Fungus on Citrus

Fungus on Citrus thumbnail
Citrus fungal diseases can range from being beneficial to being deadly to the tree.

Citrus fungus diseases can be a major problem for both commercial and consumer growers. Fungi can cause problems ranging from discoloration of leaves and fruit, to killing the entire tree. If a fungus is suspected, it is important to quickly identify the fungus and treat the tree accordingly. In some cases, the tree should be removed quickly to avoid the spread of the disease to nearby plants.

  1. Fungi Infecting Fruit

    • According to, citrus canker is a fungus that causes "yellow halo-like lesions or scabs on the fruit, leaves, and twigs of citrus trees." This infection is highly contagious and spreads easily by way of insects, birds, humans and air currents. The infection can cause blemished fruit and leaf loss. Citrus scab is another common fungus of some citrus varieties. It often effects lemons, temples, murcotts, tangelos and grapefruit. The disease begins as small pale orange spots on leaves, maturing into defined wart-like protrusions. Seven days after infection, the fruit will show scabby spots or lesions. Melanose fungus causes small brown scabs on the fruit rind, but does not effect fruit quality.

    Fungi Infecting Leaves

    • Insects such as aphids, whiteflies and mealybugs secrete honeydew, which drops onto the leaves and twigs of the tree causing sooty mold fungus. The mold will coat the leaves, block out light and cause a reduction in fruit production. Greasy spot fungus causes yellow-brown blisters, often on the underside of leaves. This fungus can cause leaf loss, and if severe, it can affect the rind of the fruit. (Yellow or red 1 1/2-inch raised spots on the underside of the leaves is a symptom of Aschersonia, a friendly fungus that helps the citrus tree by attacking whiteflies.

    Fungi Infecting Limbs & Trunk

    • Root rot, also called Brown Rot or Collar, causes "dark brownish patches of harden[d] bark on the trunk of the tree", according to Ooze will often seep out of these patches. Eventually, the bark dies and falls off leaving a "dark sunken canker." This fungus can also cause yellowing of the leaves and fruit decay. If the infection circles the trunk, the tree will die. Root rot occurs most often during periods of heavy rain and is poorly drained locations.

    Fungi Infecting the Root

    • According to the University of Arizona, cotton root rot (also known as Texas root rot) is a fungus that attacks the underground part of the tree. This fungus causes rapid wilt and death of the tree, with dead and dying leaves remaining attached to the tree. In cooler climates, the decline may be slower. Young trees are particularly susceptible to cotton root rot, although it can occur in older trees, as well.

    Prevention & Treatment

    • Prevention and treatment depends on the type of fungus infecting the tree. For cotton root rot, there is no prevention, as the fungus is unable to be detected in soil. When this fungus is discovered on a tree, the tree has to be removed and the area replanted with a tolerant species. Foot rot is best prevented by planting resistant rootstock in well-drained soil. Other methods of prevention and control include keeping the lower trunk clear of soil, budding seedlings high and avoiding wounds to the tree. Sooty mold can be washed off infected plants, but it will reoccur if the insects causing the fungus are not controlled with the proper insecticide. New citrus scab outbreaks are controlled by applying a two or three applications of fungicide. As trees grow older, a routine spraying helps eliminate the change of reoccurrence. A fungicide can help prevent citrus canker, melanose and greasy spot. Trees infected with citrus canker are often cut down to prevent the spread of the disease, while those infected with melanose and greasy spot are pruned down.

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  • Photo Credit orange tree image by Daria Miroshnikova from

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