Managing Sooty Mold on Fruit Trees
Sooty mold is an unsightly dark fungus that grows on fruit, leaving it looking spotted and dry. Sooty mold doesn’t make the fruit inedible (if you wash it off before eating), but it can be harmful to the plants themselves if allowed to grow out of control. It can completely cover leaves, preventing photosynthesis and causing early leaf-fall.
The fungus thrives on honeydew, a sticky substance left behind by certain plant-sucking insects. The insects take their fill from the plant, but can’t always use it all; what they can’t use is excreted as honeydew. Many of these insects have natural predators – aphids have ladybugs, for example – however, ants are very fond of honeydew as it’s a great source of food for them. Therefore, ants typically defend honeydew-producing insects from their predators.
Just as sooty mold can be washed off of affected fruit before eating, it can also be washed off of the plants. If it isn’t too large an infestation, sometimes a strong stream of water will do the trick; however, because the honeydew is so sticky, more effort may be needed. Often, a gentle hand washing of the leaves with mild soap and water, followed by a clear-water rinse, will work. Fruit affected by sooty mold can be washed in the kitchen with either water, soap and water or a fruit & vegetable wash.
Of course, it’s better to prevent the situation, rather than manage it after the fact. If you notice plant-sucking insects invading your garden, you can sometimes wash them away, too. If this doesn’t work on the long-term, you may try a couple different approaches. If only part of the plant is affected, try carefully pruning that section away.
If all else fails, it’s time to consider getting rid of the source – the plant-sucking insects. One way is by managing their protectors, the ants. If it’s a fruit tree, you may try pruning away any branches that come into contact with houses, etc., as this keeps ants from getting into the tree. Likewise, placing a sticky compound at the tree base does the same thing. Tanglefoot® is one consideration. Ant bait around the tree base is another option. With the ants gone, the plant-sucking insects’ natural predators will return and take care of the problem for you. Ant Pro® Bait and Station is an organic choice for home orchards and larger groves.
A more direct approach is applying insecticides to the plants. There are many types and brands available; however, because these plants are producing edible fruits for your table, an organic choice, such as Monterey® Garden Insect Spray with Spinosad, is probably best. Neem® oil is another organic choice. Always spray plants with a light coat of insecticide on a calm day. This prevents excess insecticide from blowing away in the wind or running off of the plant into the ground as well as any water sources. Be careful not to inhale any of the insecticide; you may also want to wear rubber gloves when using certain types of insecticides. Always read and follow the label for the proper dosage.
The sooty mold will likely weather away once the ants and plant-sucking insects are under control. However, it may still be necessary to wash the sooty mold off in heavier infestations to prevent harm to the plants.
If you have an unidentifiable pest problem with your fruit trees, bring in a sample in sealed zip-lock bag and ask one of our Garden Pro’s to help you identify it!