November 23, 2009

Common Citrus Problems

By Jane Morse, Commercial Horticulture Agent, Pinellas County Extension

Rust mites: A common occurrence this time of year is to see oranges that have brown, rough areas. This is most likely rust mite damage (unless it smells rotten). Rust mite damage is nothing to worry about as it doesn’t affect the taste of the fruit, it just makes the outside not look completely orange.

If you want perfectly orange rinds, spray your trees in March or April with a horticultural oil (use an oil that lists paraffinic oil as the active ingredient). Always read and follow directions exactly as indicated on the label of pesticides.

Leafminers: This is another common problem on citrus. Leaves will look misshapen and one can see serpentine tunnels on the leaf surface. Mature trees are not harmed by this slight damage to their leaves. Not much control is available. Either accept the damage and do nothing, or during the spring growth flush spray the tree with horticultural oil twice, spaced two weeks apart. Very young trees can be hurt by the loss of leaf surface that leafminers cause. Young trees should be treated with horticultural oil during the spring growth flush.

Citrus scab: Water is the single most important factor affecting the severity of this disease. This disease only needs to be controlled on fruit intended for the fresh fruit market. It is only a serious disease on certain varieties of citrus. It mainly affects lemon, Murcotts, Minneola and Temple varieties and if often a problem on grapefruit. Avoid overhead irrigation that spreads scab. Install drip or micro- sprinkler irrigation. Copper sprays also can be applied 2-3 weeks after petal fall, and again 2-3 weeks later to control scab.

Greasy spot: This can be seen on both the top or the underside of the leaf. It looks just like a spot (or many spots) of grease. The main impact of greasy spot is reduction of tree vigor. This is a disease that occurs in the summertime from infected leaves that have fallen on the ground. Once you have greasy spot, the cheapest, simplest, most effective control strategy is to remove and destroy the infected fallen citrus leaves near tree. Make sure you pick up and dispose of these infected leaves. Leaving them around the tree or using them as mulch will cause the disease to recur. Make sure the old infected leaves are destroyed before summer rains occur. Spray the tree with horticulture oil between June 15th and July 15th to control this disease. Copper sprays can also control greasy spot.

Foot rot: This is a very common disease. The first symptoms of foot rot are water soaking of the bark in irregular patches and oozing of varying amounts of gum. Over time, the diseased bark dries out, settles, cracks and weathers off, with the wood beneath the bark stained brown. The canopy may show nutrient deficiency symptoms, especially nitrogen, reduction in leaf and fruit size, leaf drop and dieback, and a general reduction in tree vigor.

The fungus that causes this disease belongs to a group of organisms called “water molds,” which do well under high soil moisture. Symptoms usually occur after heavy rains or excess watering which promote infection of plant tissue. This same fungus also causes brown rot to occur on fruit.

Although mulching can prevent weed growth and conserve water, piling mulch up against the tree trunk makes your tree prone to infection by limiting air circulation. It also provides a path for the fungus to get from the soil to the tree. Anything you can do to encourage air circulation at the base of the tree will help. Prune low hanging branches that scrape the ground (this will also help to control brown rot of fruit) and remove plant and soil debris from around the trunk. Since this fungus usually enters the tree through wound tissue, be very careful when hoeing, mowing or weed whacking around the tree. Any wounds made to the tree can allow infection to occur.

If your tree has already developed foot rot, scrape off the brown, discolored bark and surface wood until you reach healthy wood and paint the exposed area with a copper paint.

Want to know which tools to use for pest and disease management? Homeowners usually only need to apply horticultural oil, insecticidal soap or copper fungicide to control pests and disease.

There are many beneficial-bugs that help to keep pest-bug populations under control. By using horticultural oils and insecticidal soaps most of the beneficial-bugs are saved, while the pest bugs are killed. Beneficial-bugs do us a great service and help to keep most pest-bug populations under control. Using hard chemicals that kill all bugs, including the beneficial-bugs, can actually cause an explosion of pest-bugs to occur. So stay away from the hard, conventional pesticides, especially those that keep killing for a long time. You do NOT need them and they usually make the problem worse.

For other questions about plants call or visit the UF/IFAS Pinellas County Extension Service. Our Lawn and Garden help line is available Monday, Tuesday and Thursday from 9 AM to noon and 1 to 4 PM at 727-582-2110. Visit our website at:, or come to our office located at 12520 Ulmerton Road, Largo. We are open M-F from 8 AM to 5 PM.

Information for this article was obtained from: Your Florida Dooryard Citrus Guide: Common Pests, Diseases and Disorders of Dooryard Citrus.

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