It could just be coincidence, but oak trees appear at least in some cases to protect trees from citrus greening disease. I’ve photographed this situation in several locations, so it’s tempting to conclude that it’s more than just a random occurrence. The photo above is a dramatic example of this effect. Basically, trees that are under or next to oak trees do not show anywhere near the same degree of citrus greening symptoms as do citrus trees further from the oak trees. More photos after the jump. We call this phenomenon, the Oak Tree Effect.

Citrus greening is one of several diseases that cause severe problems in citrus groves[foot]The other diseases include, citrus canker, black spot, sweet orange scab and citrus variegated chlorosis. More information about these diseases is on the Save Our Citrus website.[/foot]  In most cases, the oak-proximal trees [hi]show few or no psyllid injuries or greening symptoms[/hi]. (They also show fewer leafminer injuries.) This is just an important observation at this point and it could be due to any number of reasons. But, I think it’s important to document the effect.

There’s not enough information now to know how to practically use this information in a commercial setting, but it does provide some ideas for an approach to managing the disease. A better field test could be designed to see to what extent it’s reproducible, for example. Other questions to ask include:

  • Does the healthy tree have the same root pathogen profile as the greening-affected tree next to it?
  • Does the healthy tree harbor the same level of bacteria in it’s vascular tissues? If so, why does the healthy tree not show symptoms?
  • Are there differences in soil chemical and microflora profiles between the two trees?
  • Do different types of oak trees affect the outcome?

Oak extracts are easily obtained, so laboratory experiments could be performed to test insect tolerance of oak-tree volatiles. In any case, what we see is probably due to several combined factors, including microclimate, alternate beneficial insect habitats and more. This means the answer will probably not be an easy and straightforward thing to figure out.

In each photo below, the citrus tree next to the oak tree appears to be in better health than the citrus further away from the oaks. The canopies of the citrus next to the oaks are darker, denser and fuller. The other citrus trees appear weaker, and show more signs of psyllid and greening when you look at them closely. You’ll also see more fruit drop under the weaker citrus trees than those under those under the oaks.

I’m not suggesting this a cure or even a treatment. I wouldn’t be at all surprised if this ulimately leads to nowhere. But, it’s a phenomenon we want to examine more closely. We’re going to watch this over the next few months and years to see if the effect holds up over time. If it does, it might at the least provide a clue or other idea about steps we can take to mitigate the disease.

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