A cool wet summer has produced favorable conditions for a variety of weeds and crop diseases. Now, as fall is here and harvest is ending, growers across the region should take advantage of the opportunity to control winter annual and perennial weeds.

Now is also a good time for producers to take steps to prevent the spread of diseases such as wheat streak mosaic and stripe rust.

According to Dave Wichman, supervisor of the Central Ag Research Center in Moccasin, Mont., volunteer grains are very common with the wet conditions this year.

“Downy brome responds to wet conditions as does kochia and Russian thistle,” Wichman said. These are all weeds producers should be trying to control before the temperatures drop too much.

Fabian Menalled, Montana State University Extension cropland weed specialist, knows that one of the keys to fall weed control is knowing how the different weeds grow and a few key management practices to reduce the spread and impact of weeds this fall and next spring. 

Producers will face both annual and perennial weeds this fall. Common winter annuals include cheatgrass, jointed goatgrass, field pennycress, wild mustard, shepherd’s purse, catchweed bedstraw, common chickweed and prickly lettuce.

“With more and more no-tillage acres planted with winter wheat, farmers across the region are seeing an increase in the abundance of winter annual weeds,” said Menalled.

As a general rule, winter annual species germinate from late summer to early winter. They overwinter as seedlings then grow rapidly as temperatures warm in the spring. By early summer they will flower, set seed and mature. Their seeds will then lie dormant in the soil during the rest of the summer months.

Because of that, Menalled said the best time to manage winter annual weeds is in the fall when they are smaller and more vulnerable. And earlier is best.

“Our research and experience has shown that winter annual weeds are easier to control in early fall when air temperatures are mild and weeds are still actively growing.”

Timing of herbicide applications in the fall is critical for maximum effectiveness. “Generally, treatments need to be made before the first killing frost,” he said.

A variety of herbicides can be used to control winter annuals in the fall. However, Menalled stressed that rotational crop restrictions vary between herbicides and producers should carefully read the specific restriction of the product they plan to use. This is particularly important in diversified rotations that include pulse crops such as lentils, peas, or chickpeas, as several small grain herbicides have restrictions of up to 36 months.

Fall also provides an excellent opportunity to control several problematic perennial broadleaf species. That is because the cooler temperatures trigger the movement of food reserves down to the root systems. By using this natural system to their advantage, producers can enhance the movement of herbicides to the plant’s root system and improve control.

However, Menalled pointed out that perennial weeds vary in their sensitivity to frost and the application window differs between species. Perennial weeds such as hemp dogbane and common milkweed complete their life cycles by late summer and do not tolerate frost well, so fall herbicide applications should not be delayed when controlling these species.

On the other hand, Canada thistle can survive light frosts and is effectively controlled with relatively late fall herbicide applications.

“Regardless if you are targeting a winter annual or a perennial weed, it is important to know that if plants are stressed from drought or cold temperatures, applications will not provide satisfactory control due to poor movement of herbicide through the weed,” said Menalled.

To secure active translocation, fall herbicides should be applied when temperature are expected to exceed 60 to 65 Fahrenheit during the day. Fall applications should be made only if plants still have green and pliable leaf tissue. As a rule of thumb, do not expect satisfactory control if less than 60 percent of the original leaf tissue remains.

“When designing your weed management plan, remember that the adoption of no-tillage systems has increased our reliance on herbicides,” said Menalled. “While this approach to farming has benefits in terms of reducing soil erosion and energy use, it increases the potential for selection of herbicide resistant weeds.”

Fall is also a good time to go after crop diseases that can cause problems next season. Mary Burrows, a plant pathologist with MSU, noted that the wet, cool summer has brought an increase in calls about sooty mold, wheat streak mosaic, and stripe rust.

“We’re actually getting a lot of calls right now about sooty mold,” she said. Because of the late harvest, this black, blotchy secondary fungus is showing up on crops still in the field.

“It colonizes on the dead tissue of leaves and glumes and can discolor the seed,” she said, adding that producers who haven’t been able to harvest all their crops may be seeing a problem with this.

The problem with many plant diseases is that they survive year round and, when conditions are right, they reappear the next season.

“These are green bridge issues which, in some areas this year, are common issues because of all the rain,” she commented.

In addition to controlling the weeds in their fields, producers should be spending the time necessary to eliminate the green bridges by killing off any green tissue with herbicides.

“We tell them to hit it with herbicide two to three weeks before planting but obviously that is not generally practical, given our short seasons,” she said.

Most producers go out and deal with their green bridge issues only a few days before planting a new crop, or at the time of planting. She stressed that does not give the herbicide enough time to kill the green bridge and any of the diseases those green plants may be carrying.

“That’s a good way for wheat streak mosaic and stripe rust to survive,” she commented. The state has seen a lot of late stripe rust this year and Burrows’ concern is that if it survives in green bridges that have not been properly treated, “we could have a real problem next season.”

Wichman agrees. “It’s really critical that producers take care of their green bridges this fall or stripe rust and wheat steak mosaic could get into the winter wheat crop and be devastating by the time spring rolls around.”