A couple of weeks ago now I got a phone call from the Master Gardener office asking me about a citrus problem. It is very hard to try and diagnose an issue over the phone. It was described to me as being brown all over the skin. I immediately thought of some kind of rot, but the peel was not soft or mushy. I was going to try and run by the office the next day and see if I could figure it out. In the meantime I got Maggie, the Master Gardener on duty at the time, to look up a few things online that it could be and call me if she found anything that looked like it. Well, that advice saved me a trip downtown. She figured out that it was indeed Citrus Rust Mites. Since this happened, I have actually had a couple of other people mention this same issue to me so once is a coincidence, twice or more is a problem.
Phyllocoptruta oleivora or Citrus Rust Mites are long, wedge-shaped and light yellow, measuring about 0.1 to 0.2 mm long, generally they are not visible to the naked eye.
The rust mite feeds on the outside exposed surface of the fruit. Feeding destroys the rind cells and the surface of the fruit becomes silvery on lemons, rust brown on mature oranges and grapefruits.
It looks like this:
Visible characteristics of injury differ according to variety and fruit maturity. While the primary effect of fruit damage caused by Rust Mites mainly is cosmetic, which causes there to be a reduction in grade of the fruit in the fresh market, there has been other conditions which have been associated with severe fruit injury, including reduced size, increased water loss, and increased drop.
Leaf injury caused by feeding of these mites can exhibit many symptoms on the upper or lower leaf surfaces. When injury is severe, the upper surface can lose its glossy character, taking on a dull, bronze-like color. Lower leaf surfaces often show yellow degreened patches. Complete defoliation is rarely an issue. Very high populations can reduce tree vigor.
I mentioned that these things are very tiny. The mite has an elongated, wedge-shaped body about three times longer than wide and under magnification they look like this:
Rust mites tend to seek out high humidity areas away from direct sunlight but also avoid areas where dew forms. Rust mites overwinter on foliage and in bark crevasses. On foliage, mites are most likely to be found on undersides of dry, inner canopy leaves. They reproduce very quickly also, a generation may be completed in 1 to 2 weeks in Summer, but development slows or stops in Winter, depending on temperature.
A variety of predators and diseases attack citrus rust mites. Several fungal diseases, including Hirsutella thompsonii, occur naturally and, during periods of moist weather, cause tremendous rust mite population crashes. Predators of these mites include thrips mites, coccinellid beetles, dusty wings and other insects.
If you happen to have a severe infestation, or they seem to return year after year, you can use any good miticide. Please make sure you read the label and it has listed both Citrus Rust Mite and that it can be sprayed on Citrus.
The good news is, if you do end up with fruit like the picture above, it is still very much edible. Other than the possibility of loss of tree vigor, this critter only really affects the fresh fruit market. Processed fruit, that is a whole other story. You never see the peel, so what is the difference?
Chances are you will never have a problem with the Citrus Rust Mite, and I hope you never do. I wanted to present the case, just in case you go out to your tree and find the fruit looks like it has a case of the creeping crud!