By Megan Dewdney
Greasy spot and melanose are two fungal diseases that have long needed the attention of Florida citrus growers. While they are more of a concern for fresh fruit growers, trees for processing can be damaged, too.
In terms of management priorities, greasy spot is the greater concern since it reduces tree photosynthetic capacity and causes defoliation. Melanose mars fruit, potentially leading to cracking and smaller fruit, if severe.
Considering how long fruit and leaves are susceptible to Zasmidium citri-griseum, the fungus responsible for greasy spot, is relatively simple. This is because of an unusual feature of the fungal life cycle. Unlike many fungal pathogens, Z. citri-griseum is exposed on the surface of the leaves and fruit for several months prior to tissue infection through the stomatal chambers. This vulnerable stage is targeted with spray applications.
For Valencias destined for the processing market, a single application of either spray oil at 5 to 10 gallons per acre or a combination copper and spray oil (2 pounds per acre and 5 gallons per acre) applied over the two-week period from mid-May to June should give sufficient control to avoid defoliation. Other products like strobilurin fungicides work as well, but the copper will be beneficial for melanose and canker as well.
Early and mid-season oranges are more susceptible and may require two applications. There is more disease pressure in the southern part of the state, but these cultivars can also be defoliated. The first application should be the same time as above, and the second spray should be applied when the major summer flush has expanded, usually in late June or early July. Traditionally, copper at the two described times has been sufficient for fresh fruit as well, with possibly a third in August if disease was severe the previous year. There are reports that this may no longer be the case, so researchers are looking into possible causes.
Melanose is primarily a concern for fresh fruit because it can scar the fruit peel. The inoculum is mostly formed on the recently dead twigs in the canopy that are approximately 1/8 of an inch. These twigs are not only the ones colonized the previous year, but newly dead ones as well.
While the fungus moves with rain, the spores are produced and remain viable on the twigs for up to a month in dry periods. The first rain after a dry period can be a severe infection event. Grapefruit is susceptible until it reaches a 2.5- to 3-inch diameter. This has historically been in late June or early July, but with more erratic flowering periods, this may not remain the case.
The first application for melanose is usually in mid-to-late April or when fruit are ¼- to ½-inch. Copper, the most economical product, should be applied every 21 days at 2 pounds per acre until the fruit are no longer susceptible. Check the Florida Citrus Production Guide for alternate rate and timing recommendations. For most oranges and tangerines, one or two copper applications should be sufficient.
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.