I get many questions about problems with citrus trees in late summer. Citrus trees are relatively carefree as backyard fruit trees go, but a variety of problems are fairly common.
PROBLEMS WITH FRUIT
If a citrus tree was planted within the past five years, it may not yet produce fruit or produce fruit erratically. It takes about five years, and sometimes longer, for a tree to begin to reliably set and ripen fruit.
Late summer fruit drop also is common as a tree adjusts its crop to ripeness. Stressful weather conditions, such as drought, can make it worse. The loss of some fruit means the tree will put more resources into what is left, and this will result in higher quality fruit.
As the fruits grow larger in late summer, periods of dry weather followed by rain can lead to fruit split. This occurs when the fruit swells faster than the skin. Split fruit should be discarded.
Puffy fruit can be a problem on young, vigorous-growing satsuma trees. Puffy fruit can be unusually large, have thick rinds and dry flesh. As the tree becomes older, this decreases.
Birds will peck at the fruit of citrus. The damage looks like an ice pick was driven into the rind of the fruit. Bird netting available at local nurseries thrown over the tree can help prevent damage.
Fruit with a dark brown discoloration on the skin have been attacked by citrus rust mites. They generally just damage the skin and the fruit will ripen normally. The pulp and juice are still good to eat and drink. To stop damage, spray the tree with a light horticultural oil (Year Round Spray Oil, All Seasons Oil) once a month in June, July and August.
There are a variety of fungal diseases that affect citrus fruit, such as citrus scab, sweet orange scab and melanose. By the time you notice the symptoms, however, it is too late to do anything for these diseases. However, the next year fungicide applications with copper based fungicides in the spring and early summer may help minimize symptoms.
PROBLEMS WITH FOLIAGE
Citrus leafminer is a common insect problem that first showed up in May of 1994 in Plaquemines Parish. They typically damage the late summer flushes of new growth. After the larvae hatch, they enter the leaf and feed on the inside creating a silver sheen, serpentine trails and twisted, deformed leaves. The larvae eventually emerge from the leaf and make a pupal chamber by folding the edge of the leaf down and securing it with silk.
Using insecticides is difficult in controlling the citrus leafminer, because the damage is done by the time most people notice it. Fortunately, in most situations the damage looks a lot worse than it is. Trees generally recover very well and the overall health of the tree is not greatly affected. Little or no reduction in harvest occurs when the late summer flush is damaged. So, despite the appearance, most home gardeners do not treat for this pest.
Spray the organic insecticide spinosad as soon as you see the new growth in late summer and before you see damage to reduce it.
There are several sucking insects that attack citrus trees. A common one is the citrus whitefly. These gnat-sized, white insects suck the sap from the tree. In their larval stage, they appear as flat disks on the underside of the leaves. Blackfly is similar but the adult and larva are black. As they feed on the sap the whiteflies excrete a sugary substance called honeydew. Another sucking insect, mealybugs, looks like cottony masses on leaves and fruit. Mealybugs also produce honeydew that leads to sooty mold. The wooly whitefly insect appears as fuzzy white patches on the leaves.
Deposits of honeydew on the leaves and fruit provide food for the growth of fungi that produce a condition called sooty mold. Sooty mold is a black deposit that can be rubbed from the leaves or fruit. It does not actually attack or significantly damage the tree. The sooty mold will disappear once the insects are eliminated.
Spraying trees with a light horticultural oil (such as Year Round Oil or All Seasons Oil) is effective in controlling whitefly, wooly whitefly and mealybug infestations. Be sure to spray under the leaves and make several applications following label directions.
Light horticultural oils can be used through the heat of summer, but should be applied in early morning when it is cooler. Do not apply oils any time after the end of September to the end of February as they can reduce cold tolerance of citrus trees.
The effectiveness of the oil can be enhanced by adding the insecticide Malathion. Add the appropriate amount of the oil and Malathion (following recommended amounts on each label) to the water in the sprayer tank. Use Malathion alone during the period when horticultural oils should not be used.
Finally, there is a caterpillar that feeds on citrus trees. The orange dog caterpillar is the immature form of the giant swallowtail butterfly, one of the most beautiful of our native butterflies. The caterpillar, mottled brown and white and larger at one end, is disguised to look like bird droppings.
Unless your tree is young and only has a relatively small number of leaves, the damage done by this caterpillar is generally minor, and it's worth some damage to have more of the butterflies that delight so many people. If the damage is unacceptable, pick off the caterpillars and destroy them or spray your tree with BT (Dipel, Thuricide) or spinosad. Better yet, give them to friends with larger citrus trees. They would be especially welcomed by friends that butterfly garden.
For more information on how to take care of your citrus trees, see the LSU AgCenter's "Louisiana Home Citrus Production" bulletin online.
Dan Gill is a horticulturist with the LSU AgCenter. Email questions firstname.lastname@example.org or add them to the comment section below. Follow his stories at www.nola.com/homegarden, on Facebook and @nolahomegarden on Instagram.