Citrus Issues in the Home Landscape
Article by: Jessica Warren, ANR Agent – Camden County
One of the joys of living in Coastal Georgia is being able to grow plants than can’t survive the temperatures in most of the state. Some species of citrus are among these plants and can add a delicious addition to the landscape if cared for properly. When planting citrus this far north, you’ll want to plant cold-tolerant varieties such as satsumas for success. Though other species may survive for several years, one hard winter may take them out. Another thing to consider is that citrus trees are propagated by grafting a hardy rootstock and a desirable topstock. This means that citrus will not grow “true” from seed. It also means that if the tree is damaged below the graft on the trunk, it may grow back as the rootstock variety which is usually not palatable. Today we’ll cover a handful of the most common citrus issues that I receive questions about or see in people’s landscapes.
Dead wood and twig dieback often cause concern but are usually completely normal. This is a tree’s method for maintaining the proper root to shoot ratio for health. Severe or excessive dieback can be caused by other issues, but minor amounts (think 30% or less of the total canopy) of dead wood and dieback shouldn’t be alarming. Another normal occurrence that causes concern is fruit drop. Some fruit drop is normal and healthy for the tree. Citrus trees set more fruit than they can carry to maturity. They may only retain 5-10% of the initial fruit set to maturity. Fruit production takes a lot of energy and resources from other parts and processes in the tree, and it can cause stress to the plant to carry too many fruit to maturity. The main fruit drop is usually seen in April with a second drop in May or early June. Extended warm and wet conditions can cause or exacerbate fruit drop, as can poorly drained soils and low potassium levels.
There are several environmental stressors that can cause symptoms on citrus. Sun burn is usually an issue on younger trees because the canopy is not large enough to protect the fruit. Sun burn symptoms are yellowing on one side of the fruit and a dry spot in the flesh of the fruit. Fruit splitting is caused by high humidity and rainfall (or irrigation). Potassium deficiencies can exacerbate fruit splitting, and it is more likely to occur on trees with heavy fruit loads. Premature fruit drying is common on young trees and will resolve with maturity. Fruit drying can also occur on over mature fruit or from extended warm or dry fall weather. Cold damage can also cause visible damage, dieback and death. It’s important to plant cold tolerant varieties and protect trees when temperatures drop. It’s especially important to protect the graft union.
A number of insect pests can affect citrus, but most cause more aesthetic damage than actual harm to the tree. Spider mites are tiny, difficult to see insects that cause stippling on leaves. They prefer dry weather and low humidity and are less active June-September. Spider mites feed mostly on new leaves and are usually not a significant concern in the home landscape. Citrus rust mite is another mite pest you might find on citrus. Citrus rust mites cause rusty or bronzed looking fruit that are not aesthetically pleasing but are still fine to eat. Horticultural oil is effective in helping to manage the mite population but won’t change the visible damage that has already been done. Leaf miners are a pest that is found on most citrus trees. This pest causes squiggly looking lines on the surface of the leaf and can cause leaves to curl. Leaf miners are difficult to treat since they are protected in tunnels they burrow into new growth leaves. Luckily treatment is not usually warranted in home landscape citrus unless more than 30% of the canopy is affected. My favorite citrus “pest” is the giant swallowtail caterpillar. These caterpillars are the larvae of the giant swallowtail butterfly and look like bird poop (a clever disguise to deter predators). These are pests that I would encourage you to let be, especially on larger trees where they’re unlikely to cause significant damage. You can’t have butterflies without feeding caterpillars. If treatment is truly necessary (which should only be true on young, small trees), the best option is to pick the caterpillars off and move them away or squish them. Bt, a naturally occurring bacteria that kills caterpillars, can be used but is most effective on very small caterpillars. The last three insects that we’ll cover are all sucking insects that produce a sticky, sweet waste product called honeydew. Each may also cause a secondary infection of black sooty mold which causes no harm to the tree but looks unpleasant and alerts you to the insect issue. All three can be treated with insecticidal soap or horticultural oil (following all label directions – the label is the law). Citrus whitefly is a small white winged insect usually seen on the underside of leaves. Citrus scale may be found on fruit, stems and leaves, and is often noticed in its adult form which looks like a small, hard, differently colored raised circle or bump. Aphids are small, soft-bodied insects that usually appear with the spring flush and attack new growth. Heavy aphid infestations can cause stunting, distorted growth, leaf cupping curling or twisting. Ladybugs are natural predators of aphids and will often keep aphid populations in check if broad spectrum insecticides that are lethal to ladybugs aren’t used in the landscape. The easiest way to deal with aphids is to gently grasp the plant and pull your fingers up and off the plant squishing the soft aphids. I would recommend wearing gardening gloves as their juices can stain your skin – unless you see the stain as a badge of honor gained from your war on aphids.
There are several diseases that can affect citrus, and more are most likely on the way from Florida. Citrus scab is a fungal disease that causes corky outgrowths on leaves, fruits and shoots. Citrus scab reproduces by spores which are spread by water droplets. Fallen leaf debris from diseased plants can act as inoculum and should be removed until the disease is controlled. Copper based fungicides can be used as directed on the label. Greasy spot is another fungal disease that can be treated with copper fungicides. As with citrus scab, airborne spores multiply on decomposing leaf litter and reinfect the plant. Greasy spot first appears as yellow spots on the upper leaf surface, and later causes slightly raised brown blisters on the underside of the leaf. This disease can cause defoliation. Phytophthora is common in landscapes in Camden on citrus as well as other landscape plants. Phytophthora is a soil borne pathogen that takes hold in overly wet, poorly drained soil (which we have a lot of) and causes root rot. Symptoms include yellowed leaves, leaf drop, fruit drop, dieback and death. There is no treatment for phytophthora. Exercise caution not to plant in wet or poorly drained soils and be sure to use proper planting procedures. Citrus greening is the most concerning and least treatable of citrus issues. It has decimated the citrus industry in Florida. We confirmed citrus greening in Camden County in 2016 but based on our findings, it’s likely it was here for at least a decade before we confirmed it. I see it throughout the county on the majority of citrus in home landscapes. Citrus greening is a lethal bacterial disease that is spread by a small, winged, sucking insect called the Asian citrus psyllid. There is no cure or treatment for this disease, and trees usually die within five years of infection. Symptoms of infection are mottled yellowing on the leaves, corky leak veins, bitter or salty tasting fruit and general decline. The disease cannot be confirmed without a laboratory analysis, but the visual symptoms can be pretty definitive to a practiced eye.
Nutritional deficiencies are a common citrus issue – especially during fruit production. Nutritional deficiencies cause symmetrical yellowing on the leaf, often with the leaf veins remaining green. This differs from citrus greening where yellowing is asymmetrical and blotchy. A soil test (available through our office) is recommended to determine which nutrient(s) are needed and at what rates. In lieu of a soil test, a fertilizer formulated specifically for citrus should be used. As always, if you need assistance identifying and diagnosing issues in your landscape please reach out to our office. Most troubleshooting and diagnostics can be done quickly through email if clear pictures of the issue are sent.