange and Rough Lemon exhibit strong stem pitting symptoms when infected. Mandarins can also show stem pitting symptoms, but are considered tolerant to the virus. Unlike CTV-quick decline, CTV-SP is not a disease of the bud union, but it attacks the scion varieties regardless of the rootstock variety.
How to Report a Suspected Find
If you suspect you may have Citrus Tristeza - Stem Pitting, please contact your local Florida Department of Agriculture and Consumer Service's CHRP office for further diagnostic testing. Do not move infected plant material out of the area.
Fruit Symptoms
Tree vigor declines as the infection becomes more severe, resulting in off-season bloom and poor fruit production. Fruit infected with CTV-SP do not grow properly causing them to be small and misshapen.
Leaf and Stem Symptoms
The CTV-SP virus can be found in the phloem and the severity of symptoms can vary greatly between citrus varieties. The virus symptoms can be observed by removing the bark of the branches and trunk. Symptoms can range from mild: exhibiting pitting of the wood, to severe: which exhibits a ropy appearance to the wood. These symptoms cause depressions in the bark. If severe, the symptoms can be seen without removing the bark. This distortion of the wood causes a disruption in the cambium cells, inhibiting the vascular system and the proper movement of water and nutrients in the tree. Twigs are often brittle with thick bark. Leaves become chlorotic and dieback can occur in the canopy. Tree vigor and growth is greatly reduced.
Spread
CTV-SP is most effectively spread by the brown citrus aphid (Toxoptera citricida), but can also be spread by the melon aphid (Aphis gossypii) and the spirea aphid (Aphis spiraecola) less effectively. All of these aphids are found in Florida, which increases the potential of CTV-SP being spread around the state. This virus can also be transmitted through budding and grafting, making the use of clean budwood essential to reduce an infection.
Regulations and Management
Because of the potential for spread through budding and grafting, budwood certification is essential for reducing the spread of CTV-SP. Always use clean budwood and purchase clean nursery stock from a registered nursery. Control of the brown citrus aphid can also help to reduce inoculum. Replanting with tolerant varieties is recommended if CTV-SP has become endemic in the area. Educational resources and identification tools should be utilized by grove workers, managers, and other industry professionals to increase awareness and knowledge of this disease. If CTV-SP is suspected do not move plant material out of the infected area, and contact your local CHRP office.
How to Report a Suspected Find
If you suspect you may have Citrus Tristeza - Stem Pitting, please contact your local Florida Department of Agriculture and Consumer Service's CHRP office for further diagnostic testing. Do not move infected plant material out of the area.
History
Citrus Variegated Chlorosis (CVC) is a bacterial disease caused by a subspecies of the Xylella fastidiosa bacterium, which lives in the xylem of the plant and limits the function of the vascular system. This bacterium is transmitted plant-to-plant by several species of large leafhopper insects called sharpshooters and through grafting. CVC was first discovered in Brazil in 1987 and is currently found in Brazil, Argentina, Costa Rica, and Paraguay. Sweet orange cultivars are highly susceptible to CVC. Grapefruit, mandarins, mandarin hybrids, and limes show less severe symptoms. CVC tolerant varieties are Rangpur lime, lemon, citron, and pummelo. CVC causes trees to have reduced vigor and growth. Tree death is not common from CVC, but production is greatly reduced.
Other subspecies of the causal agent are present in the United States, although this subspecies has yet to be found here. The alternate host range for this disease is unknown. Some hosts infected with X. fastidiosa show no symptoms of disease, making this bacteria and host range hard to study. Some hosts are known for example, as other subspecies of this bacterium cause Pierce’s disease in grapes and other scorch-type diseases in plants such as coffee and almond. This makes regulations difficult to design and enforce for non-citrus plant material, since it is not known how many species of plants can carry this pathogen. Another challenge with studying this bacterium is that it is difficult to culture in a lab setting and requires a very specific medium in which to grow. There is a long latent period, up to one year or longer, from when the infection occurs, and the tree starts to exhibit symptoms. During this latent period, the bacterium can be transmitted, and the spread of the disease can go undetected for some time.
Fruit Symptoms
Fruit quality and production is greatly affected by CVC. Fruit fail to grow properly and prematurely ripen, they remain small and hard and have a high acid content. This renders the fruit unusable resulting in profit loss for the grower. The fruit does remain on the tree and will color change normally. The fruit may also exhibit signs of sunburn damage. CVC also causes off-season bloom and abnormal fruit set to occur as trees become more severely affected.
