Florida citrus growers are scrambling to create new plans for their citrus groves after the recent freeze event in late January, and UF/IFAS researchers want growers to know there are several diseases that should be added to those new plans. Citrus growers need to up their scouting activities for the diseases below and plan on employing more robust management programs this spring and beyond.
Citrus Diseases to Look for Postfreeze
Melanose
Mudcake melanose is caused by the fungus, Diaporthe citri, which produces spores on dead twigs under 1/8 of an inch. The freeze likely affected most vulnerable small twigs of this size, causing twig death, so there will be more fodder for melanose to utilize. Melanose causes a scale-like growth on the surface of citrus rinds which looks like dried, caked mud. This growth keeps the rind from being able to expand as the fruit grows, and it causes damage when the rind eventually splits.
While scouting, look for a paste that looks like toothpaste, as this is how the fungus stores spores for wetter weather; rain will allow the spores to be released and spread among leaves and fruit. Mealnose is not as common on oranges and other cultivars at harmful levels, but UF/IFAS researchers believe it will likely be widespread due to the twig death caused by the freeze.
Treatment: UF/IFAS recommends copper treatments to target melanose, with an initial application in mid-to-late April prior to rainfall. Then, it should be applied every 21 days at a rate of 0.7 pounds per acre of metallic copper.
Greasy Spot
UF/IFAS citrus experts believe that greasy spot pressure will be greater than usual as affected leaves are more susceptible to cold damage; they predict leaves affected by greasy spot will fall in greater abundance this year, adding to leaf litter. The greasy spot fungus causes yellow-brown blistering lesions on leaves that can then infect fruit, causing blemishes in the rind.
Treatment: UF/IFAS experts recommend oil applications in June and July to protect citrus leaves from greasy spot, but they maintain that copper applications targeting melanose will cover greasy spot as well.
Citrus Canker
The freeze likely caused dieback in all young leaves, and the new growth that takes their places will be vulnerable to citrus canker, especially during rain. Lesions start on leaves and move to fruit later in the season.
Treatment: Again, the same copper that is used to treat melanose will also suppress citrus canker, but you will need to pay attention to timing. Once citrus leaves grow out of their protective coating, they will need protection from citrus canker as the infection can occur within just a few days.