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NEWS RELEASE
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FULL REPORT
from the NCR-25
Technical Committee
PHOTO
GALLERY
A small collection
of images portraying
gray leaf spot in a variety
of disease stages
Gray Leaf Spot of Corn: Update
A Report from NCR-25 Technical Committee
on Corn and Sorghum Diseases
Prepared by
Patrick E. Lipps, The Ohio State University;
Donald G. White, University of Illinois;
John E. Ayers, The Pennsylvania State University; and
Larry D. Dunkle, USDA-ARS, Purdue University
March 23, 1998
In February 1996, the NCR-25 Corn and Sorghum Diseases Technical Committee met in conjunction with the 7th Interregional Corn Improvement Conference in St. Louis, MO, following a major epidemic of gray leaf spot that affected the United States Corn Belt. Following that meeting, a subcommittee developed a report summarizing the most recent information on GLS. The report was distributed to interested persons and groups for use in answering questions concerning the disease. In the intervening two years considerable research has added to our knowledge of GLS, its causal agent and its global distribution. It is the intention of this report to provide an update emphasizing the most recent developments.
DISTRIBUTION: Gray leaf spot, a foliar disease of corn caused by the fungus Cercospora zeae-maydis, was first described in 1925 from lesions found on corn plants in Alexander County, IL. The disease was later reported in the 1940s and 1950s in Kentucky, North Carolina, South Carolina, Tennessee and Virginia. In the 1960s and 1970s, the disease became of concern in the eastern United States particularly in the intermountain regions of Pennsylvania, West Virginia, Maryland, Virginia, North Carolina, South Carolina, Kentucky and Tennessee. As reduced tillage became more popular in the 1980s and 1990s, gray leaf spot became common in most of the corn growing areas of the midwestern and eastern United States. Today the disease can be found as far west as eastern Colorado, Kansas and Nebraska in corn fields under irrigation and north into Wisconsin and Minnesota. The disease has become distributed internationally being reported in: Mexico and other Central American countries; Brazil, Columbia, Venezuela, Ecuador and Peru in South America; in most Sub Sahara countries of Africa including Cameroon, Zambia, Tanzania, South Africa, Malawi, Swaziland, Kenya, Uganda, Zaire, Zimbabwe; and in eastern countries including India, China, Southeast Asia and the Philippines.
EPIDEMIOLOGY: An understanding of the epidemiology of gray leaf spot is helpful in explaining why the disease has increased in intensity, severity and distribution. C. zeae-maydis overwinters in the debris of previously diseased corn plants remaining on the soil surface. In spring, conidia (spores) are produced and disseminated to corn plants by wind and rain splashing. They require several days of high relative humidity to successfully germinate and infect corn leaves. Several weeks may be needed for the development of mature lesions on leaves. Conidia have been trapped over crop debris as early as May, but abundant spore production begins in June. C. zeae-maydis is distinctly different from many other foliar pathogens of corn in that it requires a longer time to penetrate leaves and to produce mature lesions yielding another generation of conidia (secondary inoculum). Conidia for secondary spread are produced from two to four weeks after initial leaf infection. The fungus may only complete a few cycles of secondary spread in a single growing season compared to the many cycles completed by most other corn leaf blight pathogens. The importance of secondary spread in contributing to the overall development of epidemics may be more important in more southern regions than in more northerly regions where the length of the growing season and relative maturity of the hybrids limits the number of generations of conidia. If a tillage system leaves sufficient previously diseased crop residue on the soil surface, then enough primary inoculum may be available to produce severe levels of gray leaf spot. If a region has a large percentage of land in conservation tillage, corn in conventionally tilled fields or corn planted following soybeans may be damaged by gray leaf spot as a result of inoculum disseminated from fields where conservation tillage is used. Corn debris in fields planted in soybeans also is an important, and sometimes unrecognized, source of inoculum.
Gray leaf spot has increased in incidence and severity as the use of conservation tillage increased. However, the economic and environmental benefits from conservation tillage are great enough that the Technical Committee believes that its use should not be abandoned.
