**Yield Response to Foliar Fungicide Application in Winter Wheat**

Stephen Wegulo, Julie Stevens, Michael Zwingman and P. Stephen Baenziger *University of Nebraska-Lincoln USA* 

### **1. Introduction**

226 Fungicides for Plant and Animal Diseases

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Fungicides are routinely applied to control fungal diseases of wheat and other cereal crops, with the main goal of preventing yield loss (or increasing yield) and hence maximizing economic returns. In North America, the fungicides used to control foliar fungal diseases of wheat belong to two major classes with a broad spectrum of activity against fungal pathogens. These are the strobilurins and triazoles. Fungicides in both classes are used as foliar fungicides and seed treatments. The strobilurins are named in recognition of a mushroom, *Strobilurus tenacellus,* the original source of the chemical compound that formed the basis of the chemistry of this fungicide class. They are quinone outside inhibitors (QoI) and work by interfering with energy production in fungi (Vincelli, 2002). They act as local systemics by inhibiting fungal spore germination and early infection, and are highly effective when applied preventively. The strobilurins have a single-site mode of action. Examples of strobilurin fungicides used in cereal crop production in North America are azoxystrobin, pyraclostrobin and trifloxystrobin.

The triazoles are characterized by having a five-membered ring of two carbon atoms and three nitrogen atoms. They are curative and move systemically through the plant xylem. Triazoles slow fungal growth through the inhibition of sterol biosynthesis (Buchenauer, 1987). Sterols are essential building blocks of fungal cell membranes and are inhibited at a single site by triazoles. Because of their curative activity against early fungal infections and their ability to redistribute in the crop, triazoles are highly effective and reliable (Hewitt, 1998). Examples of triazoles used in cereal crop production in North America are metconazole, propiconazole, prothioconazole, and tebuconazole.

In the Great Plains of the United States, the most common foliar diseases of winter wheat are leaf rust (*Puccinia triticina*), powdery mildew (*Blumeria graminis* f. sp. *graminis*), tan spot (*Pyrenophora tritici-repentis*) (anamorph: *Drechslera tritici-repentis*), Septoria tritici blotch (*Mycosphaerella graminicola*) (anamorph: *Septoria tritici*), spot blotch (*Cochliobolus sativus*) (anamorph: *Bipolaris sorokiniana*), and Stagonospora nodorum blotch (*Phaeosphaeria nodorum*) (anamorph: *Stagonospora nodorum*). Stripe rust (*Puccinia striiformis* f. sp. *tritici*) and stem rust (*Puccinia graminis* f. sp. *tritici*) also occur, but less commonly.

Yield Response to Foliar Fungicide Application in Winter Wheat 229

winter wheat. They found that cultivar specific economic benefits were associated with improved wheat quality from fungicide treatment. Ransom and McMullen (2008) showed that within an environment and averaged across winter wheat cultivars, fungicides improved yields by 5.5 to 44.0%. Tebuconazole applied at Zadoks growth stage (GS) 37 (Zadoks, 1974) and propiconazole applied at GS 37 followed by triadimefon + mancozeb at GS 55 to control leaf rust and Septoria tritici blotch consistently resulted in the lowest

In the Great Plains region of the U.S., the prevalence, incidence, and severity of tan spot and other residue-borne diseases such as spot blotch and Septoria tritici blotch have increased over the last several decades due to a shift toward conservation tillage practices that leave crop debris on the soil surface (Watkins and Boosalis, 1994). The damage caused by these and other foliar fungal diseases has promoted the use fungicides in winter wheat

Fungicides are generally applied to winter wheat 1-2 times per season. Some farmers apply a fungicide early in the growing season during the stem elongation growth stage to control early season diseases such as tan spot. Often these early fungicide applications are done in combination with herbicide or fertilizer application. A second fungicide application is usually timed to protect the flag leaf. A high risk of Fusarium head blight may necessitate a third fungicide application at early flowering. Results from previous studies on the effect of fungicide application timing on yield in winter wheat have been inconsistent. Some studies have demonstrated yield loss from early season infections and a benefit from early fungicide application in winter wheat. Shabeer and Bockus (1988) found that about 17% of total yield loss from tan spot occurred from early season infections. Marroni et al. (2006) found that the lowest area under the disease progress curve (AUDPC) and the best level of protection against early season Septoria tritici blotch were achieved with azoxystrobin applied at the pre-stem extension stage of crop growth. They also found good control of the disease when a mixture of azoxystrobin and epoxiconazole was applied at the pre-stem extension stage or at the stem extension stage. Cromey et al. (2004) found no consistent effects of crop growth stage when the fungicides azoxystrobin and tebuconazole were applied at three alternative growth stages between flag leaf emergence and flowering to control *Didymella exitialis* (anamorph: *Ascochyta spp.*). Bockus et al. (1997) found the optimum timing to be between the boot and the fully headed growth stages. Duczek and Jones-Flory (1994) found the optimum timing to be between extension of the flag leaf and the medium milk growth stages. Wiersma and Motteberg (2005) found that across cultivars, the optimum timing for foliar fungicide application was GS 60 rather than GS 39. Because of the inconsistent results from previous studies, experiments were conducted in Nebraska, USA to investigate the effects of fungicides and fungicide application

disease severities and highest winter wheat yields (Milus, 1994).

**3. Timing of foliar fungicide application in winter wheat** 

timing on disease severity, yield and economic returns in winter wheat.

The methods used in field experiments have been described previously (Wegulo et al., 2009;

production in the region.

**4. Methods** 

**4.1 Field experiments** 

Wegulo et al., 2011).

The magnitude of yield loss caused by these diseases in winter wheat is variable and depends on several factors including environmental conditions during the growing season, cultural practices, and cultivar resistance. Leaf rust occurs every year in the wheatproducing regions of the U.S. In 2007, severe epidemics of leaf rust occurred in the Great Plains region of North America, causing yield losses of up to 14% (Kolmer et al., 2009). Stripe rust is more frequent in the western U.S., especially the Pacific Northwest (Sharma-Poudyal & Chen, 2011). However, it can be widespread in certain years, as in 2010 when severe epidemics occurred throughout the wheat-producing regions of North America. Yield losses of up to 74% due to stripe rust have been documented in experimental fields (Sharma-Poudyal & Chen, 2011). Stem rust has been effectively controlled in the U.S. through genetic resistance and eradication of barberries (*Berberis vulgaris* and *B. Canadensis*), which act as alternate hosts. Stem rust has the potential to cause 100% yield loss (Murray et al., 1998). Powdery mildew occurs wherever wheat is grown and is common where high humidity prevails during the growing season. Yield losses of up to 25% due to powdery mildew have been reported (Murray et al., 1998).

Spot blotch occurs commonly in the Great Plains of the United States (Murray et al. 1998). The causal agent, *C. sativus*, also causes common root rot and seedling blights in wheat. Spot blotch often occurs together with tan spot (Duveiller et al., 2005). In wet growing seasons, Septoria tritici blotch also can occur as part of this foliar disease complex. This leaf spot disease complex is favored by cultural practices that leave crop residue on the soil surface (Watkins & Boosalis, 1994). Yield losses of up to 50% have been documented to be caused by these leaf spot diseases in winter wheat (Murray et al. 1998; Villareal et al., 1995; Wegulo et al., 2009).
