Our Master Gardener help desk gets emails mid-summer when apple trees drop many of their leaves, except for the outer 12 inches or so. The same thing can happen to our ornamental flowering crabapples. In both cases, the cause is the same, a fungal disease that infects the foliage and fruits. What is sad is that by the time they see or report an issue, it is too late to address the problem.
When does it happen?
That sneaky foliar fungal disease is out there right now, waiting for the right weather conditions and stage of development of our crabapples and apples. When it comes to apple scab, preventing the infection and spread is up to us, the gardeners. Spray treatments need to start when the apple and crab buds are just swelling and we can see the very tiniest amount of green tissue showing. Protection is the name of game with this disease.
How long will I have to treat apple scab?
As long as the cool, moist to wet weather is with us, we need to keep up the preventive measures. The infection comes from overwintering spores in last year’s leaves that are on the ground. Spraying the ground under your crabapple or apple trees is ineffective, however, as the spores can come from anywhere in the neighborhood. Treatments can slow and then stop the spores. And as warmer, drier air arrives, the spores can no longer survive in those conditions. There, of course, is a “however” here. The cycle can continue on the tree once it is established. So you can see why preventing it is so important.
What is the process?
The goal is to protect newly emerging leaf tissue, so sprays typically are applied every five to seven days, depending on the weather and product label. The weather influences that frequency, so you use the calendar and conditions to count between applications. For example, if you treat on a Saturday and then we have heavy rains immediately, effectiveness may be lost or at least limited, and another treatment is needed ASAP. Then you begin counting those days again. Those first sprays are directed at those emerging leaves and a bit later at flower buds. The treatments are repeated until about 75% of the flower petals have fallen, this having protected very immature fruits up to that treatment. (Always read and follow produce label and safety instructions.)
Are there other protection sprays needed?
Spraying will need to continue at longer intervals to protect from other foliage and fruit diseases, if you want great looking apples or crabapple fruit, as well as the satisfaction of seeing your tree full of foliage for the whole growing year. These sprays often are called “cover sprays,” as you continue to keep the developing fruits covered and protected until a few days before harvest (if it is an apple tree). The most convenient products are sold as “fruit tree spray or home orchard spray” in a liquid form, which is easier to handle than powders. Keep in mind that at the beginning of season, you may only need a gallon or less for each tree. As the season progresses with mature foliage and maturing fruits, that could grow to three to four gallons a tree. As you think about what kind of sprayer you are going to use, keep in mind how often you want to have to refill. Find a sample spray schedule for apple trees here.
Volunteer Master Gardeners for your seasonal gardening questions
Our Master Gardener e-Help Desks are open for the 2021 growing season. Residents can get gardening and growing advice on a variety of topics. To connect with your county’s Master Gardener help desk, email your question, pictures and any additional information to: uiemg-dupage@illinois.edu, uiemg-kane@illinois.edu or uiemg-kendall@illinois.edu.
• Richard Hentschel is a Horticulture Extension educator with University of Illinois Extension, serving DuPage, Kane and Kendall counties. This column originates on his blog at go.illinois.edu/overthegardenfence. To get more tips from Hentschel, watch his “This Week in the Garden” videos on Facebook and YouTube.