Fungicides for Control of Gray leaf spot

Gray leaf spot (GLS) is a potential threat to corn production each year in Ohio. Some areas of the state are more prone to yield losses from this leaf disease than others due to the predominating production practices and environmental conditions. Generally, the severity of the disease is related to reduced tillage, especially in fields planted continuously to corn. During years with exceptionally warm and humid weather, gray leaf spot can become important throughout the state as in 1995.

Control of the disease has been achieved through the use of resistant hybrids, tillage to destroy corn residues and adequate crop rotation with soybeans, wheat or forage crops. Where conservation tillage is used, a two-year rotation away from corn is necessary because the fungus survives in corn residues left on the soil surface for up to two years.

Fungicides are available for use on corn to control GLS, but the economics of corn production have not permitted common use of these materials except on sweet corn, popcorn or seed corn. Fungicide applications have been evaluated in Ohio for control of GLS on field corn. During the growing seasons of 1996 through 2000, the yield response due to fungicide application on susceptible hybrids ranged from 2 to 32 bu/A. Obviously, the greatest yield responses occurred during years with highest disease pressure (1996 = 27 bu/A, 1997 = 2 bu/A, 1998 = 32 bu/A, 1999 = 19 bu/A, 2000 = 2 bu/A).

There are four fungicides labeled for control of GLS on corn, Tilt 3.6 EC, PropiMax EC, Quadris 2.08 SC and Stratego. Tilt (2.0 - 4.0 fl oz/A) and PropiMax (4.0 fl oz/A) are labeled for control of several foliar diseases through the end of silking (R2 growth stage). Quadris Flowable Fungicide received a federal label for control of several foliar diseases on corn at 6.2-9.2 fl oz/A applied at disease onset, and can be used until 7 days before harvest. Stratego Fungicide recently received a federal label for several foliar diseases on corn at 10.0 fl oz/A applied at the V10 to V14 growth stage. Other directions and restrictions are described on the labels.

These fungicides are labeled for more than one application during the growing season, but due to the relatively low price of corn, multiple applications during the growing season is cost prohibitive. In Ohio and other states, two applications have provided better disease control than one, but in most tests there was no significant additional yield benefit from the second application. It appears that the most economical use of these products on susceptible hybrids is a single application. Tilt has provided the best results when applied at the V14 to R2 growth stage using the 4.0 fl oz/A rate. Quadris and Stratego appear to benefit from a slightly earlier application. Apply Quadris from V10-V14 growth stage using 9.2 fl oz/A. The label suggests that an adjuvant be added to improve coverage. When using ground equipment add drop nozzles to improve coverage of the leaves at and above the ear. Apply Stratego from V10-V14 growth stage using 10.0 fl oz/A. We recommend treating susceptible hybrids, not hybrids with some degree of resistance to GLS.

In several (though not all) tests, Quadris and Stratego have provided superior disease control to that provided by Tilt. Thus, in terms of efficacy against gray leaf spot, these products may prove to be the better choice. However, because Quadris is used at an application rate over twice that of Tilt, the per-acre cost of Quadris is about twice as high as that of Tilt. The cost of Stratego is only slightly higher than Tilt

Fungicide efficacy for control of Gray leaf spot in Corn

Fungicide*	Rate	Application Timing
Tilt 3.6EC	4.0 fl oz/A	V14 – R2
PropiMax EC	4.0 fl oz/A	V14 – R2
Quadris 2.08SC	9.2 fl oz/A	V10 – V14
Stratego	10.0 fl oz/A	V10-V14

*Read label and follow all instructions