I have applied Horticultuarl cornmeal to the soil when my Hawthorns began to get black spots on the leaves. (and I also removed the bad leaves) How often should I apply the cornmeal?

Just so you understand, cornmeal on the soil will not cure the black spots on the leaves. What corn meal does is
1. Fertilize the plant
2. Kill disease causing fungi that live ON TOP OF the soil.

The reason you want to kill the diseases on top of the soil is that when it rains, those fungi splash up onto the plant. If you want to minimize airborne fungal disease you can spray with either milk or shampoo every other week. I like the milk but supposedly shampoo works, too. If you use milk, the dilution rate is 3 ounces per gallon (from an Ortho hose end sprayer). If you use shampoo, the one with the least additives seems to be Equate (Wal-Mart brand) Baby Shampoo.

Now to answer your question, monthly.

Plant Wash is also very effective: http://dirtdoctor.com/organic/garden/vi ... n/id/1863/

http://www.dirtdoctor.com/organic/garden/store/type/23/

I keep forgetting about Plant Wash. Thanks Doug.