Managing Powdery Mildew

DISEASE MANAGEMENT

Cultural practices

Cropping practices can have a significant effect on development and severity of powdery mildew. High seeding rate, high nitrogen fertility and semidwarf growth habit can increase severity of powdery mildew (Last, 1954; Tompkins et al., 1992). High nitrogen increases plant height and tillering, which reduces culm strength. This leads to increased lodging and prolonged leaf wetness favourable for infection (Shaner and Finney, 1977). Residual nitrogen from a previous crop to which high rates of nitrogen were applied and legume crops, which produce nitrogen, resulted in higher severity of powdery mildew in a following wheat crop (Parmentier and Rixhon, 1973). Variation in row spacing has been reported both to increase and decrease disease. Prolonged survival of debris-borne inoculum in reduced tillage systems has little effect on powdery mildew because most inoculum is windborne. However, volunteer wheat in reduced tillage systems can serve as an inoculum source.

Powdery mildew can develop at any growth stage. In areas where winter wheat is grown, early planting and above-average autumn temperatures favour infection although symptoms may not be readily visible. These autumn infections can contribute to yield reduction (Frank et al., 1988). Planting toward the latter part of the recommended planting period for the region can reduce early infection.

The use of cultivar mixtures to slow an epidemic of powdery mildew has been studied most intensely in winter barley (Wolfe, 1984). The anticipated benefits are to slow the rate of the epidemic to reduce or eliminate the need for foliar fungicide and thereby reduce the development of fungicide resistance in the pathogen. Deployment of a larger number of resistance genes also aims to diversify the population of B. Graminis f. sp. tritici. Mixtures of cultivars that carry several different resistance genes slowed the progress of a powdery mildew epidemic in both spring and winter wheat and improved yield by about 5 percent (Stuke and Fehrmann, 1988). Although shown to be beneficial in several wheat-pathogen systems, cultivar mixtures have been used only on a limited scale. Cultivar maturities in the mixture must be similar and the end use must be considered, especially if the crop is to be sold through typical grain marketing channels.

Disease resistance

Genetic resistance has been the primary means to manage powdery mildew. Only a brief summary of mechanisms of resistance and the genes that are used will be presented here. Bennett (1984) and Ecker and Lein (1994) have reviewed the use of several important resistance genes and their deployment in Western Europe and North America. Genes for resistance have been identified in at least 30 loci in wheat (Järve et al., 2000; Liu et al., 2001; McIntosh et al., 1998, 2000, 2001; Peusha et al., 2000; Shi et al., 1998; Rong et al., 2000). These genes often act only against specific races of the pathogen causing a hypersensitive resistance reaction in the wheat plant. A major concern is that only a few genes have been used widely in cultivar development. Resistance may be lost when new strains of the fungus develop. For example, Persaud et al. (1994) found increased virulence toward Pm17, a widely used gene from Amigo wheat (Heun et al., 1990; Lowry et al., 1984). However, the genetics of B. Graminis f. sp. tritici is complex. Higher frequencies of virulence were also found in the powdery mildew population due to Pm genes not known to be widely deployed (Niewoehner and Leath, 1998).

Resistance to powdery mildew is also accomplished by a combination of factors that slow the rate of disease progress so that plants mature before significant damage occurs. This is known as slow-mildewing or partial resistance and is race-nonspecific. Plants are susceptible as seedlings but are less susceptible in the adult stage so that this is a form of adult plant resistance. Several genes usually control partial resistance. Griffey and Das (1994) found that as few as two or three genes provided long-lasting adult plant resistance in two wheat cultivars. The factors that contribute to partial resistance include an increase in the time from infection until new spores are produced (latent period), reduced size of pustules and reduced production of spores. The infection frequency, the number of spores that successfully infect the plant, may also be reduced (Shaner, 1973). The slow rate of disease development can be quantified by calculating the area under the disease progress curve (AUDPC) based on three or more disease severity ratings during the season. AUDPC is useful to compare cultivars for differences in powdery mildew resistance (Shaner and Finney, 1977; Hautea et al., 1987). Recently, it has become possible to use molecular techniques to find quantitative trait loci (QTL) for powdery mildew resistance on gene maps to identify quantitative disease resistance (Chantret et al., 2000; Keller et al., 1999). The ability to use molecular markers associated with QTL holds promise for more rapid development of cultivars with partial resistance to powdery mildew.

