The Value of Fungicides in High-Yield Wheat Production

Summary:

• Fungicides are useful tools that can significantly protect yields
• Applications that target the flag leaf, leaf below the flag leaf, and ear, can protect up to 90% of total carbohydrates needed for grain fill
• Early applications with Nitrogen will protect the lower/mid canopy for approximately 3 weeks, but will not protect the flag leaf and above
• Applications made at flowering to target Fusarium head blight are efficacious against common fungal foliar diseases

Recent increases in commodity prices have resulted in many farming asking if fungicide use can optimize crop yield. This article will go over the basics with fungicide use in wheat and considerations. These are general guidelines. Regularly scouting fields can help determine which timing(s) will work best for your production acres.

The first thing we need to think about is what is the current yield potential? Fields with excellent tillering and establishment have greater potential for greater yields, making the choice of applying a fungicide a little easier. Obviously the higher the value of the crop, the easier it is to achieve a ROI. The first step therefore is to get out of the office and check those fields. Assess stands and quality and get a feel for what total yields may be achievable in that field given what you observe.

The second thing we need to know is the variety. Why do we need to know the variety? Varieties differ not only in yield potential but also susceptibility to different diseases. Varieties with good defensive packages against common foliar diseases encountered in the region are less likely to respond to a fungicide application, simply because the potential yield loss due to the fungal pathogen in question is low. The second thing to consider is the growth stage of the crop. Figure 1 shows the general stages of wheat growth and development. At full maturity, roughly half of the total plant dry weight is contained within the grains. This indicates that most of the photosynthate needed for grain fill is produced around and after flowering. The flag leaf (F) and F-1, and ear (Glumes + awns) together are estimated to contribute 70-90+ percent of the carbohydrates needed for grain fill. Therefore, ensuring that the tissues above F-1 are green and healthy is paramount.

Figure 1. Wheat growth stages. Fungicides are typically applied between Feekes growth stage 5 and 10.5.1, but the target and purpose of these sprays may differ.

What are the foliar diseases we may encounter and the potential for yield loss?

Powdery mildew (PM)

This is a fungal disease that likes it humid, but not wet. Recent failures in specific resistance genes (Pm6) have resulted in this disease becoming more problematic in some areas, especially the southeast / Mid-Atlantic region. Powdery mildew typically occurs in patches in the field that coincide with wetter, shaded, or low-lying areas. This disease likes it very cool (60ish°F) and therefore tends to be more problematic in dense plantings earlier in the season. Fungicide timing: FGS 5-9

Speckled leaf blotch (SLB)

This fungal disease is probably the most frequently encountered in wheat fields. Similar to powdery mildew, it does best under cool conditions, and rarely moves into the upper canopy. This often is not a disease that reduces yield to the degree that it would cover the cost of a fungicide application. Fungicide timing: FGS 5-9

Leaf blotch complex (LBC)

This includes Septoria leaf blotch and tan spot. Both like it warm and wet, and typically move up the canopy during wet periods. Both can be observed throughout the plant canopy. Fungicide timing: FGS 8-10

Stripe rust

This one is a cool season disease (60-65 F) that blows into midwestern and Mid-Atlantic regions from warmer states/areas. This rust has a rapid developmental rate and can be devestating to susceptible wheat varieties under cool, wet conditions. Fungicide timing: FGSs 8-10.5.1, highly dependent on arrival to field relative to growth stage and susceptibility of crop.

Leaf rust

This one prefers warm, humid conditions. In general, we see leaf rust start up after flag leaf emergence in susceptible cultivars, and the disease can continue to develop and spread with persistent wet weather. Fungicide timing: FGS 8-10.5.1

Septoria glume blotch (SGB)

This is the same pathogen as the one that produces Septoria leaf blotch. Spores infect at the glumes and may move into the grain, resulting in shriveled and discolored grain, as well as reduced yields. Resistance to the leaf blotch phase does not ensure resistance to the glume phase and vice versa. Fungicide timing: FGS 10.5.1

Fusarium head blight (FHB)

Moderate to hot conditions [70's-upper 80's with persistent wet weather from anthesis (FGS 10.5.1)] through maturity, with infections at initiation of flowering more severe than those later in the season. The FHB pathogen may also contaminate grain with mycotoxins, resulting in quality dockage/potential rejection of the grain on top of potential yield losses. Fungicide timing: FGS 10.5.1

References:
Sylvester,P. Dalla Lana, F., Mehl, H., Collins, A., Paul, P., and N.M. Kleczewski. 2018. Evaluating the profitability of foliar fungicide programs in Mid-Atlantic soft red winter wheat production. Plant Disease 102: 1627-1637.
Willyerd, K.T., Bradley, C.A., Chapara, V., Conley, S.P., Esker, P.D., Madden, L.V., Wise, K.A. and Paul, P.A., 2015. Revisiting fungicide-based management guidelines for leaf blotch diseases in soft red winter wheat. Plant Disease, 99(10), pp.1434-1444.