The severe problems wheat producers had with wheat streak mosaic virus this year can be traced back in most cases to a lack of control of volunteer wheat – especially the volunteer wheat that got started early after widespread hail damage to wheat just before harvest in 2016. It is important to keep that from happening again. Where wheat has been hailed out this year, volunteer wheat control should start immediately.
Producers often like to wait several weeks after harvest before making their first herbicide application to control volunteer wheat. This allows as much volunteer as possible to emerge before spraying it or tilling it the first time. Often, a second application or tillage operation will be needed later in the summer to eliminate the green bridge to wheat by making sure all volunteer is dead within ½ mile of wheat being planted in the fall. Green bridge elimination can be more difficult to accomplish when wet weather prevails through late summer because this tends to keep a lot more alternate host plants alive during the critical period when mites are host-limited. As with most plant diseases, the earlier infection occurs, the more impact on the plant and the greater the yield loss, so infections of wheat in early growth stages in the fall are most damaging.
Where wheat was hailed out and volunteer has already emerged at the time of harvest, control should begin immediately after harvest if possible. This is true even for fields that got hailed out relatively early during grain filling, as wheat grain at soft dough or later stages of development already has the potential to germinate. Hailed out fields may require one more field pass than normal to control volunteer wheat, but will help prevent even bigger problems down the road. It should be noted that grazing volunteer is not an effective option because there is green wheat material left and the mites can be living in that material.
Why the need for early control of volunteer in hailed-out wheat? Where wheat suffered hail damage after heading, volunteer often emerges even before the existing field is harvested – as much as two to three weeks or more earlier than it would normally emerge after harvest. This volunteer wheat is especially likely to become infected with wheat curl mites and lead to problems later in the season if left uncontrolled.
Wheat curl mites will move off growing wheat as the green tissue dries down and dies. After moving off the existing wheat at or near harvest time, the mites need to find green tissue of a suitable host soon or they will die of desiccation. Research has found that the mites can live quite a few hours off the plant, and up to 24 hours or more under low temperature conditions, so significant numbers of mites may be blown in from farther away than previously thought.
If there is young, volunteer wheat growing at the time the current wheat crop is being harvested in the nearby region, the mites can quickly infest those volunteer plants and survive. If volunteer has emerged and is still alive shortly after harvest in hailed-out wheat, wheat curl mites could easily build up rapidly and spread to other volunteer wheat that emerges later in the season. On the other hand, if this early-emerging volunteer is controlled shortly after harvest, that will help greatly in breaking the green bridge. However, if more volunteer emerges during the summer, follow-up control will still be needed.
Volunteer wheat is not the only host of the wheat curl mite. Over the years, multiple research studies have evaluated the suitability of wild grasses as hosts for both the curl mite and the wheat streak virus. There is considerable range in the ability of a grassy weed species to host the mite and the virus. Barnyard grass is among the more suitable hosts for both virus and mites, but fortunately it is not that common in wheat fields. In contrast, various foxtails, although a rather poor host, could be an important disease reservoir simply because of their abundance. These grasses may play an important role in allowing the mites and virus to survive during the summer months particularly in the absence of volunteer wheat.
A new K-State Research and Extension publication, Wheat Streak Mosaic MF3383, is now available. This publication includes information about grassy weed hosts of the mite and virus, and the contribution of these grassy weed hosts to the risk of severe wheat streak mosaic infections. Take note of significant stands of these grasses in marginal areas and control them as you would volunteer wheat. If volunteer wheat and other hosts are not controlled throughout the summer and are infested with wheat curl mites, the mites will survive until fall and could infest newly planted wheat at that time. Wheat curl mite infestations of wheat often lead to wheat streak mosaic infections
Another tool producers can use to help control or reduce the impact of wheat streak mosaic is the use of varieties with resistance to the disease. There are currently three varieties adapted to Kansas that have wheat streak mosaic resistance:
Clara CL (white)
Oakley CL (red)
All have the same resistance source (WSM2). Temperature sensitivity varies a bit among these, but all will tend to lose wheat streak mosaic resistance at high temperatures. In addition, there are a handful of varieties with resistance to the wheat curl mite, including TAM 112, Byrd, Avery, and T158. These varieties are actually susceptible to the wheat streak mosaic virus itself, but since they have resistance to the wheat curl mite vector of the disease, they can escape the disease pressure in many cases — depending on the severity of wheat curl mite pressure. Under light to moderate wheat curl mite pressure, these varieties held up relatively well this year against wheat streak mosaic infections. Under severe pressure, such as on fields adjacent to a field with volunteer wheat, these varieties did not generally hold up any better than other varieties that are susceptible to wheat streak mosaic.
K-State Research and Extension, Harvey County
Courthouse, PO Box 583, Newton, KS 67114-0583