Genes for avirulence in the fungus may be expressed differently depending on the host genotype. An isolate of B. Graminis f. sp. tritici may grow rapidly on one genotype but much more slowly on another genotype, so that the response of the host-parasite interaction is a partial resistance (Martin and Ellingboe, 1976).

Wild relatives of wheat have been exploited as sources of new resistance genes (Bennett, 1984). Wild emmer, Triticum turgidum var. dicoccoides, is a source of genes, some of which are expressed in both seedling and adult plants and some of which are expressed only in adult plants. Some wild emmers also possess genes for partial resistance (Silfhout and Gerechter-Amitai, 1988; Moseman et al., 1984). Triticum timopheevii var. araraticum collected in the Middle East has a gene for resistance that differs from Pm6 from cultivated T. timopheevii (Brown-Guedira et al., 1996). Genes from rye (Secale cereale), including Pm8 and Pm17, have been used widely in wheat cultivars. New genes from rye can be transferred to wheat the by use of wheat-rye translocation lines (Heun and Friebe, 1990; Merker and Forsstrom, 2000).

Regional surveys are needed to determine which virulences are present so that breeding strategies can be planned to use the most effective genes. The cultivar Chancellor and its isogenic lines containing individual Pm genes are useful to determine virulence in B. Graminis f. sp. tritici (Briggle, 1969). Recent surveys for virulence genes and identification of resistance genes in soft red wheat in the United States include procedures for inoculation and evaluation of disease reactions (Niewoehner and Leath, 1998; Persaud et al., 1994; Persaud and Lipps, 1995).

Fungicides

Application of foliar fungicides has traditionally been the only means of chemical control for powdery mildew. Seed-applied systemic fungicides are now available that control early season development of the disease. These are especially effective for winter wheat. Triadimenol seed treatment prevented excess tillering caused by mildew infection early in the season and contributed to a higher grain yield, especially when high temperatures during grainfilling reduced the amount of disease later in the season (Everts and Leath, 1992; Frank and Ayers, 1986; Leath and Bowen, 1989). Difenoconazole also has systemic activity against powdery mildew. These fungicides have a wide spectrum of activity and may be economical seed treatments when they also contribute to reduction of smuts and other foliar pathogens.

Foliar fungicides are effective but should only be applied if the cultivar is susceptible and an economic return is likely (Leath and Bowen, 1989). Pustules may develop on lower leaves early in the season on resistant cultivars but not on upper leaves later in the season. Avoid applying fungicides too early to be effective during the grainfilling period. When powdery mildew was moderate prior to flowering, early season applications of the systemic fungicide triadimefon at Feekes 6 to 8, maintained yield at 8 to 17 percent above the control (Lipps and Madden, 1989b). Comparisons must be made over several years to determine whether or not the cost of fungicide application is economical.

Fungicide insensitivity is a concern where fungicides are used intensively, such as in Western Europe. Reduced effectiveness of the triazole fungicides triadimefon and propiconazole was found in the Netherlands following intensive use (De Waard et al., 1986). More than 570 isolates of B. graminis f. sp. tritici collected throughout the eastern and southern United States, where fungicide use is much less, were sensitive to triadimenol (Niewoehner and Leath, 1998). Fungicides in the strobilurin group, such as azoxystrobin, with modes of action different from the triazoles, are currently being deployed for use against powdery mildew.

An integrated disease management system should be used with genetic resistance as the cornerstone of the programme. Cultural management, including proper management of nitrogen fertilization, is essential to minimize risk of crop damage from powdery mildew. Fungicides should be used in conjunction with a disease monitoring system employed from planting through the flowering stage of growth to estimate economic return.