Dr. Robyn Roberts
Field Crops Pathologist and Assistant Professor
970-491-8239; @RobynRobertsPhD (Twitter)
*Email is the best way to reach me
Tan spot (caused by the fungus Pyrenophora tritici-repentis) was observed in Prowers, Kiowa, Cheyenne, Kit Carson, Washington, Yuma, and Phillips counties. Tan spot appears as necrotic (dead, brown) diamond-shaped spots surrounded by yellow halos or borders (Figure 1). Tan spot was found only on the lower leaves (not advancing to the upper leaves), so it should not be a major concern if the weather continues to be warm and dry.
What should you do if you see Tan Spot in your fields? As the weather warms, tan spot does not typically continue to be a problem in Colorado, so fungicide applications are not usually recommended. The fungus overwinters in wheat residue, so tan spot appears most often in fields with high amounts of wheat residues on the soil surface. When scouting for tan spot, note which fields have higher tan spot incidence and whether there is residue build-up.
Barley yellow dwarf virus (BYDV) symptoms were observed at a few sites with low incidence. BYDV symptoms include plant stunting and yellowing of the leaves which may have purple or red tips (Figure 2). Unlike the other common wheat viruses, BYDV does not display any mosaic or streaking patterns. BYDV is transmitted by aphids, and while there were aphids observed at a few sites this week the pressure was always under the economic threshold for insecticide application, except for one case of higher Russian wheat aphid incidence in Prowers county. BYDV is not typically a major virus problem in Colorado.
What should you do if you see BYDV in your fields? Document the incidence of BYDV and scout for aphids in the coming weeks. If you observe a high incidence of aphids in your fields that is above the economic threshold for insecticide application, you may consider applying an insecticide. Do not apply prophylactic sprays because they can select for insect populations that are resistant to insecticides, causing insecticides to fail.
Wheat streak mosaic virus (WSMV) symptoms have been observed in Baca county on wheat that has abiotic stresses (drought and freeze damage). WSMV is transmitted by the wheat curl mite. WSMV symptoms appear as yellow streaks and mosaic, yellow and green patterns on leaves and appear similar to other mite-transmitted viruses (TriMV and HPWMoV), but WSMV is found most commonly in Colorado (Figure 3).
What should you do if you see WSMV symptoms in your fields? Document the event and take note of whether the affected wheat is of a resistant variety. If you think you see symptoms in a WSMV resistant variety, please send me photos. In Kansas, we have found samples from Guardian that tested positive for WSMV, TriMV, and HPWMoV, with TriMV having the highest infection levels. The resistance in Guardian is not effective against TriMV, but in Colorado TriMV has typically been less problematic compared to WSMV. We are researching whether the symptoms observed in the Kansas Guardian samples are due to TriMV, as well as many other possibilities. The Kansas samples also came from a field with high numbers of volunteer wheat, which is a major source of wheat curl mites. It is therefore crucial that growers control volunteer wheat this season to help protect the WSMV and wheat curl mite resistance in our Colorado wheat varieties.
Cephalosporium stripe (caused by Cephalosporium gramineum) disease was observed in one incidence. Cephalosporium stripe is a fungal wilt disease with symptoms appearing as one to three distinct yellow stripes near leaf veins that develop on older leaves first (Figure 4). Brown streaks eventually appear within the yellow stripes as the disease progresses. The fungus survives in wheat residue and requires cool, wet weather. High residue, limited or short crop rotations, and very early planting favor disease development.
What should you do if you see Cephalosporium stripe in your fields? This disease requires cool, wet weather conditions so it is not expected to be problematic in Colorado. Document the incidence and take note of whether there is a lot of residue build-up in your field.
There are currently no reports of stripe rust in Colorado (Figure 5). However, stripe rust is present in Texas, Oklahoma, and Kansas. The recent dry weather and freezing temperatures in Texas and Oklahoma will likely help limit the spread of the rust spores to Colorado, but be sure to continue scouting for stripe rust and look throughout the entire canopy since symptoms may appear in the lower canopy first. Please feel free to email photos if you think you see stripe rust.
Be on the lookout for virus symptoms. Wheat streak mosaic (WSMV), Triticum mosaic (TriMV), and High plains wheat mosaic (HPWMoV) are the most common in Colorado, and as insect pressures across the state increase we may start to see more virus symptoms. There are no pesticides that are effective against mites, so controlling volunteer wheat, which harbor mites, is the best management practice. If you believe you see virus symptoms on resistant varieties, please send me pictures.
Growers are strongly encouraged to regularly scout wheat fields for diseases. Particularly, scout for tan spot, stripe rust, and viruses in the coming weeks, as well as the virus vectors (mites and aphids). Be sure to look throughout the entire canopy for diseases.
Brown wheat mites were observed this past week and pressure varied across sites. However, the recent rain will likely significantly decrease the pressure below the economic threshold. Information about brown wheat mites and other mite species may be found here: https://extension.colostate.edu/topic-areas/insects/mites-in-wheat-5578/#:~:text=The%20brown%20wheat%20mite%2C%20Petrobia,usually%20disappear%20with%20significant%20precipitation.
Russian wheat aphid (RWA) was observed below the economic threshold in all cases except for one incidence in Prowers county. A few sites had thrips. Information about RWA and other wheat-infesting aphids may be found here: https://extension.colostate.edu/topic-areas/insects/aphids-in-small-grains-5-568/.
The Colorado Wheat Entomology Newsletter, written by Dr. Punya Nachappa and Darren Cockrell, covers insect/mite pests and management tips. The newsletters are published bi-weekly during the growing season and are available here: https://coloradowheat.org/category/news-events/wheat-pest-and-disease-update/
The recovery from drought stress was highly variable between sites across the state. Yield potential estimates ranged from 20-60 bushels/acre. A few sites also had freeze damage. You may find more information about freeze damage here: http://range.colostate.edu/docs/Managing%20Spring%20Wheat%20Freeze%20Injury.pdf
PLANT DIAGNOSTIC CLINIC
Do you have a disease that you would like diagnosed? Contact the Plant Diagnostic Clinic for sample submission: https://plantclinic.agsci.colostate.edu/ or email@example.com.
CONTRIBUTORS: Many thanks to the CSU Extension Cropping Systems Team: Ron Meyer, Todd Ballard, Dr. Wilma Trujillo, Sally Jones-Diamond, Dr. Jerry Johnson, and Dennis Kaan; and Kevin Larson- Plainsman Research Center, for scouting for disease and pest problems and contributing to this report.