INCREASE IN DISEASE INCIDENCE: Essentially all papers that have reported on the occurrence of the disease have indicated that the disease is most severe in reduced tillage fields, especially those in continuous corn production. Dr. David R. Smith, and most recently Dr. Jim Perkins, of DeKalb Genetics Corporation have monitored the increase in disease incidence since 1979 using plots at 24 locations throughout the corn growing regions of the United States (Fig 1). At those locations, standard sets of corn genotypes are planted and evaluated for disease incidence. Data collected at those locations indicate that gray leaf spot has greatly increased during the last 20 years (Fig 2). The disease increased through 1987 as the use of conservation tillage increased. There was a significant reduction of gray leaf spot incidence in 1988 due to widespread drought. From 1988 to 1995 the incidence increased dramatically, but somewhat leveled off in 1996 and 1997 due to the slight drop in acreage under conservation tillage and weather conditions unfavorable for widespread disease development. Gray leaf spot is now present in the major corn growing region of the United States and it will remain a major threat to corn production for the foreseeable future.
Figure 1. Occurrence (%) of gray leaf spot at 24 DEKALB Genetics
Corporation corn pathogen monitoring project plot locations and percentage reduced tillage
acres from the Conservation Tillage Information Center.
Figure 2. Gray leaf spot distribution as determined by members of the NCR-25 Technical Committee. Risk areas were identified based on reactions of susceptible hybrids, severity of past epidemics, the likelihood of favorable environmental conditions and use of conservation tillage.
The widespread increase in gray leaf spot is the result of several factors. There is general agreement among the Technical Committee that there is no relationship between the increased incidence and/or severity of disease and use of any particular susceptible commercial hybrid. In general, most widely used corn hybrids over the past 20 years had similar degree of susceptibility to gray leaf spot. Much of the germplasm that traditionally has been used as sources of commercially valuable inbreds has a low frequency of genes for resistance to gray leaf spot. The result has been that most commercial hybrids have a degree of susceptibility that may result in economic loss under conditions favorable for the disease. The combination of the widespread use of conservation tillage, planting of susceptible hybrids and weather conditions that favor rapid spread of the disease in certain years have all attributed to the fact that gray leaf spot is now endemic.
RISK AREAS: Identification of geographical areas where gray leaf spot is severe enough to potentially cause yield loss is much more difficult than identification of geographical areas where the disease occurs. In 1996, the Technical Committee identified those areas of the United States where gray leaf spot severity had been high enough to result in yield losses greater than 10% on high yielding susceptible hybrids (Figure 1). The areas were identified based on yields of susceptible hybrids in previous years, past epidemics, the likelihood of favorable environmental conditions, and the prevalent use of conservation tillage. Weather conditions in 1996 and 1997 were not highly favorable for the spread of gray leaf spot in most corn growing regions of the Midwest. Thus, there has been little expansion of the "Risk Area" since 1996. In spite of the reduced severity of disease in some areas, the "Risk Area" has not diminished in size. Yield losses in future years will depend upon environmental conditions and the overwintering of the fungus in crop residues. Yield losses also could occur in any area where the disease is present depending on environmental conditions and susceptibility of widely grown hybrids.
THE FUNGUS: When individual GLS lesions are exposed to humid conditions for a day or two in the laboratory, C. zeae-maydis within the leaf tissue produces conidia that can be transferred singly or en masse to a nutrient growth medium. The resulting pure culture that grows from each conidium can then be used for genetic, pathological and biochemical studies. Single-spore isolates obtained from GLS lesions collected throughout corn-production regions of the U.S. yield cultures that exhibit subtle differences in growth rates and production of the phytotoxin, cercosporin, but they cannot be distinguished by standard and commonly used taxonomic criteria (e.g., characteristics and dimensions of conidia and conidiophores). However, various molecular techniques (e.g., DNA fingerprinting) readily distinguish two groups that are significantly different from each other and from the sorghum pathogen, C. sorghi, a closely related fungus that is frequently isolated from corn leaves in the southeastern part of the country. The most prevalent group of C. zeae-maydis isolates is distributed throughout corn-production regions of the country. The other, less prevalent group of Cercospora isolates, is localized in the eastern third of the U.S. Isolates in this latter group tend to grow more slowly and produce much less cercosporin in culture. Both species are pathogenic on corn in greenhouse and field inoculations, but differences in virulence (or infection phenotype) are not obvious. The isolates within each group are relatively uniform genetically, suggesting that each is an asexually reproducing population or that the groups comprise separate populations in which sexual reproduction does not contribute substantially to genetic variability. Thus, the potential for the existence or emergence of races or pathotypes of the pathogen is minimal. Screening corn germplasm to select and deploy GLS resistance is likely not compromised by the composition of the pathogen population in different areas of the country.
