University of Illinois

No. 16/July 10, 1998

Fungicides for Gray Leaf Spot

Gray leaf spot is a significant concern to many dent corn producers. Since the fungicides Tilt and Penncozeb 80WP are labeled for this disease, producers sometimes ask where these products fit in Kentucky grain farm operations. Penncozeb and other mancozeb-containing products have performed inconsistently for gray leaf spot control on corn, so most of this discussion will focus on Tilt fungicide.

Which fields are at risk of yield loss from gray leaf spot? Infested corn leaf residue is the source of infectious spores of the fungus (Cercospora zeae-maydis) that causes the disease. Damaging spore levels occur very often in no-till, continuous corn fields; these are high-risk fields. Moderate-risk fields include the following: no-till fields that were in corn two years ago and were in soybeans or wheat/double-crop soybeans last year; and fields in corn last year that were disced only (if 35+ percent residue cover remains). Levels of primary inoculum are lower in these fields than in high-risk fields, although damaging outbreaks of gray leaf spot can occur, depending on hybrid, planting date and weather during the growing season. Fields with little to no risk include: no-till fields having two growing seasons without corn; disced fields with no corn last year; and fields that were moldboard-plowed, regardless of cropping history.

The only "wild card" about these comments is that corn residue from a neighboring no-till field planted to a susceptible corn crop last year can serve as a significant source of inoculum for corn planted up to 300-500 feet away, or for any field if it is planted very late (say, in June).

Can the disease be controlled adequately with hybrid selection? More corn hybrids become available each year with enough partial resistance against gray leaf spot for adequate yields. Compared to a susceptible hybrid, a corn hybrid that has partial resistance to gray leaf spot exhibits smaller lesions, a delay in lesion development, and/or reduced sporulation in lesions. While disease development is not prevented completely, partial resistance has the effect of slowing disease progress. Since gray leaf spot is principally a disease of mid- to late grain fill, resistance which simply slows disease progress can provide for acceptable yields. The greatest availability of hybrids with partial resistance to gray leaf spot is among medium-maturity and full-season hybrids.

The majority of the seed corn in stock for the 1998 season in Kentucky appears to have at least moderate resistance to gray leaf spot. For example, 77 percent of the seed stock of Pioneer Hi-Bred has a rating of "5" or higher (scale of 1-9, where 9 = excellent resistance) against the disease. Studies have shown that expected yield benefits of applying fungicides following the label directions on hybrids with these levels of resistance are generally minimal to nonexistent, even with multiple applications. Keep in mind that data used to demonstrate a substantial economic value from Tilt are usually collected in studies from no-till, continuous corn where a susceptible hybrid is planted, not a hybrid with moderate resistance.

Probability of favorable net return from fungicides:

As far as factors to consider, I’ve already mentioned the influence of crop rotation, tillage, and hybrid susceptibility on gray leaf spot development. In addition, bear in mind that late planting increases pressure from the disease. Also, field location is important. A field with good air movement will have less disease pressure than a foggy field along a creek. Coverage of the leaves is an issue, as well. On a scale of one to four, where "four stars" is excellent, Tilt is a "three-star" product against gray leaf spot. It is a good product, but not an excellent product. Furthermore, while Tilt is a systemic, its mobility in plant tissues is not as high as some systemic fungicides, so don’t count on systemic movement to compensate for poor spray coverage. Poor spray coverage (low gallonage and pressure, poor nozzle arrangement) can be expected to compromise the performance of this product.

Penncozeb, which contains the active ingredient mancozeb, provides erratic performance against this disease, since adequate coverage of this contact fungicide is very difficult to obtain using commercial equipment.

Keep in mind that the number of applications also affects profitability of fungicide usage. The research seems to indicate that most of the benefit of Tilt comes from a single, well-timed application rather than several applications. Finally, the weather and disease development pre- and post-silking determine whether is really pays to apply fungicide.

In studies conducted to date under commercial or near-commercial conditions, Tilt provided protection against gray leaf spot yield losses that ranged anywhere from 0 bu/A to 30 bu/A. In other words, some producers may see no benefit to using the product (even in high-risk fields), while other producers may avoid a 20-30 bushel loss with a susceptible hybrid in a high-risk field. There is no way to know ahead of time whether a producer will get back more than the cost of the application, since that depends on so many unknown factors, like weather, disease buildup, corn prices, etc.

Keep in mind that using a fungicide like Tilt doesn’t increase yields. All it can do is protect the yield potential of the field from losses due to one or more diseases. While one cannot predict the economic return from using Tilt in any given field, it is possible to identify fields representing extreme cases, where Tilt is highly likely or highly unlikely to be worth the cost of application.

Field A, where Tilt is highly LIKELY to be profitable:

The crop is planted no-till into corn residue in a field along a creek. The hybrid is susceptible to gray leaf spot. (For example, if the company rates their hybrids on a scale of 1 to 9, where 9 = highly resistant, we’ve planted a hybrid with a rating of 1, 2, or 3. For a company that rates their hybrids on a scale of 1 to 5, where 5 = highly resistant, we’ve planted a hybrid with a rating of 1 or 2.) The crop was planted during the first week of May. Tilt was applied once in 50 gallons per acre at 100 psi using three nozzles per row, with nozzles directed to cover the ear leaf and above.

Field B, where Tilt is highly UNLIKELY to be profitable:

The crop is planted no-till into soybean residue in an upland field. On a scale of 1 to 9, where 9 = highly resistant, the hybrid has a rating of 5 or higher. On a scale of 1 to 5, where 5 = highly resistant, the hybrid is rated 4 or 5. The crop was planted during the first week of April. Tilt was applied twice in 15 gallons per acre at 50 psi using one nozzle over the row.

Where does Tilt fungicide fit for a corn producer? Its principal use is probably in specialty corns where the hybrid is susceptible to gray leaf spot, a premium is being placed on grain quality, and the field is a moderate to high risk. It probably has value for susceptible inbreds in the few seed production fields in Kentucky. Given the rotation practices of most farms and the hybrids available for this season, most typical grain fields probably won’t benefit much, if at all, from using Tilt. Producers who are careful observers will, of course have a feel for which fields on their farms have high disease pressure most years. Producers who are unsure as to the level of gray leaf spot pressure should contact their Extension educator for information on identifying and scouting for the disease.

If a producer has decided to use Tilt, how can it best be used? If Tilt is to be used, spray the crop shortly before tasseling. The product is labeled for application prior to and through mid-silking. When silks on 50 percent or more of the plants have turned brown, it is illegal to use Tilt. Furthermore, UK studies show that efficacy declines significantly when applied after silking. High-clearance ground sprayers should be fitted with drop nozzles aimed for good coverage of the ear leaf and above. Keep application volume and pressure as high as possible (at least 50 gallons per acre at 100 psi or more). Aerial applicators should respect the 5 gallons-per-acre minimum volume indicated on the Tilt label; lower spray volumes could result in inadequate control.

What is the cost of treating with Tilt? At 4 fl oz per acre, Tilt costs about $10/acre for the material per application. Add about $4-5/acre to that for application costs.

Should the producer scout for the disease before deciding whether to treat? Ideally, yes. A very conservative spray threshold for susceptible hybrids is to consider treating if at least 50 percent of plants examined have gray leaf spot lesions (3/8 inch or longer in length) on the third leaf below the ear leaf by tasseling. Even if this threshold is reached, Tilt applications may not provide any economic benefit, since so many factors determine this. However, this is the guideline I suggest for the 1998 season, and it is very conservative--a producer who wishes to treat with Tilt will not forego a necessary application using this guideline.

Paul Vincelli, in KY Pest News, April 20, 1998