Part I. Infectious Diseases Diseases Caused by Fungi Powdery Mildew Powdery mildew is probably the most widely distributed and serious disease of greenhouse-, garden-, and field-grown roses. Although the causal fungus was first described in 1819, the disease was present long before then and is now known in all countries in which roses are grown.
Symptoms
Early symptoms are slightly raised, blisterlike, often red areas on the upper leaf surface. The white fungal growth, con sisting of mycelium and conidiophores, appears as discrete patches on the surface of young leaves, which become twisted and distorted as the powdery white growth completely covers them (Fig. 2). Older leaves may not be distorted, but circular or irregular areas may be covered with growth of the mildew fungus (Fig. 3). Mature leaves are not usually infected. When environmental conditions are favorable, the affected leaves may fall prematurely. Fungal growth may develop first on succulent, young stem tissues, especially at the base of thorns (Fig. 4). This growth persists when stems mature. On roses in landscape plantings,
Fig. 2. Leaf curl and distortion symptoms of powdery mildew, caused by Podosphaera pannosa. (Reprinted, by permission, from Horst, 2001)
the fungus, which is capable of overwintering in rudimentary leaves or bud scales, can readily infect new spring shoots that develop from these infected buds. The fungus may also attack the flowers and grow abundantly on the pedicels, sepals, and receptacles, especially when the flower bud is unopened (Fig. 5). This infection results in re duced flower quality. Severe mildew damage reduces leaf growth, aesthetic value of plants, photosynthetic efficiency and thereby plant growth, and salability of cut flowers. The number of flowers pro duced may also be reduced, but this has not been conclusively established.
Causal Organism Theophrastus (“The Father of Botany”) gave the first account of powdery mildew on roses around 300 b.c., but Wallroth in 1819 first described the fungus causing this disease as Alphito morpha pannosa. It was transferred to the genus Erysiphe as E. pannosa (Wallr.) Fr. in 1829 and finally was described and placed in the genus Sphaerotheca in 1851. Although the fungus has remained identified as S. pannosa (Wallr.:Fr.) Lév., some authorities recognize a division of this species by Woronichin
Fig. 3. Heavy, white mycelial growth of Podo sphaera pannosa, the cause of powdery mil dew, on upper leaf surfaces.
Fig. 4. Symptoms of powdery mildew, caused by Podosphaera pannosa, on a cane and thorns.
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