Hi! One of my knock-out rose bushes has developed brown spots and now the leaves are turning yellow and falling. Can you recommend a product/remedy to treat it? Thank you!
Determine what the problem is before buying anything or applying anything. Look at the undersides of the leaves for any stippling or discoloration. I'm thinking that maybe you might have Spider Mites. Take a clean piece of white paper, hold it under that affected leaves and tap lightly on the leaves and stems. Do this a couple of times, then examine the paper for anything that resembles dust particles that are moving. If you see anything like this, then you have Spider Mites. Use a miticide or Neem Oil and spray all leaf surfaces. You'll probably have to repeat a few times at about 5-day intervals.
Another possibility is that your plants are staying too dry during this heat wave. Check the soil and make sure that cracks haven't developed in the area of your planting area and your water isn't flowing into the cracks and not around the roots.