Leaf and Stem Symptoms
Symptoms can occur on one branch or in the entire canopy of the tree. Although the bacterium is living in the vascular tissue, stems are considered asymptomatic, but the tree can have twig dieback in the canopy. Leaf symptoms include severe chlorosis between veins and lesions. In the early stages, leaf lesions can resemble zinc deficiency. Lesions become gummy, reddish to brown in color and occur on the underside of the leaf. These lesions correspond to the yellow chlorotic areas on the upper side of the leaf. The symptoms of CVC are most pronounced in younger leaves but can also be expressed in older leaves. As the infection becomes more severe, tree health declines and the thinning of the canopy will occur.
Spread
The bacterium that causes CVC is transmitted by sharpshooters (leafhoppers, Cicadellidae), tree hoppers (Membracidae), and spittlebugs (Cercopidae). In Brazil, 11 species of sharpshooters are known to transmit CVC. There are two of these species currently found in Florida; the glassy-winged sharpshooter, Homalodisca vitripennis, and the blue-winged sharpshooter, Oncometopia nigricans. The presence of these insects in Florida increases the potential risk of an infection, if the pathogen should enter the state.
CVC can also be transmitted through grafting, making the use of clean budwood essential to reducing the spread of the disease. In Brazil, the main source of infection was from the use of diseased budwood, but with the implementation of undercover greenhouses in the early 2000’s, this is no longer the case. It is also believed that natural root graphs between adjacent trees can spread the disease within the grove. Studies have shown that the pathogen is not transmitted through seed.
Regulations and Management
Although the complete host range of X. fastidiosa, subspecies pauca, is still unknown, it is important to keep up regulations and restrictions on the plant material that enters the United States and Florida, especially from those countries known to have this disease. The use of clean budwood from registered sources and the planting of disease-free nursery resets, is essential to limiting the spread of this pathogen should it enter Florida. The current Florida nursery industry is well structured to be able to produce clean trees for the citrus industry should CVC arrive in Florida. Growers in Brazil prune out diseased wood on mild to moderately affected trees to reduce inoculum sources. In cases of severe symptoms, the entire tree is removed. Educational resources and identification tools should be utilized by grove workers, managers, and other industry professionals to increase awareness and knowledge of this disease.
How to Report a Suspected Find
If you suspect you may have Citrus Variegated Chlorosis, please contact your local Florida Department of Agriculture and Consumer Service's CHRP office for further diagnostic testing. Do not move infected plant material out of the area.
History
Pseudocercospora Fruit and Leaf Spot (PFLS), formerly known as Phaeoramularia Fruit and Leaf Spot, is a fungal disease caused by Pseudocercospora angolensis. This disease is currently found in Sub-Saharan Africa, the Comoros Islands, and Yemen, but not in South Africa, a major citrus producing country. PFLS was first found in 1952 in Angola and Mozambique and has since become a serious disease in Sub-Saharan Africa. The fungus thrives in warm humid environments which increases the potential of a disease establishment in Florida. This disease causes extensive damage, not only to the fruit, but also to the tree canopy. Without the use of fungicides, crop losses have been known to reach 50-100%. This disease affects all varieties of citrus, but grapefruit, oranges, pummelos, and mandarins are highly susceptible. Lemons are less susceptible than other types of citrus, followed by limes which are the least susceptible. At this time no alternate hosts of PFLS are known.
Fruit Symptoms
Lesions on young green fruit begin as small, individual, protruding swellings with yellow halos. Mature fruit lesions are dark brown to black and are generally ½ inch (10mm) in diameter. They are flat or sunken with a yellow halo and eventually develop necrotic centers. Their shape can be circular or irregular and the lesions can coalesce. Premature fruit drop can occur, but fruit may also remain on the tree and become dried up and mummified, leading to yield and profit loss. The infected fruit has poor juice quality and therefore cannot be used for processing or sold for the fresh market.
Leaf and Stem Symptoms
Leaf lesions of PFLS are circular to irregularly shaped. They can either be individual or coalescent with a brown or grayish center surrounded by a yellow halo. Young flush may look bleached in color and can be killed resulting in leaf drop. Leaves are the main source of inoculum. The lesions are brown to gray when the fungus is dormant, and when sporulating the lesions become black in color. The leaf lesions of PFLS can be mistaken for citrus canker, but unlike canker lesions which are raised, PFLS lesions are flat or sunken.
