Citrus Diseases

Four serious citrus diseases found in the United States include Huanglongbing (also known as citrus greening or HLB for short), citrus canker, citrus black spot and sweet orange scab. Today, one or more of these diseases threatens every acre of citrus in America.

Leaf Diagnosis


Tips on Distinguishing the Leaf Symptoms of Citrus Greening Disease
The most commonly reported symptoms of suspected citrus disease are related to discoloration of the leaves. More than 60 percent of reports submitted through the Save Our Citrus program cite spotted or blotched leaves, or leathery leaves with yellow or clear veins.While these symptoms are telltale signs of citrus greening disease, abnormal looking citrus leaves can also be the result of other factors such as unfavorable weather or overwatering. Additionally, other severe citrus diseases such as citrus canker and sweet orange scab can cause lesions on the leaves. This can make it difficult to distinguish serious disease from harmless environmental conditions. Remember, if you do suspect your tree may be infected, please don’t hesitate to report it. A mistaken case of disease is harmless compared to an undetected disease that spreads to other healthy trees. Here are some tips to tell the difference:

Cold weatherCold weather weather can cause citrus leaves to curl. Cold can cause the tissue in the leaves to dry out leaving a burnt appearance. If possible, cover citrus trees if you expect a freeze.

SunburnSunburn can leave yellow or brown leathery spots on the fruit and leaves, likely on the south and west sides of the tree. Too much heat can cause leaves to curl.

Over wateringOverwatering is another common cause of unhealthy looking leaves. Too much water can cause leaves to curl, turn yellow, and drop. If you suspect overwatering, reduce your watering schedule and look for signs of improvements. Also make sure your tree has proper drainage.

Sooty MoldSooty Mold is the result of massive, superficial fungal growth commonly occurring on the upper surfaces of leaves, fruit, twigs and small branches. Sooty mold develops on the honeydew produced by insects such as aphids and white flies. Controlling insect populations is the best way to control sooty mold.

Citrus GreeningCitrus Greening Disease. Check leaves and fruit. In addition to leathery leaves with yellow veins, leaves may show an asymmetric blotchy mottling, which is the most characteristic symptom. Fruit of trees infected with citrus greening may be small and lopsided, or retain green color on the bottom rather than fully ripening to the expected yellows and oranges.

Citrus CankerCitrus Canker Disease is a bacterial disease of citrus that causes premature leaf and fruit drop. The main symptoms are raised, corky, water sunken lesions on the leaves, stems and fruit of citrus trees. A yellow halo maybe be evident on leaf and fruit, but not on stems.

Sweet Orange ScabSweet Orange Scab is a fungal disease that results in unsightly, scab-like lesions developing on fruit rinds and, less often, on leaves and twigs. Rusted colored lesions have been observed on the lower side of the affected leaves.

Magnesium DeficiencyMagnesium (Mg) deficiency results from translocation (the transport of all dissolved material within a plant) from the leaves to developing fruit of new leaves on the same shoot. Mature leaves may show a triangular green pattern along the mid-rib (an inverted V). Mg deficiency may be caused by the presence of citrus greening disease. If you suspect this is the case, check for the presence of the blotchy mottle (uneven spots) symptom on leaves of previous flushes (fresh leaf growth).

Zinc DeficiencyZinc (Zn) deficiency is one of the most damaging and widespread citrus nutritional disorders. A distinctive yellow mottle (uneven spots) is observed on reduced and pointed leaves: green veins with interveinal yellow spaces. This deficiency may be caused by the presence of citrus greening disease. If you suspect this is the case, check for the presence of the blotchy mottle symptom on leaves of previous flushes (fresh leaf growth).

Manganese DeficiencyManganese (Mn) deficiency occurs in young, enlarging leaves on which dark green areas along the mid-rib and main lateral veins can be observed accompanied by light green interveinal areas. This deficiency may be caused by the presence of citrus greening disease. If you suspect this is the case, check for the presence of the blotchy mottle symptom on leaves of previous flushes (fresh leaf growth).

