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Prevent Black Spot Disease On Your Roses With A Handy Pantry Staple
By KATE NICHOLSON
Caused by Diplocarpon rosae fungus, black spots can cover the leaves of a rose and cause it to fall off. To prevent this, several experts suggest using a pantry staple: cornmeal.
Dr. Joe McFarland's research in the 2010s found cornmeal doesn't kill the disease but encourages the growth of cannibalistic fungi, which feed on the other harmful fungi.
The Texas Organic Research Center had positive results when using cornmeal drench to treat fungal infection in oak trees. Cornmeal needs to be activated by water to work.
However, this research might not apply to rose black spot. There's no scientific evidence to prove it is an effective fungicide, and it's not legally allowed to be sold or used.
If you want to experiment, spread a handful of whole-ground cornmeal around the base of a rose bush in spring as the buds start to appear, and water thoroughly. Repeat every month.
Use 2, 20, or up to 50 pounds for every 100 square feet of garden or half a cup per plant. You can also make a foliar spray "tea" for easier application.
To prepare this tea, put a cup of cornmeal into an old sock or cheesecloth bag and soak it in 1 to 5 gallons of water. This concoction works well as a soil drench, too.