We want our plants to do their best: to grow, to be healthy, to flourish, to bloom, and in the case of fruit trees, to produce. As the Commercial Tropical Fruit Extension Agent for Miami-Dade County, I am often presented with what the grower sometimes believes are major problems, when in fact, the pest (or problem) in question is not what it appears to be.
Lately I have received quite a few questions about lichens as 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 calloused finger at the innocent lichen. “How can I kill this? It is affecting my fruit production.” You do not need to kill or remove lichens, but you may need to fix the true problems affecting your fruit trees. See “Why Won’t My Tree Fruit?” for an explanation of true problems.
Sooty mold is so obvious to the naked eye, that the questions, “What is all this black stuff on my trees? And how can I get rid of it?”, are also questions I often receive. Sooty mold (see photo) is a fungus that grows in the carbohydrate rich secretions of piercing and sucking insects such as scales or aphids. These pests attack the plant, and 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.
“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.
A sudden change in leaf color is often blamed on pests or pathogens when many times the cause is much more benign. Cooler temperatures will sometimes bring guava 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 simply reacting to the weather and your trees will be completely 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 sometimes be difficult to differentiate from pathogen or pest damage. This type of damage is often caused when a pesticide or fertilizer is used incorrectly. Remember to always read the label of any chemical you intend to use and make sure that the crop you intend to treat is listed on the label. In all cases, the label is the law and must be followed.
Sometimes it is very difficult to determine what is the cause of damage on your fruit tree, but by paying attention to your trees and by frequently scouting your garden or grove, you will have a much easier time figuring out the cause of your plants distress.
Source: UF/IFAS Pest Alert
Note: All images and contents are the property of UF/IFAS.