The primary reason for controlling weeds is to protect crop yield potential from the negative effects of weed competition. Weeds compete with crops for limited resources such as light, water, and nutrients. Weed management is also essential for another important, but often overlooked, reason. Several common weed species can serve as host plants for insect and disease problems. If not controlled in a timely manner, these weedy volunteers can sustain large numbers of migrating insect pests, as well as plant viruses transmitted by insect vectors, and as alternate hosts for other types of plant pathogens.
What corn pests may be more problematic in fields where weed control is lacking?
Black Cutworm: In much of the corn growing region, moths start migrating from the southern United States in early April. Strong southerly winds influence the transport, distribution, and severity of black cutworm infestations. Female moths deposit eggs onto weeds (henbit, common purslane) and crop residues prior to corn planting. Upon hatching, black cutworm larvae feed on weeds, when the weeds are removed with tillage or herbicide the larvae will move to corn when the seedlings emerge in May and early June. Black cutworm larval feeding results in cutting of corn seedlings, which may occur at or below the soil surface. Usually, larvae do not cut the seedling at the V5 and beyond as the plant is too large. However, the larvae can tunnel into the plant, causing hollow heart and in some cases destroy the growing point. For more information about cutworms and how to manage them, please read this Q&A, on How to Manage Black Cutworms.
Nematodes: Most plant-parasitic nematodes of corn have a wide host range or a host range that is not well-studied. For these reasons, it can be difficult to find suitable rotations for managing nematode problems. If sting, stubby-root, or lesion nematodes are problematic on corn, grass species, such as sorghum, sugarcane, or forage grasses, should be avoided as these nematodes thrive on grasses. Weed management is an important supplement to crop rotation because plant-parasitic nematode population densities can be maintained or increased on some weed hosts, including volunteer corn, growing in a non-host crop.
Can tillage impact problematic pests of corn?
An environment where the soil is not disturbed between harvest and planting is favorable for winter annual weed survival. Weeds can harbor diseases that can be transferred to crop plants by insects feeding first on weeds and then on the crop. Weed species can serve as alternate hosts and sources of inoculum of insect vectored, viral plant pathogens including maize chlorotic mottle virus, maize dwarf mosaic virus, sugarcane mosaic virus, or by mites in the case of wheat streak mosaic virus. Aphids, whiteflies, leafhoppers, and thrips are the most common vectors of plant pathogenic viruses due to these insect’s piercing sucking mouthparts. Elimination of the source of the disease and vectors is one of the most effective approach to management, and this often includes weed control.
No-till, continuous corn has the potential to increase nematode problems. Rotation to a non-host crop is one of the best strategies for reducing nematode populations below economic thresholds. Different nematodes have different host ranges. Some nematodes are able to parasitize a wide range of field crops (e.g. corn, cotton, soybean), so it is important to identify the species of nematodes present in a field before implementing a crop rotation strategy. Many weed species can serve as alternate hosts for nematodes that attack corn, which makes weed management particularly important for nematode control in no-till cropping systems.
What are some considerations for managing the weed-insect-disease relationship?
Weeds that harbor plant diseases vectored by insects are a special concern and should be controlled at the earliest possible stage before insect vectors infest them. This would include weeds in field margins and other non-crop areas. However, care should be taken when controlling weeds that are used by butterflies (e.g., monarch) or other pollinators.
The first line of defense against corn diseases is genetic resistance, especially in corn-on-corn situations. Diseases such as seedling blights, gray leaf spot, northern corn leaf blight, many common stalk and ear rots, and in some areas Goss’s wilt are potentially more severe in corn-on-corn production, and some can be harbored by weeds. Late season weeds, such as giant ragweed, that are serve as pollen source for western and northern corn rootworms can attract females into corn fields, where eggs can be deposited impacting next year crop in a continuous corn production system.
Starting with a clean field by utilizing burndown herbicide applications and preemergent herbicides with multiple modes of action will help reduce the number of weed species present to harbor problematic insects and diseases.
Seed treatment fungicides are a critical component of an integrated disease management strategy in corn, especially when problematic weed species are present in large populations. The fungicides present in Acceleron® Seed Applied Solutions protect against the top three seedling infecting pathogens, including Fusarium, Pythium, and Rhizoctonia solani, as well as Colletotrichum gramincola in Acceleron Seed Spplied Solutions ELITE offerings. Acceleron® Seed Applied Solutions products also contain an insecticide that protect against early season insect damage that may lower the threat of root injury that serves as entry point for disease infection.
For more information on scouting for weeds and identifying weeds, check out: Making Decisions Based on Weed Scouting in Corn
Table 1. Corn pathogens hosted by common weed species.
Capinera, J.L. 2005. Relationships between Insect Pests and Weeds: An Evolutionary Perspective. Weed Science, 53 (6), 892-901. http://www.jstor.org/stable/4046991.
Ikley, J., Wise, K., and Johnson, W. 2015. Annual Ryegrass (Lolium multiflorum), Johnsongrass (Sorghum halepense), and Large Crabgrass (Digitaria sanguinalis) are Alternative Hosts for Clavibacter michiganensis subsp. nebraskensis, Causal Agent of Goss's Wilt of Corn. Weed Science, 63(4), 901-909. doi:10.1614/WS-D-15-00028.1
Mohler, C.L. and McGrath, M.T. 2009. Crop Rotation on Organic Farms, APPENDIX 5: Crop Disease Pathogens Hosted by Common Agricultural Weeds, Natural Resource, Agriculture and Engineering Service (NRAES) and Sustainable Agricultural Research and Extension (SARE). https://www.sare.org/resources/crop-rotation-on-organic-farms/.
Norris, R. and Kogan, M. 2000. Interactions between Weeds, Arthropod Pests, and Their Natural Enemies in Managed Ecosystems. Weed Science, 48(1), 94-158. http://www.jstor.org/stable/4046162.
Thomas, S., Schroeder, J., and Murray, L. 2005. The Role of Weeds in Nematode Management. Weed Science, 53(6), 923-928. http://www.jstor.org/stable/4046996.