Gray leaf spot (GLS) is a foliar disease of corn caused by the residue-borne fungus, Cercospora zeae-maydis. This disease has become a widespread problem affecting corn production in the United States over the past two decades. The increase in GLS prevalence has accompanied an increase in the use of conservation tillage for corn production especially in areas that grow continuous corn. Reported ﬁeld losses from production ﬁelds have varied widely with the higher levels ranging from 50% to 80-100%. Field surveys during the late 1980s and early 1990s indicated that GLS was slowly moving to the west and northwest part of the state. The extremely favorable weather conditions during 1995 enhanced the spread of the disease throughout most of the state and resulted in statewide yield losses of up to 10%. However, ﬁelds with economic levels of the disease generally were conﬁned to the southern 2/3 of the state.
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 1995-96, 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 this 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 cooperators in 1995.
Field Plot Locations and Production Practices
In 1995 ﬁeld plots were established at six sites with a history of GLS in four east central counties (Coshocton, Fairﬁeld, Knox and Richland). Hybrids were planted in 4 to 6 row strip plots, at least 300 feet in length. Disease assessments were made two to three times by estimating the percentage ear leaf area covered by lesions using disease assessment scales. The incidence of stalk rot was estimated using the "squeeze" method (i.e. squeezing the stalk above the brace roots and recording the number of stalks that crushed easily). Additionally, a subset of 10 hybrids was also planted at one location in Coshocton Co. in a randomized block design with four replications. Each plot consisted of four rows, 24 feet long. Disease and performance data were recorded for this plot as described above.
In 1996, a similar on-farm study was conducted at six sites in six central and northeastern counties (Ashland, Coshocton, Fairﬁeld, Knox, Ross and Wayne) using hybrids submitted by commercial seed companies. Pioneer 3394 again was used as the check hybrid at each site. Similar procedures were used to obtain disease and production data as in 1995 except disease was assessed 3 to 4 times from early grain ﬁll to 1/2 milk line growth stages.
Weather conditions in 1995 were highly favorable for GLS development. The season was characterized by wet conditions early in the season followed by warm humid weather and associated morning fog that extended into late September. Weather at four of the six sites provided conditions for sufﬁcient disease pressure to evaluate the reaction of test hybrids to GLS. To determine if differences in hybrid strip-plot performance were signiﬁcant, data from these four test sites with 13 hybrids in common was combined and evaluated as a randomized complete block design with four replicate blocks. At each location, the susceptible check hybrid exhibited greater GLS ratings than 11 of 12 hybrids designated as having some resistance (Table 1). The one exception was Pioneer 3335 which had disease ratings not signiﬁcantly lower than the susceptible check. Six hybrids (Asgrow RX770, Pioneer 3335, Great Lakes GL581, DeKalb 634, ICI 8541, and ICI 8321) produced grain yields signiﬁcantly greater than the susceptible check, Pioneer 3394. Moreover, some hybrids did not differ signiﬁcantly in yield from the check hybrid and had GLS levels signiﬁcantly lower than the check hybrid.
|Table 1. Agronomic performance and reaction to Gray Leaf Spot of 13 commercial hybrids evaluated in on-farm strip tests at four locations in East Central Ohio, 1995.|
|Brand/Hybrid||Yield Bu/A @ 15.5% Moisture||% Moisture||Harvest Population Plants/A||% Lodged||% Stalk Rot**||% Ear Leaf Affected (Month-Day)|
|Great Lakes GL581||151||20.5||26370||3||18||2.9||27.9|
|Northrup King N5866 1||40||21.8||26920||1||12||3.3||28.6|
*Values for hybrids are means of four replications.
**Stalk strength determined by "pinch" method.
Relatively dry conditions at the replicated study in Coshocton Co. restricted the development of GLS and possibly affected yield as well. Average GLS levels on Pioneer 3394 reached 47% ear leaf area affected by 21 September, whereas much higher disease levels (61-89%) occurred at on-farm strip plot sites planted on approximately the same date. Although Pioneer 3394 consistently exhibited signiﬁcantly greater disease levels on each assessment date during grain ﬁll than the other hybrids, its yield was not signiﬁcantly different than the top-yielding, more resistant hybrids (Table 2). Apparently, grain ﬁll was complete before most of the foliar damage occurred.
|Table 2. Agronomic performance and reaction to Gray Leaf Spot of 10 commercial hybrids evaluated at West Lafayette in Coshocton Co, Ohio, 1995.|
|Brand/Hybrid||Yield Bu/A @ 15.5% Moisture||% Moisture||Test Wt||Final Stand Plants/A||% Lodged||% Stalk Rot**||% Ear Leaf Affected (Month-Day)|
|Northrup King N5866||176.5*||22.0||55.0||30025||1||13||1.6||10.5||27.5|
|Great Lakes GL581||163.5||21.0||55.8||30325||2||73||2.8||20.7||39.6|
*Values for hybrids are means of four replications.
