Climate Makes Citrus Susceptible to Many Diseases, Disorders

Florida’s citrus can fall prey to many diseases and disorders. Last time, we discussed Citrus Greening, which has already infected residential trees in Palm Beach and Martin counties, and learned that it has cost the state’s economy more than $3.6 billion in the last five years (The Greening of Florida Citrus, July 10, 2012). Because of Florida’s humid climate, fungi and germs multiply easily, which lead to citrus tree damage. The good news is that these predators don’t change very much and that by recognizing common symptoms, such as infections on tree leaves, including blemishes, roughness and damaged color patterns, gardeners can treat problems before more harm is done.

Many of citruses’ disease producing agents become active and increase in numbers in the spring when citrus trees develop new leaves, shoots and fruit. The most common fungal diseases of citrus in Florida are scab, melanose, greasy spot and foot rot. The first three affect fruit, leaves and twigs while the last affects the trunk of the tree and over time can weaken the entire tree.

Citrus Scab

Scabs appear as a rash of scabs or warts on leaves, twigs and fruit. Early symptoms include well-defined, cone-shaped growths on one side of the leaf, with a corresponding depression on the opposite side. Immature fruit on infected trees becomes misshapen with warty growths, although the interior fruit quality is usually not affected. Since new generations of the infectious material can be produced within five days, scab can spread rapidly, given the right conditions, from overwintering infections on leaves and stems. According to the University of Florida Extension, water from rain, overhead irrigation and dew is the most important factor affecting disease development.

Varieties susceptible to scab include Temple oranges, lemons, Minneola tangelos, Murcotts and Page oranges. Other varieties like satsuma tangerines, Orlando tangerines, Tahiti limes and grape fruit are less susceptible.

The best scab management strategy is to remove and destroy infected leaves, twigs and fruit in order to control the disease before it can spread. Since scab appears on some rootstocks, this disease is commonly introduced into the home landscape on already infected nursery trees. Carefully examine the leaves for disease lesions when buying a nursery tree. Copper sprays may be applied 2-3 weeks after petal fall, and again 2-3 weeks later to control scab.

Melanose

A fungal disease that attacks mainly young grapefruit, melanose can be recognized by small, dark brown, raised lesions on leaves that have a rough, sandpaper texture. These lesions are actually scar tissue that is formed when the the host plant walls off infections. Fruit injury from melanose is often confused with damage from rust mite. Both rust mite blemishes and melanose lesions on fruit are brown but melanose lesions have a rougher texture. Trees that are 10 years and older are typically susceptible to melanose and because melanose develops in dead wood, clearing dead wood can help control the problem. The disease, however, has little serious impact on mature trees.

All varieties are affected by melanose, but it does not cause leaf drop and damage is only superficial. Leaves are susceptible until fully expanded and fruit is susceptible for 12 weeks after petal fall. Another form of the melanose fungus causes a post-harvest fruit rot, so greater control may be necessary.

Greasy Spot

According to the University of Florida, greasy spot mostly occurs in Southwest Florida from late May through September.If allowed to survive, it will reduce tree vigor and ruin an entire fruit crop. During the summer, infected, fallen leaves decompose and produce additional fungus for one to four months that will infect leaves that will drop during the following spring. The cheapest, easiest, most effective control is to remove and destroy fallen leaves near the infected tree. Make sure they’re destroyed before summer rains occur. Oil and copper sprays can also control greasy spot.

Look for pinpoint black specks on the rind, especially on grapefruit, lemons and tangelos. Greasy spot can be distinguished from melanose by using a magnifying glass to see where the black spots occur. Greasy oil lesions don’t cover the oil glands on the fruit rinds while melanose lesions do.

Foot Rot

Foot rot is caused by two soil borne organisms. Symptoms include rotted roots; cracked bark and oozing of varying amounts of gum; water-soaked, reddish-brown to black bark at the soil line; discolored tissue in the lower trunk; yellowing, sparse foliage and death of the tree. The fungus that causes this disease belongs to a group of organisms called “water molds,” that do well under high soil moisture, but not flooded conditions. If planting is done in soil that has naturally poor drainage, especially in the flatwood areas of South Florida, the University of Florida recommends planting trees higher than usual by creating a mound or raised bed that will allow good drainage, even under flooded conditions.

Any actions to encourage air circulation at the base of the tree will help. However, if the tree has already developed foot rot, scrape off the brown, discolored bark and surface wood until healthy wood is exposed and paint the exposed area with a copper paint. Systemic fungicides are also available.

The trained experts at Insect Protection Pest Control are available to advise you on the many diseases and pests affecting your citrus trees. Call now for a FREE consultation.

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