By Megan Dewdney
The recent freeze events of the last few weeks have scrambled the plans of citrus growers. It will take several weeks to months to understand the extent of the damage caused by the unusually cold weather.
MELANOSE
There is at least one disease that is highly likely to be worse than in other years. Melanose is likely to be damaging even to oranges and other cultivars that do not usually see harmful levels. After a freeze, young fruit can split from mudcake melanose. When the rind is badly damaged by melanose, it can no longer expand as the fruit grows.
The fungus responsible for the disease, Diaporthe citri, produces spores on dead twigs under 1/8 of an inch. These twigs can be colonized by the fungus two ways: 1) as young alive twigs or 2) after the twigs have died. The small twigs are vulnerable to damage from extreme cold. Once it is warmer, these twigs can be colonized by fungus and produce abundant spores.
While the current forecast is for lower than average rainfall in Florida during the spring, this is not necessarily relief from melanose. The fungus can viably store the spores for a month in a paste that looks like it has been squeezed from a toothpaste tube. When a rain event occurs, large numbers of spores are splashed onto vulnerable leaves and fruit, where they cause large areas of symptoms.
Copper is still the most economical product for managing melanose. The first application is usually in mid-to-late April, with the timing depending on predicted rainfall. Copper should be in place before rain events, or it will be ineffective. Applications should be every 21 days with a rate of 0.7 pounds per acre of metallic copper. Most oranges and tangerines should be adequately protected with one or two applications. Grapefruit should be protected until the fruit are 3 inches in diameter, usually by late June or early July.
GREASY SPOT
Leaves with greasy spot symptoms are more susceptible to cold damage. They will fall, contributing to more leaf litter than usual. It is highly likely that the greasy spot pressure will be greater than usual. On most oranges and tangerines, oil applications in June and July will keep the leaves protected. If you are applying copper for melanose at that time, it will also cover greasy spot.
CITRUS CANKER
Canker is likely to create headaches with all the vulnerable new growth that will push this season, particularly if rain occurs when the tissue is vulnerable. The copper for melanose will also keep this suppressed somewhat, but keep in mind that leaves grow out of their protective coating quickly and could be vulnerable to infection within a few days. What becomes lesions on leaves will move to the fruit later in the season.
Megan Dewdney is an associate professor at the University of Florida Institute of Food and Agricultural Sciences Citrus Research and Education Center in Lake Alfred.
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