If you think about the best ways to get your apple tree leaves infected with fungus, just continue to block all those conditions and the spores will have no place to germinate and grow. Fungal spores travel through the air, and when they land on a leaf, they need moisture and food. Constant rain (or watering) and aphids leaving honeydew on the surface of the leaf practically guarantees germination and growth of fungal spores. Lots of bacteria (from a compost tea application) may outcompete the spores and you don't get any growth. Lime sulfur raises the pH to where the spores have difficulty growing. Unfortunately that can also burn leaves, which is why people try the plain sulfur.
The best way to get rid of fungus problems is to have a drought. Apples grown in the high desert of California, where they rely on irrigation, have next to no fungus problems because the rainy season is when the leaves are off the trees.
I found this JPG of a guild. I think it's from Gia's Garden.
Calcium deficiency can also make them more susceptible, and apple trees are calcium-lovers. Under the apple trees is a good place to throw eggshells and bury bones (if you don't have too many wild animals to dig them back up at night!)
I've got 3 apple trees. On has the cedar apple rust pretty badly, one has spots but it doesn't seem to be affecting the health of the tree and is only on a few of the fruits, the third barely has spots; so the kind of tree may have something to do with it, too. I read apples can't pass it to other apples, it goes cedar => apple and apple => cedar only.
I had a poor fruit set this year, so maybe that is boron deficiency, maybe something else, but I don't have a chemistry lab at my disposal to make that determination. I will continue to mulch and add manures and sprinkle the compost tea around hope that the chemistry of my garden environment is improving. As I think back to how it was since I first moved in, it seems a lot is improving: more spiders, more anoles and skinks, more toads, less loss to Fusarium diseases. Qualitatively, it looks better, but it's frustrating my scientific side not to quantify it.
I will start putting my egg shells out with my trees instead of my compost now.
C Quint wrote:Is it cedar-apple rust fungus (orange spots on the leaves)? Mine have that and the organic advice I read was to chop down all of the Eastern Redcedars within 2 miles of my trees, which is not possible. Two of my trees, which are heirloom apples local to our area and planted in guilds with comfrey, bee balm, dandelions, false indigo, New Jersey Tea, gooseberries, and mountain mints did not succumb to the fungus, but the columnar apple trees from a local chain nursery, planted near only comfrey, have the orange-spotted leaves now. I'm hoping to minimize the damage for next year by removing cedar-apple rust galls from all of the Eastern Redcedars in our yard (we only have 1 acre) next winter, but we'll see. In the future, I plan to plant only disease-resistant varieties.