In addition to diseases caused by bacterial, fungal, and viral pathogens, roses may display similar symptoms that are the result of chemical toxicities, mineral deficiencies, or environmental problems. Such problems are termed abiotic disorders and can often be corrected by changing environmental conditions. Leaves may distort and drop. Powdery mildew does not require free water on the plant surfaces to develop and is active during California's warm, dry summers. Glossy-foliaged varieties of hybrid teas and grandifloras often have good resistance to mildew as well.
Plants grown in sunny locations with good air circulation are less likely to have serious problems.
The best time to apply this solution to avoid problems with phytotoxicity is during cool weather. Sodium bicarbonate is deleterious to maintenance of soil pH and soil structure and may leave white foliar deposits, so numerous applications with resulting runoff should be avoided. Commercial formulations of neem oil will also control powdery mildew. Interveinal, angular purple, red, or brown spots appear on leaves, followed by leaf yellowing and abscission. Fruiting bodies of the fungus occasionally may be observed on the undersides of leaves.