By Theresa Badurek, UF/IFAS Extension Agent, Pinellas County Extension
Greasy spot is not that new burger joint in town- we’re talking about a citrus problem called “Greasy Spot”. This unsightly disease is caused by a fungus called Mycosphaerella citri. While the symptoms for this disease are usually evident during the fall and winter, now is the time to treat a diseased tree. So, when you chomp down on the greasy burger at the 4th of July picnic, remember “greasy spot” and you will remember to care for your infected citrus.
How to tell if your citrus has greasy spot:
Symptoms often start as yellow mottling on the underside of the leaves, as seen in this picture:
Later this tissue swells and eventually turns brown and the symptoms become visible on the top side of the leaf as well. The next photo shows the symptoms on both leaf surfaces:
Infected leaves will often fall from the tree before these lesions (brown to black greasy-looking spots) are visible. The appearance of these lesions can vary according to the type of citrus, both in the timing and appearance of the lesions. Some citrus varieties will show symptoms as early as November or as late as January. Grapefruit is more susceptible than other types of citrus.
Greasy spot infection can also show symptoms on the fruit itself. This looks like tiny black spots on the surface of the rind. Larger specks may even coalesce (grow together) on grapefruit and look like pink pitting on the fruit surface. This is called greasy spot rind blotch (photo below).
The leaf drop caused by this disease can reduce the yield of the tree the following year, and cold damage has been noted to be more severe on defoliated trees.
How does greasy spot spread?
Greasy spot follows a seasonal pattern. First a few leaves drop in the summer and fall, then more in the winter, and finally by spring there is an accumulation of fallen leaves under the tree. By May and June the more frequent rainfall will encourage spore development. Remember that this is a fungus and spreads through the production of spores. Unfortunately this is also a great time for the infection to spread as the infected leaf litter below the tree releases spores. These spores can be transported several hundred yards on a breeze. There is an optimal window of temperature and humidity that encourages germination of greasy spot spores; between 77-86 degrees (F) and near 100% humidity. In Florida that means almost every night throughout the summer is just right for the growth of this fungus.
What to do if your citrus has greasy spot:
If you have a citrus tree with this fungus, the first thing you need to do is to practice good sanitation. This means removing all of the dead leaves from around the tree. This will reduce the spore development that will spread the infection. Then, a single application of copper fungicide between mid-May and mid-July is usually all it takes in our area to control greasy spot. A great way to remember this each year is to think about greasy spot when you bite into that juicy, greasy burger at your 4th of July picnic. If you haven’t treated by then, you will remember to do so by the mid-July window of opportunity. A treatment of horticultural oil can also help control greasy spot by preventing the spores from penetrating the leaf surface. If you treat with a copper fungicide, wait at least ten days before treating with a horticultural oil. Always be sure to read and follow all label instructions!
For more information on greasy spot: http://edis.ifas.ufl.edu/pdffiles/CH/CH01500.pdf