Problems with Cold

Citrus trees can be damaged by subfreezing temperatures. Generally, temperatures that drop briefly into the mid to upper 20s are not a problem. Temperatures in the mid to low 20s or below that persist for many hours, however, can damage the fruit and trees. Harvest all fruit anytime temperatures below the mid-20s are predicted for 8 or more hours. Protect young, small trees, if possible, by covering them. From most hardy to least hardy citrus can be ranked as follows: kumquat, satsuma, orange, grapefruit, lemon, lime.

Harvesting

Citrus fruit take a long time to ripen. From the spring blooming, satsumas and kumquats ripen first beginning in October. Oranges generally begin to ripen in December, and some do not fully ripen until January. Grapefruit ripen in December. Lemons ripen in mid October, and limes about the same time. Valencia and blood oranges do not ripen until January. Citrus that are harvested before fully ripe will not be as sweet. The fruit does not ripen off the tree.

Citrus fruit may be left on the tree once ripe and harvested as needed (barring freezes). However, quality does eventually begin to suffer. Harvest all ripe fruit by the end of January – early February at the latest. to ensure good blooming and fruit set in the spring.

Problems with Fruit

If your citrus tree was planted within the last three to five years, don’t be dismayed if it’s not yet producing fruit or produces fruit erratically. It often takes three years, and sometimes longer, for the tree to become established and begin to reliably set and ripen fruit. Time will solve this problem, along with your good care.

Young trees in pots at the nursery typically have fruit on them when you purchase them. But, this is because the roots are confined to a small space in the pot. When the tree is planted into the ground and the roots have room to grow, the tree reverts to its true juvenile nature and stops reliably producing fruit. This is good, actually.

A young tree should be putting its efforts into growing strong roots, stems and leaves, not fruit. This makes for a stronger, more productive tree in the long run. You should strip any fruit a young tree tries to produce the first year or two after planting for that very reason.

Older, well established trees may set more fruit than they can carry and ripen. Some fruit falls off early when quite small, but many trees will also experience a late summer fruit drop in August and September. This later fruit drop is generally minor and involves a relatively small percentage of the fruit. Stressful weather conditions, such as drought, can make it worse.

Sometimes older trees will skip a year of blooming and producing fruit. This generally occurs after a tree has produced a really large crop the year before.

As the fruits grow larger in late summer, periods of dry weather followed by rain can lead to fruit split. The rapid uptake of water by the tree causes the fruit to swell faster that the skin can stretch causing it to split. Split fruit should be removed from the tree and discarded. Your best defense is to water the tree deeply once a week during dry weather, but this will not absolutely prevent it.

Puffy and misshapen fruit are mostly a problem on young vigorous growing satsuma trees. As the tree becomes older, the occurrence of puffy fruit decreases. Puffy fruit on older trees are the result of fruit set on late blooms during periods of warm weather. Little can be done to prevent puffy fruit. Good growing conditions, proper fertility and pest control and time will help to reduce the amount of this condition.

Birds will peck at the fruit of citrus. The damage looks like an ice pick was driven repeatedly into the rind of the fruit. This may cause rough, brown areas on the rind. If the damage is not too severe, it is mostly cosmetic and the fruit will ripen normally. If you have a serious problem with this, and the tree is not too large, 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. Just to make sure, moisten your thumb and rub it forcefully across the fruit several times. If the dark area does not rub off, it is mites (if it does rub off, it’s sooty mold, see below). Citrus rust mites, too tiny to be seen with the naked eye, generally just damage the skin. Fruit will often ripen normally, perhaps smaller, with the pulp and juice still good to eat. To stop damage, spray the tree with a light horticultural oil (Year Round Spray Oil, All Seasons Oil) at the first sign of damage on the green fruit in summer. If you want to spray preventatively, spray once a month in June, July, August and September.

There are a variety of fungal diseases that affect citrus, such as citrus scab, sweet orange scab and melanose. These are generally not a major problem. 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

Spider mites can infest the leaves causing them to become pale, sickly and drop from the tree. A severely attacked tree can become completely defoliated. Malathion and light horticultural oils are useful for mite control. It is best to spray before severe damage occurs, but trees generally recover even if completely defoliated.

