cjccmc, This sounded interesting to me also, so I did a search with 'cornmeal' and found what you maybe looking for.

Nandina's thread on cornmeal for blackspot is on this page!

I was going to suggest looking at Nandina's experiment, too.

I use one handful per rose plant. That's about a half cup. In the past two years, since I started using it, I have had no fungus problems. Not only that, I have not had any aphids in two years. This follows 8 consecutive seasons of aphid problems. So, from hundreds of aphids on all plants to zero aphids on all plants, all at once following the application of corn meal...I just have to connect the events. Don't ask why. Other than possibly the corn meal created a plant with more resistance to natural pests??? I don't know.