June 11, 2021
Introduction
Insects and diseases can affect the establishment and early seedling growth of cereals. Seed and soil-borne fungal diseases, Hessian fly, aphids, and wireworms are major pests of cereal crops at planting and early in their growth. Insecticide and fungicide seed treatments are effective management tools to minimize the risk of damage by these pests to early seedling cereal crops.
Seedling Diseases
Seed and soil-borne fungal diseases can affect cereal establishment and severely impact yield and quality. Seed rots, seedling blight and common root rot are part of the cereal seedling disease complex. Seed becomes infected when spores released from diseased plant tissue or residues land on the developing heads and infect the structures. Highly infested seeds often have lower germination and vigor and are more susceptible to seed rot. It is not unusual for the infection to move from the seed to the seedling, causing lesions or necrosis of the developing roots and coleoptile which may lead to seedling death before emergence or shortly after (Figure 1). Infections caused by soilborne spores or mycelia arising from plant residues may cause similar symptoms. Above ground symptoms are often nondescript and may include stunting, chlorosis, reduced tillering, smaller heads and premature ripening.
Fungi in the genus Fusarium, Pythium, and Rhizoctonia can cause seedling damping off or seedling blights, root and crown rots, leaf spots, and diseases that affect the head of cereals. There are other species of fungi and secondary pathogens that can impact seedlings of cereals. These fungi can overwinter and survive in crop debris and in soil for several years. Moisture is critical for fungal spore germination and growth, and more disease problems generally occur in wet years. Because different fungi require different temperatures for their optimum growth, some fungal issues may be more prevalent during early season rather than late season. Some may be suppressed by either low or high temperatures.1 The pathogen’s preferred environmental conditions are highly variable depending on species. Small grain crops differ in their susceptibility to these pathogens.2 Pythium seed decay, damping-off, and root rot affect all small grains. Fusarium crown rot occurs on spring and winter cereals and can damage wheat, barley, and oats. Rhizoctonia root rot (bare patch) is more damaging to spring cereals than to winter cereals, and more damaging to barley than to wheat.
Figure 1. Seedlings infected with C. sativus showing characteristic lesions on the subcrown internode.
Insects
The three most important early-season insect pests of cereals in the United States are the Hessian fly, cereal aphids, and wireworms. Although widespread economic outbreaks of these insect pests occur infrequently, local outbreaks can happen nearly every year for each pest.3
Hessian fly can be a serious pest in wheat, barley, and rye with wheat being the preferred host (Figure 2).4 The insect does not attack oats or ryegrass. Hessian fly can be a problem in all major wheat production areas of the United States, infesting both winter and spring wheat. The pest is generally more troublesome in the southeast where it has a longer period of activity and undergoes more generations than in other areas. The maggot or larvae stage of Hessian fly causes the injury to plants. Maggots can damage seedlings in the fall and overwintered plants in the spring by feeding on lower stem tissue. Stunted or dead tillers and thin stands are typical signs of early season infestation. A thin stand consisting of stunted wheat plants with few heads is the sign of an extremely heavy Hessian fly infestation that began when plants were small. Wheat often lodges in seriously infested fields. A moderate infestation can result in yield loss with less obvious symptoms. Multiple generations of Hessian fly can cause damage throughout the growing season. Maggot feeding on small seedlings in the fall can cause severe injury to plants which may not grow past the four-leaf stage and generally die during the winter.
Figure 2. Hessian Fly. Image courtesy of Scott Bauer, USDA Agricultural Research Service, Bugwood.org.
Cereal aphids are small pear-shaped insects that obtain their nutrition by sucking sap from plants and can attack wheat, barley, oats, and other cereal crops (Figure 3).5 Aphids also inject salivary secretions into plant tissues during feeding. Symptoms of aphid injury include leaf stippling, discoloration or striping, and wilting, premature browning, and death of the plants may result. The most serious cereal aphid pests are Russian wheat aphid (RWA), greenbug (GA), bird cherry-oat aphid (BCOA), and the English grain aphid (EGA).3 Wheat injury varies by species with RWA and GA providing visually striking symptoms of leaf chlorosis, whereas BCOA and EGA do not. Regardless, all can lead to yield loss in wheat.
