Decision Time For Foliar Fungicides on Wheat

By Jane Fyksen, Crops Editor
Thursday, June 5, 2008 9:30 AM CDT

Lured by strong prices, some Wisconsin growers have stepped up acres devoted to winter wheat. Some, who've grown it in the past, are returning to the crop. Others are growing winter wheat for the first time. UW-Madison wheat specialist Shawn Conley and UW-Extension plant pathologist Paul Esker are keeping growers briefed on winter wheat issues as they leaf out.

By the middle of last week, winter wheat was at or near the flag leaf (Feekes 8) growth stage pretty much throughout the state. It's an important time to scout wheat fields to determine whether foliar fungicide will need to be applied.

The main diseases Conley and Esker have seen so far include Septoria leaf blotch, powdery mildew and tan spot. So far, the incidence and severity of these three diseases have been quite variable and dependent on wheat variety and environment. With susceptible varieties, they've seen Septoria leaf blotch and powdery mildew as high as 100 percent in some of their plots.

It's during flag leaf emergence that questions need to be asked to determine if foliar fungicide is in order. This UW winter wheat team advises growers to ask four questions:

1) Is there a fungal disease or diseases present?

2) Does the variety planted have resistance, and to which wheat diseases, or does it appear that disease pressure in the canopy is increasing?

3) Does the expected crop yield warrant the cost of applying a foliar fungicide?

4) Is the crop under stress?

If your first three answers are "yes," even if the answer to the last question is "no," application of a foliar fungicide will most likely be warranted.

Besides identifying foliar diseases in wheat, it's important to understand if disease pressure is increasing. At http://soybean.uwex.edu/documents/FlagLeafEmergenceandFoliarFungicidesinWinterWheat.pdf, they share two examples - one of rust, the other for powdery mildew - that can be used to determine the amount of disease on leaves.

Further, if it appears disease is moving into the upper canopy, that's another indicator disease is increasing.

It should be noted though that not all wheat diseases, especially viruses and Take-All, will be impacted by foliar fungicide applications.

Many different approaches can be taken for sampling wheat fields, but it's "absolutely necessary" to avoid just sampling field edges or worse yet, from the vehicle.

Noting growth stage is important, as many of the foliar fungicides labeled for wheat have specific requirements as to the last growth stage when an application can be made. Scout the entire field - 10 locations within a field and 10 plants per location, avoiding field edges. The 10 plants should be selected randomly; simply close your eyes, reach into the wheat and assess the first plant you grab. This will help avoid a biased sample, as "our eyes often naturally see the lesions and flecks, and we would want to only assess 'diseased' plants," they say. Do not expect to just find one disease in the field.

A field assessment is average disease severity across all sampling locations and plants. For example, incidence is the number of plants with a specific disease divided by total number of plants assessed (here, 100), while severity would be the average area of the leaf covered by foliar diseases (again, averaged over the 100 plants assessed). For example, 22 of 100 plants have leaf rust (22 divided by 100 is a 22 percent incidence), and say those 22 plants has 10 percent of the leaf area covered for 10 percent severity.

As noted proper scouting, especially for the flag leaf, is crucial for determining if a foliar fungicide application is warranted. The flag leaf, which starts to become visible during Feekes 8, is the most important leaf in terms of yield, as upwards of 50 percent or more of the final yield depends on its health. The presence of disease on the flag leaf may indicate it's too late for a fungicide to reduce the effects of a disease.

Septoria leaf blotch initial symptoms are light green to yellow spots between the leaf veins on the lower leaves that are in contact with the soil. Under favorable conditions, symptoms elongate, forming irregularly shaped lesions that are tan to red-brown, parallel to one another. As lesions age, black speckles (fruiting bodies that contain spores) can be seen on the lesion; those black speckles are a good diagnostic sign.

There are two phases to this disease, the first during the fall in wheat just sown, the second in the spring and summer on the upper leaves. The inoculum can survive two to three years in residue. Infection is favored by cool conditions (59 to 68 degrees). Also, at least six hours of leaf wetness is required for infection, with maximum infection when there is 48 hours leaf wetness.

