Risk Factors for Virus Disease Spread and Development in Winter Wheat
In considering the risk factors for mite-transmitted viruses in winter wheat, mite presence overrides all other risk factors. If mites are not present in sufficient numbers, other risk factors will have minimal impact on the level of risk. Thus, primary efforts to manage viruses should target managing green bridge hosts — those plants growing from harvest of one wheat crop to emergence of the next, enabling mites to over-summer in sufficient numbers to create a significant risk to the next wheat crop.
Risk factors that can be managed (highest risks listed first):
- Hail storms within the last three weeks before harvest (early to late dough stages) shell out seeds and result in pre-harvest volunteer wheat. If left uncontrolled until wheat emergence in the fall, this will result in serious mite/virus buildup and spread into the new crop. See Surviving the Green Bridge for more information.
- Poor control of volunteer wheat emerging before wheat harvest within summer crops, such as sunflowers, millet, and corn, can result in volunteer wheat growing during the wheat pre-harvest period when mites are very active and will have a high likelihood of serious mite infestations. Risk level will depend on volunteer density and subsequent mite density.
- Mites survive well on corn so an overlap of growing (green) corn adjacent to emerged winter wheat in the fall has increased risk, and the risk increases with the degree of overlap.
- Mite-host crops (for example, wheat and foxtail millet) or these crops used as companion crops or cover crops that emerge before wheat harvest and grow through the over-summering period risk mite infestations and subsequent spread into new crop wheat.
- Planting winter wheat earlier than the recommended planting window for your area increases virus risk by providing a longer growing window for mite infestation and virus infection in the fall.
- Grassy weeds that host the mites and bridge the over-summering period can increase risk if weed and mite density are great enough.
- The risk from volunteer wheat emerging after wheat harvest will be much lower than from pre-harvest volunteer. The risk from post-harvest volunteer increases the longer it grows through the green bridge period.
When mite presence is great enough to increase risk, several environmental factors can increase virus risk and the potential impact. Wet summers encourage volunteer wheat and enhance mite survival. Warm, dry weather through the fall, particularly extended warm conditions that continue through October and into November, results in increased mite reproduction and movement of the wheat curl mite and buildup of the virus in the new crop wheat. In addition, warm spring conditions will accelerate virus symptom development and yield impact.