Figure 3. Russian Wheat Aphids. Photo courtesy of Frank Peairs, Colorado State University, Bugwood.org.
Severe infestations of RWA can lead to stunting of plants and small heads with poor grain quality. Infestations begin in the fall when they move out of alternate grass hosts such as volunteer wheat and into newly emerging cereals. Severe fall infestations leave plants weakened and vulnerable to winter kill. Late spring movement generally does not result in severe damage to winter wheat. Spring barley, however, is at severe risk for damage from aphid movement in the spring because it is infested at a much younger stage. Greenbug aphid is distributed throughout the United States, and a perennial pest of wheat in the southern Great Plains with droughty weather favoring infestations. Greenbug can be found in the whorl of seedlings. They move early in the spring to infest winter wheat as well as other cereal grains. They rarely damage wheat in the spring but can develop damaging populations in the fall. Bird cherry-oat aphid is one of the most common and widely distributed aphids found in wheat in the United States. Yield loss from BCOA is primarily from infestations on younger wheat plants. English grain aphid is a sporadic pest of wheat in the United States. Economic injurious levels of EGA are most likely in spring wheat in the Pacific Northwest and northern Great Plains. Importantly, plant viruses can be transmitted to the plants during aphid feeding. Barley yellow dwarf virus (BYDV) is one of the most important viral diseases of wheat in the United States with BCOA, EGA, and GA the primary vectors of the virus.
Wireworms are the larval stage of click beetles that feed on germinating seeds and young seedlings of cereal crops (Figure 4).6 These soil-dwelling insects feed on emerging cereal sprouts resulting in failed germination. Feeding on emerged plants is visible in the form of a brown spot on the root and/or at the very base of the stem just above root. Necrosis and desiccation of the flag leaves may indicate feeding that occurred after plant emergence. Wireworm damage in the field can appear as patches of missing and/or weakened plants. Affected areas also suffer from delayed growth that can cause patches of green plants in a maturing cereal field. Wireworms are particularly problematic in the Northwest where heavy infestations can cause severe yield loss and even total destruction of individual wheat fields.3
Figure 4. Wireworms in soil. Photo courtesy of R.J Reynolds Tobacco Company Slide Set, R.J. Reynolds Tobacco Company, Bugwood.org.
Management Strategies
Tillage and crop residue management helps to break down residue that harbors diseases and insects. Plowing or disking wheat stubble after harvest effectively reduces populations of Hessian fly and wireworms.
Crop rotation provides a means of reducing disease and insect carryover into the growing season following wheat, barley, and other cereal crops. It is best to rotate to a non-grass crop.
Cover crop choice can be important to reduce disease and insect populations. Oats is not a favorable host for Hessian fly making this grain preferable over wheat as a cover crop in wheat producing areas.
Weed control is important to help prevent disease and insect pest survival during the period between crops. Use tillage or herbicides to help control volunteer wheat and other weeds that can become hosts to insects and diseases.
Plant certified seed for assurance of seed quality, germination, purity, and genetic identity that has consistently outperformed seed from noncertified sources.
Product selection is important considering what is available in a given area. Planting multiple products with different maturities helps to minimize risk. Resistant products can provide the best protection from diseases, but not all products are resistant to all diseases. Planting resistant products after the “fly-free” date when Hessian fly is generally no longer active in the fall can be an effective strategy. However, Hessian fly can overcome host plant resistance mechanisms and, in areas with severe infestations, resistant products may not be sufficient enough to manage populations of the insect. Aphid-resistant wheat products are also available in some areas, but biotypes can overcome resistance requiring the use of other tactics to minimize infestations. Barley yellow dwarf virus (BYDV) tolerant or resistant products are also available, but the choice of cultivars must be based on knowledge of the prevalent biotypes of greenbug and Russian wheat aphids in an area. Multiple biotypes can complicate the use of resistant products which can limit their availability as a viable management option.
