Citrus trees are considered some of the easier fruit trees to grow in our area, but sometimes weather conditions create challenges and citrus is not without its fair share of problems.
Unlike previous years that have been hot and dry, frequent rainfall and high humidity have created more disease problems than normal. Diseases that are more active in wet weather are scab, greasy spot, melanose and brown rot. Unfortunately, for most of these, it is too late to implement control measures, but it’s still a good idea to identify the problem so corrective actions can be taken next spring.
Scab ( Elsinoe fawcetti ) is one of the most common citrus diseases in Northeast Florida during wet springs. It affects leaves, twigs and fruit. The disease is more problematic on lemons, temples, murcotts, minneola tangerines and grapefruit. Symptoms first show up as pinpoint circular raised spots on either side of young leaves. After several days, the bumps get bigger and develop a cream to yellow orange color on the tips. Over time, the bumps develop a cone shape resembling a volcano. Affected fruit become scabby in appearance with abnormal growth on the rind.
IT’S MOSTLY COSMETIC
Scab is more of a cosmetic problem and will generally not damage fruit unless it becomes severe. To control, keep irrigation from wetting the leaves, especially during the first 2 to 3 weeks of spring growth. If the disease is limiting production, consider spraying with copper next spring when new growth is 2 to 3 inches long and repeat at petal fall. A third application may be applied three weeks later.
Another disease is greasy spot, Mycosphaerella citri. Leaf symptoms start as yellow mottled spots with a dark blister on the underside of the leaf. Over time, the spot becomes dark and is visible on both sides, plus is greasy in appearance. Severe infestations will cause leaf drop in the winter, which will affect production the following year.
On the fruit, infection starts as tiny pink spots on the peel between oil glands that later change to brown or black. Fruit will not develop a uniform color and some areas will remain green. The best control for the homeowner is to remove leaves that drop to reduce the inoculum. If sprays are needed, apply an oil spray in June and another in August, but make sure to follow label directions regarding temperature restrictions.
Melanose, Diaporthe citri can occur on all citrus but is more serious on grapefruit. Melanose fungus creates small, dark brown depressed dots on leaves with a yellow margin. As the leaf cuticle explodes, a sticky material is released that turns brown and the yellow halo disappears.
Symptoms on the fruit begin as small raised dots. Spots become so plentiful that they join together, affecting a large portion of the fruit. Rain (or irrigation) causes the spores to run down the fruit, creating a tear-streaked appearance. Fruit has a muddy appearance, which is often confused with damage from rust mites. A good way to distinguish between the two is that fruit infected with melanose will have a rough texture like fine sandpaper whereas fruit damaged by citrus rust mites is smooth.
Melanose is more common on older trees and the disease infects new fruit and growth from the fungus stored in dying twigs or infected fruit left on tree. To control, remove dead twigs before the spring growth flush, bag and get them away from plants. If infected grapefruit are still on the tree before new growth emerges next spring, harvest fruit to reduce the spread.
To prevent the disease from occurring on new fruit, spray with a copper fungicide two to three weeks after petal fall and repeat in two to three weeks.
Twelve weeks after petal fall, fruit is immune to infection. This year, we are seeing an increase in fruit drop due to brown rot, a disease caused by Phytophthora palmivora. The disease is most active from August through October when temperatures are warm and fruit are wet for more than 12 hours for several days in a row. Early maturing fruit is more susceptible. How disappointing to discover that fruit has developed leathery, olive-brown peels and become rancid just before harvest.
The disease splashes up from the soil and infects the low hanging fruit. The easiest fix for a homeowner is to remove fruit close to the ground and avoid wetting the fruit by irrigation.
AND DISTORTED LEAVES
Despite the increase in disease problems this year, the most common citrus question is “why are my leaves all distorted?” Although there are several possible causes for this, the most likely culprit is citrus leaf miners. These insects quickly disfigure new growth by tunneling through leaves and sometimes fruit. The adult leaf miner is a tiny, silvery-brown moth. The moth hides among the leaves during the day and is only active at night, laying eggs on the underside of new leaves. Eggs are very small, dome-shaped and almost clear in color.
As eggs hatch, the young larva bores into the leaf and creates narrow, white tunnels referred to as mines while feeding. Once the larva matures, it exits the leaf edge (margin) and rolls up the leaf for protection while pupating. The entire life cycle can take as little as two weeks.
Citrus leaf miners are not life-threatening to mature trees but could be problematic for young citrus. The best control is to spray with a horticultural oil spray as late in the day as possible, ensuring good coverage on the underside of leaves. Repeat in 10 to 14 days. These insects only affect the new growth, so once leaves fully expand and harden off, they should not be attacked.
Avoid using broad spectrum foliar insecticides to encourage populations of beneficial insects. Many of the problems discussed can be avoided by simply taking good care of the citrus trees. Irrigate when needed using drip or low-volume irrigation and fertilizer with a citrus fertilizer during the growing season from March to October. Check soil pH to make sure it’s in the correct range because this will affect the availability of some nutrients in the soil.
For more information on citrus problems, check out this great publication that will help identify many look-a-like diseases: edis.ifas.ufl.edu/pdffiles/PP/PP26100.pdf.
Terry Brite DelValle is a horticulture extension agent with the Duval County Extension Service and the University of Florida/IFAS.