|I have an althea bush that is suddenly full of white powdery substance on alot of blooms. What is causing this? Planted in a semi-shady spot.|
That may be powdery mildew if it is laying right on the leaf and bloom surface and not sticking up off of the surface. Powdery mildew can be managed with sprays of neem oil, potassium bicarbonate (GreenCure is one brand), or bayleton.
You may also be seeing an infestation of the giant whitefly. These pests leave white waxy residue on the leaves and even create long hair-like strands that stick out from the plant surfaces. The following web site has more information and photos of this pest:
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