Time of planting, seed placement, and fertility can be important management considerations. Avoiding early planting of winter wheat and planting spring barley as early as possible helps to minimize the risk of economic injury from cereal aphids and the incidence of BYDV. Delayed planting of winter wheat can reduce exposure of seed and emerging seedlings to large injurious larvae of wireworms that can be present in late summer and early fall. Proper seeding depth helps to facilitate quick emergence and reduce sprout exposure time to wireworms. Maintaining a balanced fertility and avoiding excessive nitrogen fertilization that can promote lush growth and disease development is important in small grain production.
Biological control with natural enemies to pests helps but current production practices can limit their effectiveness.
Insecticides and fungicides through the use of seed treatments, in-furrow and foliar applications are effective tools to help manage early seedling cereal pests.
Scouting early and frequently makes a big difference in identifying pests before they become severe. Yearly scouting helps to build a field database so pests can be proactively managed in future crops.
Seed Treatments
Seed treatments are available to provide early-season protection against insects and diseases in cereal crops. Fungicide seed treatments can provide a healthy start for seedlings, especially in cool and damp spring conditions. Systemic insecticidal seed treatments based on neonicotinoid chemistry can also be used to help manage populations of Hessian fly, aphids, and wireworms.
Bayer provides several options for seed treatments in cereal crops:
EverGol® Energy Seed Treatment Fungicide is a combination of prothioconazole, penflufen, and metalaxyl for control of seed and soil-borne diseases from Rhizoctonia, Fusarium, Pythium and other fungi. The seed treatment provides control of seed rot, damping-off, and seedling blight while suppressing root rot, crown rot, and seed decay.
Raxil® PRO MD Seed Treatment is a combination of prothioconazole, tebuconazole, and metalaxyl for broad spectrum early-season seed and seedling disease protection in cereals. The seed treatment is formulated with MD (micro-dispersion) technology to help provide protection against seedling and soilborne diseases and promote the development of robust plants.
Gaucho® 600 Flowable Seed Treatment contains imidacloprid for early season protection of seedlings against injury by aphids, Hessian fly, and wireworms. The seed treatment also helps to reduce potential spread of BYDV due to aphid vectors.
Gaucho® XT Insecticide/Fungicide seed treatment is a combination of imidacloprid, metalaxyl, and tebuconazole for early season protection against both insects and diseases.
Raxil® PRO Shield Seed Treatment is a convenient all-in-one combination of imidacloprid, prothioconazole, metalaxyl, and tebuconazole that provides advanced broad-spectrum control of seed, seedling, and soilborne diseases in addition to early season seedling injury protection against aphids, Hessian fly, and wireworms.
For more information about early season cereal pests and seed treatments, contact your local Bayer Crop Science representative.
Sources
1 French, R. 2015. Potential wheat disease issues on seed, seedlings, and heads during a wet year. Texas A&M AgriLife Extension Service. http://sickcrops.tamu.edu.
2 Smiley, R., Paulitz, T., and Marshall, J. 2012. Controlling root and crown diseases of small grain cereals. Pacific Northwest Extension Publication PNW 639. Oregon State University. http://extension.oregonstate.edu.
3 Hesler, L., Sappington, T., Luttrell, R., Allen, K., and Papiernik, S. 2018. Selected early-season insect pests of wheat in the United States and factors affecting their risks of infestation. Journal of Integrated Pest Management 9(1): 17, 1-8.
4 Flanders, K., Reisig, D., Buntin, G., Winslow, M., Herbert, D., and Johnson, D. 2014. Biology and management of the Hessian fly in the southeast. University of Kentucky ENTFACT-155. https://entomology.ca.uky.edu.
5 Hein, G., Kalisch, J., and Thomas, J. 2005. Cereal aphids. NebGuide G1284. University of Nebraska-Lincoln. http://extension.unl.edu.
6 Rashed, A., Etzler, F., Rogers, C., and Marshall, J. 2015. Wireworms in Idaho cereals: monitoring and identification. University of Idaho extension bulletin 898.
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