GRAY LEAF SPOT MONITORING PROJECT: In 1996 and 1997, the NCR-25 Technical Committee established a project to monitor the development of gray leaf spot across the corn growing regions of the United States. The purpose of the study was to document the level of gray leaf spot at different locations, to evaluate the reaction of a diverse set of corn genotypes and to examine the variability in disease reactions among hybrids due to differences in populations of C. zeae-maydis at the different locations. Two nurseries, representing hybrids of different relative maturities (RM) (95-110 day RM and 110-117day RM) were planted at up to 11 locations from Virginia to Nebraska. Standard disease assessment protocols were followed at each location. Gray leaf spot severity ranged from very severe to light across the locations both years. The 20 hybrids tested ranged from highly susceptible to moderately resistant. Rankings of the disease reactions of the hybrids were similar at the different locations both years regardless of the severity of the epidemic at each location. Results indicated that hybrids responded similarly across environments and there was no evidence for difference in ability of local populations of C. zeae-maydis to cause differential reactions on the hybrids tested. These findings are significant because corn breeders now know they can develop corn hybrids with resistance to local fungal populations and these resistant hybrids will have a resistant reaction at other locations as well.
YIELD LOSS: There is general agreement among the Technical Committee members that gray leaf spot must damage leaves at or just after flowering to cause severe yield reduction. Early blighting of the leaves above the ear leaf leads to severe yield losses. Blighting that does not occur until well into the grain fill period probably results in very little yield loss. Research has shown that there is not an exact relationship between susceptibility to gray leaf spot and yield loss. Hybrids that have similar resistance to gray leaf spot may have different levels of yield loss in the presence of disease. Several factors may contribute to this response, including yield potential of the hybrid, and the ability of leaf blighting to predispose hybrids to stalk rots. Premature stalk death and lodging is enhanced by severe leaf blighting. Hybrids differ in their ability to maintain green stalks when under severe leaf blighting. Those hybrids with stalks that remain green and functional under stress by gray leaf spot will continue to translocate photosynthates to the ear and have less yield loss. A recommended approach to screening for reaction to gray leaf spot is to place test plots in areas where the disease is severe and select hybrids on the basis of leaf blighting, delayed stalk death, stalk rot and yield.
MANAGEMENT OF GRAY LEAF SPOT: Fields planted continuously to corn using conservation tillage methods have the greatest risk for yield losses due to disease. Gray leaf spot has been difficult to control in areas where the disease is endemic because fungal spores produced on corn residues left on the soil surface can be dispersed by wind to fields planted in rotation with soybeans or other non-host crops. In areas where the disease occurs sporadically, it can be effectively managed by reducing the amount of fungus surviving over winter in crop debris. Tillage accomplishes this goal rather quickly and the fungus dies within a few months of being buried in the soil. Crop rotation takes longer to reduce inoculum levels, usually requiring two years for the fungus to be reduced to low levels. The Technical Committee recognizes the importance of using conservation tillage for erosion control and the likelihood of yield losses from gray leaf spot in areas where conservation tillage is practiced. The committee recommends the use of resistant corn hybrids in combination with crop rotations to restrict yield losses. Results from numerous studies indicate that none of the commercially available hybrids have adequate resistance to gray leaf spot to completely avoid yield loss with severe disease development. However, significant advances have been made in incorporating resistance into commercially acceptable corn hybrids. Several hybrids have been marketed in the southeastern United States with good resistance to gray leaf spot. Recently, gray leaf spot resistant hybrids suitable for the Midwestern states have also become available. Research has shown that these resistant hybrids have higher yield potential than susceptible hybrids when gray leaf spot is moderate to severe. Results also indicate that when gray leaf spot development is less severe or develops late in the season after grain fill is complete, certain more resistant hybrids have yields similar to more popular susceptible hybrids. Farmers should be aware that some very late maturing hybrids may be reported as being resistant because they appear green due to late maturity. These hybrids may actually cost more to grow due to high grain moisture at harvest resulting in increased drying costs.