Stem symptoms of PFLS are rare and are usually the result of leaf lesions extending past the petiole. These lesions can become corky and crack. A collection of lesions in an area can cause stem dieback and results in secondary shoots growing at that point.
Spread
The causal agent of PFLS is the fungus Pseudocercospora angolensis and leaves are the main source of inoculum. The asexual spores (conidia) multiply on leaf lesions, become airborne, and are dispersed long distances by windblown rain. Conidia can also be dispersed short distances by rain splash. Humans can be another means of long distance spread when infected plant material and fruit are moved from area to area. Although the conditions in Florida are favorable for PFLS, more information is needed to determine the extent to which this disease could become established and spread in the state. There is no known sexual stage of this fungus.
Regulations and Management
There are no specific regulations for this exotic disease. However, it is always important when replanting in the field, to use only clean nursery stock from registered nurseries. Monitoring of groves and early detection are essential to reduce the spread of the disease and aid in eradication efforts, should this disease enter Florida. Educational resources and identification tools should be utilized by grove workers, managers, and other industry professionals to increase awareness and knowledge about this disease. In countries affected by PFLS, fungicide sprays (copper hydroxide, chlorothalonil, and flusilazole) are used to manage this disease. Sprays are applied every 10-14 days and are required to form a protective coating on the fruit until several months after bloom when the fruit is most susceptible.
How to Report a Suspected Find
If you suspect you may have Pseudocercospora Fruit and Leaf Spot, please contact your local Florida Department of Agriculture and Consumer Service's CHRP office for further diagnostic testing. Do not move infected plant material out of the area.
History
Sweet Orange Scab (SOS) is a fungal disease caused by the fungus Elsinoë australis. Sweet Orange Scab is found in many of the citrus growing regions of South America: Argentina, Bolivia, Brazil, Ecuador, Paraguay, Uruguay, and in Korea. In the United States, the disease was believed to have been detected on a residential tree in Texas in 2010. This disease can be easily confused with common Citrus Scab which is found in Florida and caused by a different fungus Elsinoë fawcettii. The host range for Sweet Orange Scab are sweet oranges and tangerines. This contrasts with Citrus Scab which affects, grapefruit, tangerines, and limes, but does not commonly affect sweet orange varieties.
Fruit, Leaf, and Stem Symptoms
Young fruit exhibit wart-like pustules, which are tan to gray in color, raised, corky, and rough to the touch. As the fruit matures the lesions become smoother. Fruit infected with SOS cannot be sold for the fresh market because of appearance. They can be sold for juice because the internal quality if the fruit is not compromised by this disease Unlike Citrus Scab, Sweet Orange Scab does not exhibit symptoms on the leaves and twigs, only on the fruit.
Spread
The presence of moisture is necessary for Elsinoë australis to produce spores and spread. The asexual spores (conidia) are produced in lesions on the fruit and are spread short distances by rain splash. The movement of infected fruit out of quarantine areas causes the fungus to travel long distances. The fungus also requires the presence of susceptible young fruit, which are susceptible from 6-8 weeks after petal fall. Unlike the spores of Elsinoë fawcettii which survive in old leaves and twigs, it is not known how the spores of Elsinoë australis survive in the absence of susceptible fruit, but infections can still occur the following season even when all fruit are removed.
Regulations and Management
Do not use overhead irrigation, as this can cause the fungal spores to be carried in by water drops and spread from fruit-to-fruit. The use of azoxytrobin, trifloxystrobin, pyraclostrobin, ferbam, and copper fungicides create a barrier on the fruit during its most susceptible growing period, reducing the amount of inoculum. A typical spray program for areas infected with SOS is for two fungicide sprays: one at 2/3rd petal fall and a second spray applied 2-3 weeks later. Educational resources and identification tools should be utilized by grove workers, managers, and other industry professionals to increase awareness and knowledge of this disease. If you suspect the presence of SOS in your grove, do not move the fruit out of the area.
How to Report a Suspected Find
If you suspect you may have Sweet Orange Scab, please contact your local Florida Department of Agriculture and Consumer Service's CHRP office for further diagnostic testing. Do not move infected plant material out of the area.
