Diagnosing and controlling citrus diseases


As citrus season approaches, gardeners and farmers are checking their trees and finding symptoms they want explained.

Curling leaves are commonly seen on citrus trees. Curled or distorted leaves are usually the result of damage caused by insects such as leaf miners, aphids or mites feeding on the leaves. On younger trees a soap and oil solution applied directly to the insects can kill them and reduce future damage. High heat may also cause leaves to curl.

Cracking and splitting bark is another symptom that is often reported. This can be caused by the natural growth process or rapid changes in temperature during various parts of the growing season. If this sloughing is not accompanied by other symptoms such as gumming, oozing, discoloration or dieback, it will likely not affect your tree’s health.

Sun scald can also cause cracks or splits in the bark. The sun hitting a tree’s bark in the same place can cause cracks or splits over time. Wrap the tree with a trunk guard where the sun hits, if this is the case.

If cracked and split bark is accompanied by additional symptoms, you may have a disease called Phytophthora gummosis which is characterized by cracked, dry bark with reddish brown gum exuding from the cracks as a defense against the fungal disease, usually on the lower parts of the tree and sometimes near the ground. It is encouraged by wet weather and the fungus will often die in drier conditions. The best control for this disease is prevention. Avoid letting water hit the trunk of the tree where possible. Keeping the trunk dry is essential to preventing this disease. A practical control measure is to cut out the discolored bark near the gumming and about an inch into the healthy bark and paint the area with a registered copper fungicide.

Sometimes citrus trees will bloom but no fruit will form or flowers will be followed by new fruit drop. These symptoms can be caused by pollinator decline or stress factors such as vog, excessive or deficient nitrogen, sudden high temperatures, lack of water, too much water, heavy insect or mite infestations, hot dry winds or constant rain during the bloom period. If any of these conditions exist you may see reduced or no fruit production.

Several conditions and diseases can cause twig dieback and dead limbs in citrus trees.

Tristeza means sadness in Spanish and Portuguese. Citrus tristeza virus symptoms vary greatly depending on the growing conditions and the abundance of the aphid vectors. Typical tristeza symptoms often appear first as stem pitting followed by a quick or chronic decline of trees starting with leaf and eventually branch dieback that starts at the tip. Stem pitting appears as longitudinal grooves or depressions in the stem paralleling the grain of the wood. Infected trees will usually die though some diseased specimens can linger for many years. In other cases, affected trees die quickly after the leaves wilt and fall off. The brown citrus aphid is by far the most efficient vector of CTV. Controlling the aphid with soap and oil can help prevent the spread of this deadly disease.

Keeping your trees healthy is the best prevention against diseases or insect attacks. An adequate but not excessive water supply can be difficult to achieve. In a dry climate several drip emitters might be needed or a weekly soaking to allow water to penetrate several feet into the soil. Keeping the roots adequately but not overly watered and the trunk dry can pose a challenge. Mulching can help to conserve water, but avoid mulching too close to the tree trunk.

Proper fertilizer application can also help with plant health as well as flower and fruit production. Follow a fertility plan that includes fertilizers especially designed and suited to the needs of citrus trees for best results.

Diana Duff is an organic farmer, plant adviser and consultant.

Tropical gardening helpline

Bonnie asks: I have a “Kary” star fruit tree that has been in the ground for about eight years. It bloomed and fruited profusely a few years ago but has not fruited since. I’m wondering what I might do to get it to fruit again.

Answer: It sounds like you have a mature star fruit tree on your farm. Though some trees tend to have heavy and light fruiting years, several years with no fruit is unusual, especially since it fruited well one year.

Maintaining ideal growing conditions can make a big difference. Star fruit trees do best in an area protected from the wind in full sun with warm daytime temperatures between 70 and 80 degrees. Nighttime temperatures below 50 can slow the tree down.

Trees will thrive in soil that has organic matter as long as it drains well. They prefer not to have “wet feet” though they do well with regular watering except during rainy times.

You can fertilize your star fruit tree four times a year with fertilizer high in micronutrients. A soil pH around 6.5 will ensure that nutrients in the soil are available to your tree. If you suspect a soil problem, get a soil test to be sure that nutrients like phosphorous and potassium are adequate for good fruit production.

Kary was developed by the University of Hawaii Department of Horticulture in 1980 and has been a dependable variety of sweet fruit here in Hawaii. Most of the literature about this star fruit mentions that mature trees benefit from pruning. If your growing conditions match the star fruit’s preferences, you might try pruning your tree to encourage new growth and perhaps flowers and fruit.

Trees often flower in summer and produce fruit two months later, but some trees will have two or more fruit sets in a year. Prune now and watch for flowers in a few months. Be sure not to remove more than one-third of the leaves at once so the plant can continue to photosynthesize without stress.

Plant advice lines

Call UH-CES in Kainaliu at 322-4892 between 9 a.m. and noon Thursday.

Email questions to master gardeners at konamg@ctahr.hawaii.edu.Some questions will be chosen for inclusion in this column.