Diseases of rose (
Powdery mildew
Many foliar diseases common to rose in other areas are not problematic in Arizona because of the dry climate and adaptability of roses to the desert environment. However, powdery mildew of rose, caused by Sphaerotheca pannosa, is a common problem throughout the state. Disease occurs in the spring in the low deserts and usually subsides in June when maximum daily temperatures are above 95 F. In higher elevations, disease may persist throughout the summer. Roses grown in shady areas often have more severe infections.
Symptoms in rose (photo 1) are typical of powdery mildew infections, with a white powdery-looking growth on the surface of leaves and occasionally bracts and stems. When the fungus is active, severely infected leaves appear almost white. Young leaves may be twisted and curled. Older leaves are not affected as severely. Plants are rarely killed by powdery mildew infections, but leaves and flowers may be smaller and plants unattractive.
The fungus produces spores abundantly on the leaf surface, and they readily infect new sites as long as environmental conditions are conducive to germination. Unlike most fungal spores, powdery mildew spores do not germinate well in free water. However, they germinate within a couple of hours at high relative humidity and at about 72 F, conditions common in early morning hours, even in the low desert. In Arizona, frequent foliar sprays with water alone may reduce powdery mildew on roses. However, once disease has been observed, it is usually certain to re-appear to some extent every year.
Preventive fungicide sprays can be used effectively in early spring in the low desert and at higher elevations throughout the spring and summer. Effective fungicides include wettable and dusting sulfurs, mylobutanil (Systhane), azoxystrobin (Heritage), triadimefon (Strike), and thiophanate-methyl formulations (Cleary’s 336, Zyban) at recommended rates. Follow manufacturers instructions for resistance management to prevent development of fungicide resistance.
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February 21, 2013