Extension Connection

These Are Not the Pests You’re Looking For

By Jeff Wasielewski / Photography By Jeff Wasielewski | May 15, 2016
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Lichens

I’m often presented with what the grower believes are major problems, when, in fact, the problem in question is nothing to worry about.

LICHENS  Lately I’ve received questions about lichens. They are being blamed for everything from plant decline to poor fruit set. Lichens are a symbiotic combo of a fungus and an alga (or sometimes a cyanobacteria) and in almost all cases, are completely harmless. Lichens do sometimes like (see what I did there?) areas that are somewhat shaded or moist, and can be found on older, decaying wood. If your fruit tree is being shaded out, getting too much water, or contains dead wood, there is good chance lichens have found a home on your tree’s trunk.

Too much water and not enough sunlight can lead to poor fruit production, causing the grower to point his or her finger at the innocent lichen. “How can I kill this?” You do not ever need to kill or remove lichens. You may need to fix the true problems affecting your fruit trees (see “Why Won’t My Tree Fruit?” in our winter 2016 issue).

Sooty mold on a mango tree

MOLD  Sooty mold is so obvious to the naked eye that I often hear: “What is all this black stuff on my trees? And how can I get rid of it?” Sooty mold, above right, is a fungus that grows in the carbohydrate-rich secretions of piercing and sucking insects like scales or aphids. These pests attack the plant. Then the secretions, called honeydew, drip down on to the leaves or branches below them, allowing the relatively harmless black fungus to colonize the plant.

Bidens alba

 “Oh, so it’s a fungus. What fungicide should I spray to get it to go away?” You do not treat the fungus. You treat the pest causing the fungus. In many cases, particularly for homeowners or commercial growers wanting to move towards sustainable practices, natural predators (the good bugs) will do an excellent job of removing the pests. Keep your grove or garden natural-predator- and pollinator-friendly by promoting plant diversity and leaving some of the less aggressive weeds, such as the native Bidens alba.

Leaf color change because of chemical damage

LEAF COLOR  A sudden change in leaf color is often blamed on pests or pathogens, when the cause is much more benign. Cooler temperatures in January brought growers to my door asking what was wrong with their guava trees. “Some of the edges of my tree’s leaves have bright red coloration. Should I be worried?” Not at all. That color change was the result of the cool temperatures. These leaves were reacting to the weather and your trees will be fine. Sudden leaf color change can also occur due to chemical damage. The damage may not appear until a few days after an insecticide, herbicide or fertilizer was applied on or near the plant, so this type of damage can be difficult to differentiate from pathogen or pest damage. Such damage is often caused when a pesticide or fertilizer is used incorrectly. Always read the label of any chemical you intend to use and make sure that the crop you want to treat is listed on the label. In all cases, the label is the law and must be followed.

Sometimes it’s difficult to determine the cause of damage on your fruit tree. But by ruling out these non-problems, you’ll have an easier time figuring out the real cause of your plant’s distress.

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