Several fungicides have shown effectiveness in controlling gray leaf spot and increasing yield as compared to unsprayed control plots. Propiconazole, mancozeb, thiophanate-methyl, azoxystrobin, chlorothalonil and tebuconazole have been evaluated by various Technical Committee members. Most of these fungicides have been effective in reducing disease severity levels, but the level of yield response varies among the fungicides as to degree of control. Propiconazole (Tilt by Novartis) is labeled for use on corn before the end of silking for gray leaf spot control and has provided an economic advantage particularly when used on susceptible inbreds in seed production fields. Mancozeb (labeled under several names by several companies) is also labeled for use on corn, but results have been variable due to partial control and inconsistent yield improvements. Of the non-labeled materials tested, azoxystrobin has provided good residual activity and the greatest yield responses of the fungicides tested. The use of fungicides for control of gray leaf spot has been limited in commercial field corn production due to the high cost of fungicides, the relatively low value of the commodity and the lack of adequate application equipment. Fungicide application will be most practical for high value corn crops. Read product labels and check with state University Extension offices for product information and recommendations for individual states.
RECOMMENDATIONS: The Technical Committee recommends that growers continue to use conservation tillage methods wherever practical. At this point, unless environmental conditions are extremely favorable for gray leaf spot development, the economic and environmental advantages of this practice clearly outweigh the risk of loss due to the disease. Growers should consider planting more different crops in rotation with corn in their farming system. A one or two year rotation away from corn would help reduce inoculum levels of C. zeae-maydis. Growers should select the newer gray leaf spot resistant hybrids for use in fields where the potential for gray leaf spot is high. Good progress has been made by seed corn companies in developing hybrids with improved resistance to gray leaf spot. Selection should be based on yield potential and standability under gray leaf spot pressure. Fields should be monitored throughout the growing season for disease development and harvested early if high amounts of disease develop during grain fill. The economic benefit of controlling the disease with fungicides in grain production fields is still marginal except in high risk areas with significant yield losses each year. The labeling of more efficacious fungicides with longer residual activity and the availability of adequate application equipment would improve the economics of chemical control. Read product labels for specific use rates and restrictions. Mention of specific products or companies does not imply endorsement over similar products or companies. All uses of pesticides must be registered by appropriate state State or Federal agencies and the pesticide must be used in accordance with the label on the product.
Selected References:
Beckman, P. M. and Payne, G. A. 1982. External growth, penetration, and development of Cercospora zeae-maydis in corn leaves. Phytopathology 72:810-815
Coates, S. T. and White, D. G. 1994. Sources of resistance to gray leaf spot of corn. Plant Dis. 78:1153-1155.
de Nazareno, N. R. X., Lipps, P. E. and Madden, L. V. 1993. Effect of levels of corn residue on epidemiology of gray leaf spot of corn in Ohio. Plant Dis. 77:67-70.
Freppon, J. T., Lipps, P. E., and Pratt, R. C. 1994. Characterization of the chlorotic lesion response by maize to Cercospora zeae-maydis. Plant Dis. 78:945-949.
Latterell, F. M. and Rossi, A. E. 1983. Gray leaf spot of corn: a disease on the move. Plant Dis. 67:842-847.
Ringer, C. E. and Grybauskas, A. P. 1995. Infection cycle components and disease progress of gray leaf spot on field corn. Plant Dis. 79:24-28.
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