By mid-April citrus trees are growing profusely and the fruit set on the trees is getting larger by the day. It will not be long before the fruit of the yield will be ready to harvest!
There are two problems associated with late spring on citrus trees. The first disease has been seen on several mature citrus trees over the last few weeks. Foot rot is the most frequently encountered disease on the trunks of citrus trees in Louisiana. A soil-borne fungus, Phytophora parasitica, causes the disease. Members of these genera of fungi also cause brown rot on the fruit later in the year, usually in October.
Foot rot produces motile spores that usually invade the trunk at the bud union. Wet conditions during the spring or summer of the year favor foot rot development. Initial symptoms include water soaking of the bark that appears as a dark spot on the trunk. At first the bark appears firm, but with age it becomes cracked and may shed as it dries. Gumming often accompanies advanced stages of foot rot.
This disease can be managed by providing adequate drainage when planting citrus trees. There are three commercial fungicides labeled for controlling this disease, Ridomil Gold, Ridomil 5G and Aliette 80 WP. They are expensive and difficult to find but work effectively. It is important to minimize the use of mulch around the base of your citrus trees. Excessive mulch can increase moisture levels, especially next to the truck of the trees. This particular pathogen is a water mold that really likes free moisture, so do not mulch citrus trees, especially next to the base of the tree.
The second disease is the sooty mold fungus (Capnodium citri), which is not a parasitic organism. It does not penetrate the tissue of the plant but grows superficially on the honeydew excretions of white flies, aphids, mealy bugs and scale insects. Sooty mold can cause damage by preventing the sunlight from reaching the leaf and by making the fruit black and unattractive. Fruit covered by sooty mold is smaller and does not color well.
Sooty mold is controlled indirectly by controlling insects (white flies and scale), which excrete the honeydew on which the sooty mold fungus grows. When these insects are killed, the sooty mold disappears after a few weeks. You can speed up this process by mixing one tablespoon of a mild liquid detergent with one gallon of water and spraying this mixture on the affected areas. Wait 30 minutes and wash off with a pressure hose and repeat until the sooty mold breaks up and washes off. The use of ultra fine oil as per label directions will also break up sooty mold. Do not apply oils to your trees after mid August as this can impact the natural maturing of your fruit. Always read, understand, and follow all label directions.
Citrus Disease Guide:
http://www.lsuagcenter.com/NR/rdonlyres/1621D6A9-45FA-4D50-99F8-15499E0A494A/42817/citrus1.PDF
For a copy of the Louisiana Home Citrus Production guide, call the LSU AgCenter Extension office at 985-446-1316 in Thibodaux or you can also find it plus the citrus insect and disease spray schedules at http://www.lsuagcenter.com/.