![]() ![]() Do not apply oils to water-stressed plants or within two weeks of a sulfur spray. ![]() Oils work best as eradicants but also have some protectant activity. Except for the oils, these fungicides are primarily preventive, although potassium bicarbonate has some eradicant activity. In addition to synthetic fungicides, other materials having fungicidal properties are available, including horticultural oils, neem oil, sulfur, potassium bicarbonate, and the biological fungicide Bacillus subtilis. Plants grown in sunny locations with good air circulation are less likely to have serious powdery mildew problems.įungicides may be available, but generally you must apply them to prevent rather than eradicate infections, so timing is critical and repeat applications may be necessary. Glossy-foliaged varieties of hybrid teas and grandifloras often have good resistance to powdery mildew as well. Rose varieties vary greatly in resistance to powdery mildew, with landscape (shrub) varieties among the most resistant. However, these sanitation practices may not entirely eradicate powdery mildew since airborne spores from other locations can provide fresh inoculum. The pathogen requires living tissue to survive, so pruning, collecting, and disposing of leaves during the dormant season can limit infestations. Overhead sprinkling, such as irrigation or washing, during midday may limit the disease by disrupting the daily spore-release cycle, yet allows time for foliage to dry before evening. Powdery mildew does not require free water on plant surfaces to develop and can even infect roses during California’s warm, dry summers. ![]() rosae, produces white-to-gray powdery growth on both sides of leaves, as well as shoots, sepals, buds, and occasionally on petals. Powdery mildew, caused by the fungus Podosphaera (previously Sphaerotheca) pannosa var. LEAF AND SHOOT DISEASES AND DISORDERS Fungal diseases Although some rose enthusiasts consider regular application of fungicides a necessary component of rose culture, many gardeners can grow plants with little to no use of fungicides, especially in California’s dry interior valleys. This publication helps readers manage rose diseases and problems by offering an integrated approach which includes choosing varieties and irrigation practices carefully, promoting air circulation by pruning correctly and providing sufficient space between plants, and removing severely infested material promptly. Such problems are termed abiotic disorders and improving environmental conditions can often correct these symptoms. In addition to diseases caused by plant pathogens, roses may display damage symptoms resulting from chemical toxicities, mineral deficiencies, or environmental problems. The most common rose disease in California gardens and landscapes is powdery mildew, but other diseases including rust, black spot, botrytis, downy mildew, and anthracnose may cause problems where moist conditions prevail. Rose mosaic disease has damaged this foliage.Ī variety of plant pathogens (bacteria, fungi, and viruses) can attack roses and lead to diseases.
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