Principal Investigators:

Abstract:

Although hybrid resistance has been a very effective means for control of a number of corn diseases, the level of resistance to GLS currently available in commercial hybrids is limited. All hybrids currently on the market will develop high levels of GLS when the fungus is present on corn residue within the field and the environmental conditions are highly favorable for disease development. The resistant hybrids suitable for production in Ohio are considered to be moderately resistant at best. Since GLS may not be a problem every year it is important to consider how resistant hybrids will perform in the absence of disease pressure. Will resistant hybrids be competitive with more susceptible hybrids when GLS is not a major limiting factor? In the past some hybrids characterized as GLS resistant, especially those of early to mid maturity, were not used because of their relatively low yield potential. Growers were often willing to risk some degree of GLS damage in order to use a higher yielding but more susceptible hybrid.

In 1996 we conducted field tests to evaluate the performance of a limited number of corn hybrids known to have some degree of resistance to GLS with maturities acceptable for production in Ohio. The goal of our study was to help corn growers identify profitable corn hybrids for use in regions of the state where GLS can be a major yield limiting factor.

Commercial seed corn companies were requested to submit only those hybrids known to have some level of resistance to GLS and to be of a maturity acceptable for production in Ohio. Pioneer brand 3394 was chosen as the check hybrid because of its high yield potential, adaptability, popularity and GLS susceptibility. In several on-farm strip tests additional hybrids were included by the cooperator.

Field plots were established at six on-farm sites in Wayne, Holmes, Coshocton, Knox, Fairfield and Ross Counties that have a history of heavy gray leaf spot pressure. Plots were located in fields which have been continuously cropped to corn. Hybrids were planted in strip plots at least four rows wide (in 30-inch or 38-inch row spacings) and 300 feet in length.

A set of 20 hybrids compared in the on-farm strip tests (described above) was also planted at the Ohio Corn Performance Test site at West Lafayette in Coshocton county, a site previously cropped to corn with a history of gray leaf spot. The experimental design was a randomized block with four replicate plots. Each plot consisted of four 30-inch rows 25-feet in length.

Disease assessments were made in each plot location two to three times at approximately two week intervals. Assessments were made on 21 August, 29 August, 11 September, and 25 September, but each plot was not assessed on each date. Hybrids were assessed for GLS by estimating the percentage of the ear leaf covered by lesions on six plants per plot. Disease assessment scales were used to estimate severity percentages. The average severity was calculated for the six plants per plot and plot means were used for statistical analysis. Area under disease progress curve (AUDPC) was calculated from percentage ear leaf are affected to determine differences in disease increase over time. The incidence of stalk rot was determined using the "squeeze" method, i.e. squeezing the stalk above the brace roots and recording the number of stalks that crushed easily. At least 10 plants in each plot were checked for stalk rot. Stalk lodging (stalk breakage below the ear), test weight, and grain moisture were recorded at harvest and yields were adjusted to 15.5% moisture.

None of the hybrids were highly resistant to GLS. Hybrids with resistance delayed onset of the disease and this allowed plants to retain more green leaf tissue during grainfill.

As of 5 December, 1996 certain agronomic performance data including grain yield had not yet been obtained for several test sites due to harvest delays caused by inclement weather. Table 1 shows disease reactions of hybrids common to four of the test sites in Wayne, Holmes, Knox and Coshocton Counties. In each of the strip plot evaluations, the susceptible check exhibited higher GLS disease ratings, as much as two times more leaf tissue affected, than hybrids characterized as having some resistance. One of these hybrids, Pioneer 3335, had disease ratings that were not significantly lower than that of the check.

TABLE 1. Reaction of commercial hybrids to gray leaf spot in on-farm strip plot test at four locations in east central Ohio, 1996**.

	Ear Leaf
Brand/Hybrid	Affected	AUDPC**
Pioneer 3352	16.3	5.9
ICI 8342	22.7	7.6
Northrup King N6800	26.5	8.4
Porter 5111	25.5	8.5
Doebblers 75X-2	24.8	9.9
LG Seeds V2504	31.5	10.6
LG Seeds V2524	26.7	10.5
Asgrow RX701	33.4	10.7
Doebblers 66XP	31.1	10.6
Porter 5408	33.9	10.9
ICI 8541	32.5	10.4
DeKalb DK634	37.3	11.6
Northrup King N7070	37.0	12.6
Asgrow RX770	34.0	11.9
Pioneer 3335	50.5	16.7
Pioneer 3394	57.2	21.1
LSD (P = 0.05)	10.7	5.3

*last assessment for Holmes, Knox, Coshocton Counties on 25 September , 1996 and for Wayne County on September 11, 1996. All assessments were estimates of the percentage ear leaf area affected.

**AUDPC = Area under disease progress curve

Extension Program Implementation:

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