Q: The leaves on my crabapple tree are turning yellow and getting spots on them. What's the best way to treat that? The tree is about 10 years old.

A: You're dealing with a leaf disease called apple scab. It's more of an aesthetic problem than a serious tree problem. There are different fungicides that you could spray to keep that off, but the trouble is once it's on there, there's nothing you can do that'll take it off. All of the fungicide sprays are preventative. They have to be in place whenever the weather conditions are favorable for that disease to hit.

The way everyone in our industry is getting around that is planting varieties of flowering crab that are resistent to the disease. Some varieties are very, very resistent to it. Some can catch it just by having a couple of spores blow by. It's more of a weakness in some crabs than others. The better places only carry varieties of flowering crab that are resistent to the apple scab. It comes from cool, damp weather. As long as you have that type of weather and a variety that is susceptible to that particular disease, you're going to deal with that disease. It's similar to black spot on roses; some roses are not resistent at all. So you want to seek out plants that have a natural resistence to start out with if you want to avoid future problems down the road.

Collect as many of the fallen leaves as you can. When it gets hot and dry, there won't be any apple scab. It doesn't like that kind of weather. To have any disease, you have to have three things in place: the spores, a plant the disease wants to land and grow on, and environmental conditions, which is why you have certain diseases in warm weather and certain diseases in cool weather. But you won't have the same disease in both. And most diseases are aesthetic problems. They don't cause health problems to the plant; they just look bad.