Powdery mildew

Powdery mildew, caused by the fungus Blumeria graminis (DC) E.O. Speer f. sp. tritici Em. Marchal (syn. Erysiphe graminis DC f. sp. tritici Marchal), is one of the most important foliar diseases of wheat worldwide. It is an obligate parasite, growing only on living tissue. Spores of the fungus germinate on the leaf surface and invade the plant. The fungus then colonizes the epidermis of the leaf by obtaining nutrients from the plant cells without killing them. A vast literature exists on powdery mildew of wheat and other cereals. This chapter presents an overview of the disease and its management, with emphasis on recent literature.

THE DISEASE

Symptoms and signs

Powdery mildew produces white to grey, cottony fungal growth mostly on the upper leaf surface, although some pustules may develop on the underside of the leaf (Daamen, 1989; Wiese, 1987). Pustules begin as small white circular patches of fungal mycelium often surrounded by chlorosis, most visible on the underside of the leaf. 'Green islands' appear near infected areas as the plant transports nutrients to non-diseased cells (Schafer, 1987). Pustules often join together to form large masses of fungal growth on the upper surface of leaves when conditions are favourable (Plate 46, Plate 47). As the diseased area expands, tissue around older pustules dies and turns brown. Severely diseased leaves collapse and die. Powdery mildew is most prevalent on lower leaves but can cause blighting of the upper leaves, heads and awns of susceptible cultivars. Within a few days after they appear, the white powdery pustules produce large quantities of small asexually produced conidia (also called oidia) in long chains, which are easily dislodged by wind or rain. These spores are single-celled, oval (8 to 35 µm) and colourless. As lesions age, the mycelium becomes more dense and turns grey (Wiese, 1987). Dark round cleistothecia (135 to 280 µm in diameter) develop in the fungal mass as the pustules get older. Ascospores develop in the cleistothecia and serve as the long-term survival structures. Cleistothecia may be empty if examined when wheat is harvested because ascospores often do not mature for several months.

Geographic distribution

Powdery mildew occurs almost everywhere wheat is grown. It is important economically under humid rainfed conditions and in dryland areas when irrigation is used for production of improved cultivars with high yield potential. Powdery mildew has increased in importance in some regions because of increased application of nitrogen fertilizer, which favours the disease. The disease is important in regions where rain occurs early in the season and where temperatures are relatively cool, such as regions with maritime climates, and in cooler regions with a humid continental climate (Bennett, 1984). Powdery mildew is important in the cooler regions of China, Japan and other areas in Asia, in North and East Africa, in northern Europe and eastern North America (Roelfs, 1977; Saari and Wilcoxson, 1974). It is also important in warmer, humid regions with mild winters where wheat is planted in the autumn, such as parts of the Southern Cone of South America and the southeastern United States. In regions or seasons in which rain is frequent and heavy, the occurrence of powdery mildew may be very low because spores are washed from the leaves or they burst in water (Merchan and Kranz, 1986).

Epidemiology

Powdery mildew typically begins rapid growth on the lower leaves and sheaths when plants begin to joint. It is usually the first leaf disease of the season because it is favoured by temperatures between 10 and 22°C. Infection and disease development decline after flowering when temperatures increase above 25°C. Conidia are the primary inoculum source for dissemination of the fungus. They are easily dislodged from lesions by wind and rain. Production of conidia is optimal at 20°C and declines rapidly above and below that temperature (Ward and Manners, 1974). Although conidia only survive for several days, they are capable of disseminating the fungus long distances. New pustules with conidia are produced every seven to ten days at optimal conditions and provide repeating cycles of spores. Conidia germinate most rapidly at 97 to 100 percent relative humidity, but their high water content allows them to germinate when humidity declines below 50 percent. However, germ tube growth and appressorium production are greatly reduced below 92 percent relative humidity (Friedrich and Boyle, 1993). Frequent light rain removes conidia from leaves and thus reduces the number of new colonies that form. Periods of heavy rain slow the development of established pustules (Merchan and Kranz, 1986). Conidia do not germinate in free moisture, which can cause them to burst. After crop maturity, ascospores in cleistothecia serve as survival structures, but their role in initiating disease is much less important than that of the conidia in most environments. Conidia produced on grasses and volunteer wheat also maintain inoculum until wheat is planted. In autumn-sown wheat, infections that do not result in visible symptoms can maintain the fungus in leaves through the winter (Frank and Ayers, 1986).