By Megan Dewdney
Spring is the time to start thinking about foliar disease management in Florida. Depending on the type of citrus being produced and the market destination, different diseases will be important.
POSTBLOOM FRUIT DROP
The disease that affects all citrus, particularly oranges, is postbloom fruit drop (PFD). This disease is highly dependent on the weather during bloom. Currently, we are in a neutral El Niño Southern Oscillation phase with warmer than average temperatures and below average rainfall, which reduces the PFD risk. However, if we have warm temperatures and a series of wetting events during the main bloom event, the disease could be problematic.
Only the strobilurin [Fungicide Resistance Action Committee (FRAC) 11] fungicides have been consistently effective in University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) trials. But these fungicides are prone to resistance development, so they should be used sparingly.
GREASY SPOT
Greasy spot, also known as rind blotch, can be problematic under individual protective covers, in citrus under protective screen (CUPS) and in conventional grapefruit. On oranges, foliar disease can be managed with a well-timed application of 435 oil in the late May to early June period as the flush become close to fully expanded. Rind blotch on grapefruit responds better to other fungicides such as copper (FRAC M 01), strobilurins or demethylation inhibiting (DMIs) fungicides (FRAC 3).
Heat in CUPS can make using oil or copper very challenging because of phytotoxicity, so it is important to time applications for cooler weather or alternate chemistries. Keep in mind that the fungus causing greasy spot can develop resistance quickly, so fungicide rotation is important. If working in a closed system, leaf litter management in the early spring is a good idea to eliminate some of the inoculum.
BLACK SPOT
Citrus black spot still plagues groves in the southwest region of Florida as well as Polk and Manatee counties. There are quarantines in blocks with the disease.
Black spot is difficult to scout for when ripe fruit are not present. The disease is generally asymptomatic on leaves. Weather conditions appear to have favored the disease last season, so new blocks may show symptoms this year and lead to additional fruit drop.
Black spot management usually starts in the beginning of May with fungicide applications to substantially suppress the disease. Applications should continue to September or October, depending on the amount of rainfall. If blemishes are not a concern, early-season applications should be prioritized. Copper will suppress the disease but works better combined with oil. Good results have also been observed in trials with DMIs and strobilurins.
CITRUS CANKER
A canker management program with copper will help suppress all of the above diseases except PFD, but timing will matter. Canker is still causing substantial fruit drop in badly affected blocks in Florida and should not be ignored. Injected antibiotic treatments have not shown canker suppression and should not be relied on for this disease.
MORE INFORMATION
Consult the Florida Citrus Production Guide for specific information on these diseases and details on pesticide applications and timing.
Megan Dewdney is an associate professor at the UF/IFAS Citrus Research and Education Center in Lake Alfred.
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