Our Coreopsis (don't know its cultivar) has this white spots on its leaves. I'm not sure whether it is "powdery mildew"? Please can you tell me what they are? And how to get rid of it? Thank you very much.
Looks like powdery mildew.....You can buy fungicides to treat it.
I agree powdery mildew- Try spraying 2 taspoons of baking soda in a quart of water. Too much baking soda is not too good for the soil so use it as a stopgap measure until you get a fungicide. Neem oil is a good product to have on hand it is an effective fungicide and insecticide.
There are many varieties of Correopsis. Some are taller than others. I'd say if yours is shorter and has different leaves that you both have different varieties.
You might try a milk/water solution. 1 part milk, 9 parts water. Spray thoroughly.
Powdered milk may work. Coffee whitener did not work for me.
Coffee whitener is not milk, nor does it have any part of milk in it. Here is a list of coffee whitener's ingredients--nothing I'd want to put on my plants or in my stomsch
GLUCOSE SYRUP, HYDROGENATED VEGETABLE OIL (MAY CONTAIN COCONUT, PALM KERNEL AND/OR SOYBEAN OIL), SODIUM CASEINATE (A MILK DERIVATIVE), DIPOSTASSIUM PHOSPHATE, SODIUM ALUMINUM SILICATE, MONOGLYCERIDES, ACETYLATED TARTARIC ACID ESTERS OF MONO- AND DYGLYCERIDES, ARTIFICIAL FLAVOUR, COLOUR
Here is a link that might be useful: Baking soda
Using milk for disease control.....
Here is a link that might be useful: Milk