Certified, disease-free seed that has been treated with seed fungicide reduces the risk of infection. Some wheat varieties carry moderate levels of resistance. Rotation helps reduce pathogen carryover, but requires at least two years out of wheat. The use for foliar fungicides can be beneficial if the weather is favorable for disease development.

Powdery Mildew is a powdery white to gray fungal growth that can occur on leaves, stems and heads. Typically, pustules are first detected on the lower leaves, and as the plant matures, the white growth becomes gray-brown. Also, leaf tissue on the opposite side of the leaf from the white mold growth becomes yellow, later turning tan or brown. Late in symptom development, small, black fruiting bodies can be found that serve as the inoculum that further infects wheat. Infection can often occur first in the fall. Spores are dispersed by wind and infection is favored by cool (59 to 71-degree) wet weather. This disease tends to be more severe in dense stands of heavily fertilized wheat.

Soft red winter wheat generally has resistance to powdery mildew, but the level of resistance can be variable. Fungicide seed treatments can help reduce the risk of infection during the fall. Also, foliar fungicides in the spring are effective for managing this disease, especially when applied between Feekes 6 (first detectable node) and stage 8 (flag leaf first visible).

Severity increases under high rates of nitrogen, so balanced fertility can help reduce the disease.

Tan Spot shows up, as the name implies, as small tan spots. The characteristic symptom is their round to slightly elongate shape, surrounded by a yellow halo. The center is a dark brown spot that often appears diamond-shaped. When lesions aggregate, the leaf may appear yellow. As the plant matures, the fungus will invade the straw, where tiny black, raised fruiting structures are formed. In severe infections, a red smudge symptom can occur on the seed, leading to a downgrading in quality.

Initial infection of wheat occurs under cool, cloudy, humid weather with frequent rains early in the spring. Also, infection of wheat seed has been found to be positively correlated with the severity of tan spot. Foliar fungicides are effective, although application may be required earlier than for rusts. Tillage and rotation can help reduce survival and infection.

Photos of these diseases and others that impact wheat are at http://ipcm.wisc.edu/Portals/0/Blog/Files/17/488/WCM_15(5).pdf.

These specialists say that the decision to apply a foliar fungicide shouldn't be driven by the current market alone. Again, it's critical to identify growth stage. The major disease of concern in Wisconsin that may be controlled by fungicides include: rust (leaf, stripe or stem), Fusarium head scab, powdery mildew, Septoria leaf blotch, Glume blotch and tan spot.

Further, understand your fungicide of choice, especially pre-harvest interval, particularly if wheat straw will be used as a forage. Not all fungicides have a feeding restriction, they say, but it's still important to look closely at the label. For instance, Quilt has a specific use restriction, they note, not to harvest the wheat for forage, or graze or feed livestock treated forage or cut green crop for hay or silage.

Another factor in the decision to apply fungicide is "the cost in bushels per acre of the wheel tracks" following an application. UW researchers don't have data but will be conducting work in this area this year to define yield loss due to wheel track damage in wheat. Given a sprayer with 13-inch tires, they assume for now that you'll run down four rows (two for each tire pass). In a 90-foot spray pass, there are 144 7.5-inch rows. If you lose four of those rows (four divided by 144) that's 2.8 percent of the area that's run down. If you normally raise 70 bushel wheat, the yield loss (assuming all plants are killed) is 70 bushels times 2.8 percent or 1.96 bushels. That's probably worse-case scenario though.

"This estimate assumes we spray with the rows and not perpendicular to the rows. If we spray across the wheat rows, yield loss is decreased," they report, adding, too, that they don't factor in any compensation from adjoining rows, which may or may not occur depending on the application timing and year.

They suggest the decision to apply a fungicide "must be made for more than prophylactic control." Variety differences should also be noted for disease resistance. The P 25 percent 78 has excellent leaf rust resistance but moderate powdery mildew resistance, while Kaskaskia has resistance to some races of leaf rust but is susceptible to stem rust and powdery mildew.

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