**Stalk strength determined by "pinch" method.
Weather conditions in 1996 were much different than in 1995. Much less disease developed due to cooler temperatures throughout the season and fewer days with high relative humidity. Four of the six on-farm locations developed similar levels of GLS, but disease development was retarded until relatively late in the season after late dough stage. As in 1995, the susceptible check hybrid developed more disease by the end of the season than any of the other hybrids designated as being resistant, except Pioneer 3335 (Table 3). Six hybrids (Porter 5408, ICI 8342, Pioneer 3352, Northrup King N6800 and N7070, and Dekalb DK634) produced grain yields significantly greater than the susceptible check. Although several hybrids had GLS levels significantly lower than the check, they did not differ significantly in yield from the check.
|Table 3. Agronomic performance and reaction to Gray Leaf Spot of 16 commercial hybrids evaluated in on-farm strip tests at four locations in East Central Ohio, 1996.|
|Brand/Hybrid||Yield Bu/A @ 15.5% Moisture||% Moisture||Harvest Population Plants/A||% Lodged||% Stalk Rot**||% Ear Leaf Affected|
|Northrup King N6800||137.0||26.5||24360||1||38||26.5|
|Northrup King N7070||136.9||23.7||25790||5||42||37.0|
|LG Seeds V2504||130.3||24.2||25155||2||26||31.5|
|LG Seeds V2524||129.2||27.4||25340||3||47||26.7|
*Stalk strength determined by "pinch" method.
**Last assessment for Holmes, Knox and Coshocton County locations 9/25/96; for Wayne County location 9/11/96.
Differences in disease increase over time were detected among hybrids both years. Comparisons of disease progress curves indicate that GLS increases much more slowly on the more resistant hybrids than on the susceptible hybrid. By the 7 September 1995 assessment, most of the hybrids were in the dent stage of development. At this time the more resistant hybrids Asgrow RX770, Great Lakes GL851 and Dekalb DK634, had only one third the percentage ear leaf area covered with lesions as did Pioneer 3394. In 1996, the difference between the more resistant hybrids and the susceptible hybrid was not as dramatic. Disease development was very slow on all hybrids until 11 September when most hybrids were in late dough stage. However, by 25 September at 1/2 milk line stage, the more resistant hybrids, Pioneer 3352, Asgrow RX770, and DeKalb DK634, averaged nearly half the percentage ear leaf area affected as did Pioneer 3394. These results support previous evidence that resistance to C. zeae-maydis is due to factors that reduce the rate of GLS development.
The response of Pioneer 3335 was not typical of the other hybrids in the test. This hybrid had disease levels not signiﬁcantly less than those recorded for the susceptible hybrid by the end of the season (Table 1 and 3), but its yield was not signiﬁcantly less than the more resistant hybrids in 1995 (Table 1). This hybrid is apparently tolerant to C. zeae-maydis as evidenced by its relatively high yield in spite of high disease levels.
Results of 1995-96 strip tests indicate that there are commercial moderately resistant hybrids available that have higher yield potential than the susceptible check when GLS pressure is severe (Table 1). However the results of the replicated plot test at West Lafayette in 1995 (Table 2) suggest that when GLS development is less severe, resulting in less leaf blight or developing late in the season after grain ﬁlling is complete, certain resistant hybrids may offer no yield advantage over popular susceptible types. Nevertheless some of the hybrids with low disease levels exhibited yields that were not signiﬁcantly different from the susceptible check hybrid suggesting that there are hybrids available now that perform well under varying levels of GLS pressure.
- Peter Thomison, Extension Agronomist, Horticulture and Crop Science
- Patrick Lipps, Extension Plant Pathologist, Plant Pathology
- John Barker, Ext. Agt., Agr. & Nat. Res, Knox Co.
- Allen Geyer, Research Assistant, Horticulture & Crop Science
- Ray Grifﬁth, FFA/Riverview High School, Coshocton Co.
- Dave Jordan, Supervisor, Ohio Corn Performance Test, OARDC
- Howard Siegrist, Ext. Agt., Agr. & Nat. Res. Licking Co.
On-Farm Test Cooperators
Darren Carroll, Ross Co.; Mike Lemmons, Knox Co.; Dave Mauer, Wayne Co.; Dave Miller, Fairﬁeld Co., and Kenny Stitzlein, Ashland Co.
Acknowledgements: We are grateful for the assistance provided by Sandy Thomas and Dennis Wickham, Pioneer Hi-Bred Int'l., Inc.