Powdery Mildews in Your Backyard – Part 1
Powdery mildew is caused by a group of fungi that consists of several genera and hundreds of species. There are only a few plant groups and species that are not hosts for powdery mildew, for example conifers and gingko. A sign of powdery mildew is a white layer of fungal mycelium and spores on the surface of leaves, green shoots, and fruit (Figs. 1a-b). The fungus grows on the surface of the tissue and absorbs nutrients by extending haustoria (specialized cells) into plant cells without destroying them.
Powdery mildews produce two types of spores – during the spring, summer, and fall most of them produce conidia (asexual spores) that allow for fast and wide spread (if picked up by wind currents spores can travel many miles) to neighboring plants, and ascospores that survive adverse environmental conditions (winter, dying plant tissue) in a round fruiting body. As conditions improve, the ascospores germinate and new fungal growth starts. The new growth will then produce conidia for widespread dispersal, completing the life cycle.
Without a microscope, powdery mildews all look the same and even with a microscope it is sometimes not easy to identify the genus or species. Conidia of powdery mildews look barrel-shaped. To identify the species, diagnosticians view stained spores under a microscope, or use molecular tools such as PCR (polymerase chain reaction). The best stage for identification is the fruiting bodies, which will contain ascospores. The fruiting bodies have appendages of various shapes (Fig. 3) that are specific to each genera, and the number of asci (sac-like structures containing the ascospores within the fruiting body, see “Picture of the Quarter”, last page) vary by species as does the number of ascospores in an ascus.
To every rule there is an exception. There is one powdery mildew, Leveillula taurica that can infect tomato, pepper or onion leaves but does not grow on the plant surface and is therefore not easily visible. It grows spore-bearing structures out of the stomata (openings in the leaves for gas exchange by the plant). The spores are cylindrical or spear shaped and fruiting bodies are rarely produced. If you see yellow spots (Fig. 4) developing on these plants, please submit a sample to the UPPDL.
Host specificity of powdery mildew fungi varies. Some species require a specific host whereas others have a wide host range. For example, maple powdery mildew (Sawadea sp.) will not attack any vegetables or flowers. It is specialized to maple species. In contrast, Erysiphe cichoracearum infects many weeds, ornamentals (such as phlox, penstemons, black-eyed susan and many others), and some vegetables. It is one of two powdery mildews infecting cucurbits such as melons, cucumber and squash. I will cover powdery mildew species frequently found on vegetables, ornamentals, and fruit trees in Utah in the next newsletter.
-Claudia Nischwitz, Plant Pathologist