CITRUS GREENING


citrus greening leafCitrus greening is one of the most serious citrus plant diseases in the world. It is also known as Huanglongbing (HLB) or yellow dragon disease. Once a tree is infected, there is no cure. While the disease poses no threat to humans or animals, it has devastated millions of acres of citrus crops throughout the United States and abroad.

Citrus greening disease was first reported to have occurred in Asia during the late 1800s and the disease has already caused devastation in Asia, Africa, the Arabian Peninsula and Brazil.

Named for its green, misshapen fruit, citrus greening disease has now killed millions of citrus plants in the southeastern United States and is threatening to spread across the entire country. See all of the affected areas.

Citrus greening is spread by a disease-infected insect, the Asian citrus psyllid, and has put the future of America’s citrus at risk. Infected trees produce fruits that are green, misshapen and bitter, unsuitable for sale as fresh fruit or for juice. Most infected trees die within a few years.

The Asian citrus psyllid, which spreads citrus greening, is no bigger than the head of a pin. The infected insect spreads the disease as it feeds on the leaves and stems of citrus trees. Once the Asian citrus psyllid picks up the disease, it carries it for the rest of its life. Citrus greening is then spread by moving infected plants and plant materials such as bud wood and even leaves.

CITRUS CANKER


citrus canker leafCitrus Canker causes the citrus tree to continually decline in health and fruit production until the tree produces no fruit at all.

Citrus canker is a bacterial disease of citrus that causes premature leaf and fruit drop. The main symptoms are lesions on the leaves, stems and fruit of citrus trees. While not harmful to humans, the disease affects the health and marketability of infected fruit.

Canker causes the citrus tree to continually decline in health and fruit production until the tree produces no fruit at all. It affects all types of citrus and is highly contagious. Citrus canker can be spread rapidly by:

  • Windborne rain
  • Lawnmowers and other landscaping equipment
  • People carrying the infection on their hands, clothing, or equipment
  • Moving infected or exposed plants or plant parts

While citrus canker is believed to have originated in South East Asia, the disease was first found in the United States in 1910. It was discovered in Florida, but reached as far as the Gulf States and South Carolina before it was eradicated. Citrus canker was detected again on the Gulf Coast of Florida in 1986 and declared eradicated in 1994.
The most recent outbreak of the disease was discovered in Florida in 1995 and after a 10-year eradication effort, the USDA declared that eradication was no longer possible.

CITRUS BLACK SPOT


citrus blackspotCitrus Black Spot is one of the most devastating fungal diseases of citrus worldwide.

Citrus black spot, or Guignardia citricarpa, is a fungal disease characterized by dark, speckled spots or blotches on the rinds of the fruit. As one of the most devastating fungal diseases of citrus worldwide, it causes early fruit drop, reduces crop yield and leaves the highly blemished fruit unsuitable for sale.

While all commercial citrus is at risk for citrus black spot, lemon and late-maturing citrus varieties like Valencia are most vulnerable. The disease is transmitted through spores released from fallen, decomposing citrus leaves.

Citrus black spot is common in subtropical regions of the world with summer rainfall, including numerous countries in Asia, South America, Africa and Australia. It was first detected in the United States in 2010 during a pest survey of Valencia sweet oranges in the Immokalee area of Florida.

SWEET ORANGE SCAB


sweet orange scabThe disease has been reported by a number of countries in South America and Oceania. In June 2010, it was first confirmed in the United States in Texas.

Sweet orange scab is a fungal disease that results in unsightly, scab-like lesions developing on fruit rinds and, less often, on leaves and twigs. The damage produced is superficial and does not affect internal fruit quality or taste.

The fungus is easily dispersed by rain or irrigation and can be carried on infected nursery stock such as ornamental citrus plants and fruits. Insects also can spread the fungus when carrying fungal spores on their bodies.

Infected fruit is more likely to drop prematurely, and the scabby lesions reduce the fruit’s fresh market value. In addition, the disease may stunt young citrus seedlings.