If you see these symptoms of common plant problems, there are many excellent chemical and natural products on the market which can help. Your local garden center should be able to offer suggestions. Itís always okay to use an insecticidal soap, such as Saferís Soap, for insects. A general purpose fungicide can be a good preventative measure if the summer is particularly soggy.
Red Spider (and other mites): Plants are yellow and weak. The undersides of the leaves are always dirty when examined (from soil sticking to the fine webs.) Tap a leaf over a piece of white paper and you will see tiny specs. If they start crawling, then you have spider mites. The bigger mites can be seen by the eye. Often first seen with the heat of summer.
Powdery Mildew: Leaves are disfigured by a white coating resembling mold. Begins when there is little air movement or when night dews become prevalent in late July. Use fungicides listing Powdery Mildew by midsummer until late August (in Ohio). Leaf damage remains even with treatment, unfortunately.
Leafminer: White curling trails are seen in the leaves, most often in late spring only.
Aphids & Chewing Insects: Holes in leaves or along leaf margins, or visible insects along stems.
Leaf Spots: Dark brown spots on leaves can be caused by a fungus or by over-watering. Red pustules (bumps) can indicate rust.
Spittle Bugs: Bubbly masses in the leaf axils.
White Fly: Small white insects fly when disturbed. Responds well to insecticidal soap.