Maddening Mildew
An Extensive Family Powdery mildew has a common symptom— the development of fine, whitish, powdery deposits that form mostly on the upper leaf surface. These deposits look like a sprinkling of talc and the early stages are often missed by growers as they tend to first form inside leafy canopies. Despite this common symptom, powdery mildew is not one disease, but can be caused by a number of different genera of fungi. Some of the powdery mildew fungi species are specific to certain plants—for example, Sphaerotheca pannosa var. rosae is confined to roses—while others have a much wider host range. In greenhouses and grow rooms, the most common species of fungi that cause powdery mildew symptoms are Erisyphe, Leveillula, Microsphaera, Podosphaera, Odium and Sphaerotheca. It’s not usually possible to tell exactly which of these fungal pathogens are causing powdery mildew symptoms in different situations, although Leveillula—which occurs mostly on tomatoes and peppers and is sometimes called internal powdery mildew—may produce fungal spores on the lower leaf surfaces as well as the upper surfaces. This poses another problem, as downy mildew, which is a separate, less common disease, also produces grayish-white powdery spores on the undersides of leaves. Mistaking downy mildew and Leveillula powdery mildew creates problems with control as different sprays need to be used for these two very different diseases. To get the diagnosis correct, both leaf surfaces should be checked carefully for the presence of powdery spores. If these are present on both the upper and lower leaf surfaces, then Leveillula powdery mildew is likely to be the cause; if grayishwhite spores are only on the lower surface, then downy mildew is more likely. Neither Leveillula 64
Maximum Yield USA | October 2010
Powdery mildew not only forms on the foliage, but also on stems, buds, flowers and fruit.
nor downy mildew should be confused with Botrytis or grey mould, which has brownish-grey spores and is also a common fungal pathogen in enclosed growing areas. Apart from Leveillula or internal powdery mildew, most of the other powdery mildew species have a simple cycle of infection. The spores are often carried into the grow room on wind currents from infected plants. They are introduced to the growing area on infected seedlings or cuttings or develop from survival structures on plant debris. Most infections come from airborne spores landing on leaf surfaces. The spores do not need an open wound or injured plant to infect their host; they happily germinate on dry leaves, penetrating the leaf tissue and sending food absorbing haustoria into the cells. The threadlike fungus (hyphae) grow over the leaf surface and eventually produce more spores, which trigger further infections. This infection process takes anywhere from three to seven days depending on conditions, but in as little as 72 hours from landing on a leaf surface, the fungi can send fresh spores into the air creating a high degree of infection over a wide area.
Powdery mildew can develop rapidly from a few isolated spots to complete plant coverage in a short period.