Farmers seeing yellow winter wheat in their fields need not worry, at least not yet, according to Montana State University Associate Professor Mary Burrows.
The yellowing could be harmless frost damage. The mild spring has winter wheat greening up in some areas of the state, but because the nights are still cold frost damage is a risk.
Recognizing frost damage in winter wheat is fairly straightforward. Often, the lower leaves die and the next upper leaves have burned tips; other times, all the new growth is yellow.
Still not sure? Look to see if the yellowing is all at one level. If it is, that’s frost damage. That’s because that’s where the plant tissue was young and tender when the frost event occurred. Sometimes it can be seen in the middle of a leaf on all the plants.
The good news is that little or no yield losses are expected from early frost events, Burrows said.
“I’m asking guys to be patient,” Burrows said. “When it starts to grow actively it will grow out of these yellow symptoms if it’s healthy.”
While frost damaged winter wheat is rife this spring, other more serious ailments can cause yellowing as well, including powdery mildew, nutrient deficiency, Cephalosporium stripe, and wheat streak mosaic virus.
Powdery mildew brought on by a mild fall is prevalent on winter wheat throughout the state. Powdery mildew is characterized by so-called green islands and, of course, powdery white lesions.
“With the early season, we have a lot of guys worried,” Burrows said. “We’re getting samples already. They’re scouting earlier because we had disease issues last year. They’re looking more.”
Experts aren’t entirely sure what effect the powdery mildew will have on the crop. Continued drought and temperatures ranging from 59 to 72 degrees F will cause the disease to flare up. Free moisture and high temperatures will shut it down.
Yield losses can be 20 to 45 percent in more humid wheat growing areas of the United States. Burrows said there’s not too much information about potential yield losses here. Fungicide trials will occur this spring. Anecdotal reports from spring fungicide applications last year varied.
Yellowing can also be caused by nutrient issues. Cold spring temperatures trigger nitrogen, iron, and various other deficiencies. Usually the crop grows out of them with some moisture and sun, but a complicating factor could be high plant densities and the incredible amount of growth due to mild fall conditions.
Yellowing in winter wheat may also be caused by Cephalosporium stripe, a disease increasing in importance according to Burrows.
The plants will have a yellow or lime-green cast from far away. A close look will reveal a distinctive stripe from the base of the leaflet to the tip, with a thin brown border on one side. Burrows has not yet seen these in the clinic this year, but said she expects samples to start rolling in by early April, if not before.
Likely plants will recover, but symptoms will reappear at flag.
Finally, the big daddy: wheat streak mosaic virus (WSMV). Widespread hail in parts of Montana last year, an extended fall, and the increasingly common practice of grazing hailed wheat and keeping hailed wheat for a crop the following year, together with the uptick in WSMV samples last year spells trouble, Burrows said.
“It’s going to be a fairly severe year for wheat streak according to data, monitoring and hail streaks through Havre last year,” Burrows said.
The key for identifying WSMV is to look at the youngest leaves to see if they have yellow streaks. This symptom can be confused with bad powdery mildew infections or iron deficiency.
Then, look for wheat curl mites with a hand lens, and think about the field history: was there a lot of volunteer or cheatgrass that was not killed prior to planting? Was the upwind crop infected or was it a late maturing spring wheat? Both can be good sources of the disease.
Many factors affect management decisions for WSMV: variety, how much of the field was infected and when, and the timing of crop replacement or ability to rotate to a non?-host of the virus (dicot crops such as mustards, safflower, and legumes).
In most cases, a limited infection of winter wheat will not cause severe yield loss. The grower is better off keeping the crop rather than terminating it to plant spring wheat or barley, Burrows said, cautioning that the grower would need to be attentive to the green bridge that fall.
Crop termination using glyphosate will allow the mite 5 to7 days (or up to 3 weeks) to move downwind into anything green. Tillage is a better option for crop termination, but is not always feasible. Swathing is an option later in the season.
Spring wheat and barley are more susceptible than winter wheat, and have larger yield losses. Given the burn down time of glyphosate and planting windows with weather, growers often end up with more of a disaster on their hands when replacing winter wheat with a spring cereal. If nothing else, do not apply any additional nitrogen to the crop in order to limit spread of the disease. Any yield gain from nitrogen will be taken away by the virus, and the virus will spread more with increased mite replication on fertilized plants.
“It’s a little early to see symptoms, but they should sure be scouting pretty soon,” Burrows said. “I don’t want them to immediately terminate the winter wheat crop and plant spring wheat, but they should consider their long term management options.”
(Source – http://www.farmandranchguide.com/news/crop/yellow-winter-wheat-in-field-don-t-worry-quite-yet/article_2c181562-ec6b-11e5-80ff-e32530b09a8e.html)