Sphaerotheca is a group of filamentous fungi that belong to the order Erysiphales (the powdery mildew fungi). These fungi form all of their structures on the surface of host plants and are obligate parasites which means that they cannot survive outside a host plant.
The fungus produces white mycelium that grows on the surface of the plant tissues and forms a weft of hyphae (threads), some of which develop into short, erect conidiophores (structures that bear fruiting bodies). At the tip of each conidiophore 5 to 10 egg-shaped conidia (fruiting bodies) are produced that cling together in chains.5
Members of Sphaerotheca genus cause powdery mildew in many crops, fruits, vegetables, and ornamental plants. Sphaerotheca fuliginea causes powdery mildew on summer squash, pumpkin, and cucumbers. Sphaerotheca mors-uvae causes gooseberry mildew and sometimes affects currents. Fruits dry up with a brown, felty covering. Leaves and canes are stunted with white coating. The disease can result in premature defoliation. Most of the commonly grown cultivars of gooseberry and black current are susceptible.
Sphaerotheca pannosa causes peach mildew called rusty spot of peach. Peach buds serve as overwintering structures of the fungus, with leaves becoming infected as they emerge from buds. Peach trees may serve as a source of infection for plum trees. Peach fruit are susceptible from the early stages of growth to about the beginning of pit hardening. The disease is also common on almond, apricot, and nectarine.1,2,3
The mycelium and conidia of S. pannosa are common on leaves and shoots of cultivated and wild roses. Overwintering is by means of spores and mycelium within dormant buds. Shoots arising from buds containing mycelium become infected and provide inoculum (mycelium and spores) for subsequent secondary infection and disease development on foliage and flowers. Most popular rose varieties are highly susceptible to powdery mildew.4,5
Sphaerotheca macularis causes hop mildew. It also occurs on fruits, blackberry, dewberry, gooseberry, raspberry, strawberry, rose, and other ornamentals. This mildew is sometimes serious on strawberries. The edges of affected leaves curl upward, exposing the lower surface where the powdery frosty growth is seen. Fruit, stems, and berries may be affected, with fruit often failing to color.1
- Westcott's Plant Disease Handbook. Cynthia Westcott
- Pest and disease management handbook. D. V. Alford, British Crop Protection Council
- Integrated Management of Diseases Caused. Fungi, Phytoplasma and Bacteria By Aurelio Ciancio, K. G. Mukerji
- Introduction to Fungi. John Webster and Roland W.S. Weber
- Plant Pathology. George N. Agrios