Why dusting with alkaline corn meal works?
Keep the surface of leaves either dry and alkaline, or VERY Acidic ... both suppress microbes. Here's dusting with cornmeal, after 7 days of non-stop rain in late fall, zone 5a. Radio Times rose below has a reputation of black-spot-prone. It's 100% clean, dusted with whole-grain cornmeal, very acidic at pH 3.5.
At the same time, my Impatient annual flower came down with black spots in 7 days of non-stop rain:
See the abstract below:
"The influence of pH on the two principal decomposer groups in soil, fungi and bacteria, was investigated at Rothamsted Research in the United Kingdom. ...The growth-based measurements revealed a fivefold decrease in bacterial growth and a fivefold increase in fungal growth with lower pH. .. Below pH 4.5 there was universal inhibition of all microbial variables. "
See below link on how to dust your roses, plus the ground with Whole-grain cornmeal, pH 8. It's sold for $2 to $3 per lb. at the Organic Section of grocery stores. Walmart has it as Bob's Mill brand. The advantage of corn meal is it has no salt. Milk, baking soda, or potassium bicarbonate (Green Cure) all have salt. Corn meal also supplies nitrogen to the soil, and host the beneficial fungi Trichoderma that suppress the pathogenic black spot fungi.
Here is a link that might be useful: How to dust roses with whole-grain corn meal
This post was edited by Strawberryhill on Mon, Jul 21, 14 at 15:16