Citrus leafminer is an increasingly common problem that first showed up in May of 1994 in Plaquemines Parish. The adult citrus leafminer is a tiny moth that is believed to be native to India and Southeast Asia. The female moth lays her eggs on tender, new growth. This rarely occurs on the new growth that emerges in the spring, but is common on the late summer flush 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 pesticides is difficult in controlling the citrus leafminer, since they may not be particularly effective by the time most home gardeners realize there is a problem and decide to do something. Fortunately, in most situations the damage looks a lot worse than it is. Affected trees generally recover very well and the overall health of the tree is not greatly affected. Since flowering and fruit production occur on the spring flush of growth, little or no reduction in harvest occurs when the late summer flush is damaged. So, despite the terrible appearance, most home gardeners do not treat for this pest.

If you begin spraying with the organic insecticide spinosad as soon as you begin to see the new growth emerging in late summer, it will reduce the amount of damage. (Gardeners may obtain spinosad at local garden centers under different commercial names such as Conserve, Naturalyte Insect Control, Green Light Spinosad, Success, Ferti-lome Borer, Bagworm, Leafminer & Tent Caterpillar Spray, etc.).

Or, treat the tree with a drench of imidacloprid (Bayer Advanced Fruit, Citrus and Vegetable Insect Control) once in spring or early summer to prevent citrus leaf miner damage.

There are a variety of sucking insects that attack citrus trees. One of the most common pests of citrus trees 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. 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. Another insect, mealybugs, looks like cottony masses on leaves and fruit. Mealybugs also produce honeydew that leads to sooty mold. Finally, the wooly whitefly insect appears as patches of slightly fuzzy white patches on the leaves.

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. Blackflies are not so easily controlled, but repeated applications will help reduce their numbers. 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 not be used after 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 to the mix. 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.

The sooty mold will eventually disappear once the insects are eliminated. Light oils are used during summer when daytime highs are above the mid-80s.

Greasy spot disease causes yellow spots on the foliage in summer. It’s often worse when the weather has been rainy. Fungicides can be applied during the early summer for control. To prevent this disease, spray with a copper fungicide in early May and early June.

Heavy infections occurring in wet years may require 2 to 3 fungicide treatments. Since infection occurs on the lower leaf surface, thorough spray coverage is essential for effective control. Collect and dispose of any infected leaves that fall from the tree. These leaves if left on the ground can serve as a source of infection in the future.

Finally, there is a caterpillar that feeds on citrus trees. Called the orange dog caterpillar, it 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, Biological Worm Control). 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 here’s a link to our online “Louisiana Home Citrus Production” bulletin.

Louisiana Home Citrus Production

Description: The different types of leaves, abundance of blooms, aroma of flowers and color of mature fruit of different types of citrus add to the aesthetic value of the landscape. Citrus in the landscape also can provide excellent quality, nutritional fruit. So citrus is the ideal fruit for the homeowner. Recommended varieties and descriptions, as well as tips on site selection, planting, spacing and pruning, are included. (Rev Feb 2007)

Publication #:1234

Citrus Problems - Sprouting Rootstock

The citrus trees you purchase at the nursery have all been grafted. That is, a desirable, named citrus variety, such as Owari satsuma or Meyer lemon, is grafted onto a rootstock that is a completely different type of citrus. Trifoliata orange is often used as the rootstock.

The point where the graft was made (called the graft union) will generally appear as a swollen point or crook in the lower part of a trunk. When you purchase a young citrus tree, look for and find the graft union. Everything above the graft union is the desirable citrus tree – the satsuma, lemon, kumquat, orange or grapefruit – called the scion. Everything below the graft union is something else entirely – either trifoliata orange (Poncirus trifoliata Rubidoux) or Swingle citrumello – called the rootstock.

The purpose of the rootstock is to provide a strong, vigorous root system that will produce a robust growing, productive tree. The advantage of the trifoliata root stock is that is also imparts increased cold hardiness to the upper part of the tree

Once you have located the graft union on the trunk, you must never allow any shoots to sprout and grow from below the graft union. If you do, you will allow something that is not your desirable citrus variety to grow. When a citrus tree produces atypical fruit, it generally means the rootstock has been allowed to sprout and grow. The trifoliata rootstock produces poor quality, seedy, sour, round yellow fruit. The Swingle produces a large fruit with thick skin. The growth from the rootstock often has different shaped leaves from your citrus and is thornier (although, many citrus produce thorns).

All you can do to correct this is to saw off all of the shoots growing from below the graft union back to the trunk. Allowed to go on for too long, the growth from the rootstock can take over the tree and crowd out the desirable citrus until there is none left. Don’t let this happen. (See diagram above)

Dan Gill
LSU AgCenter
Consumer Horticulturist