Stripe rust, wheat streak mosaic, barley yellow dwarf and leaf spot diseases were seen developing in several wheat fields scouted in Hughes, Stanley, Pennington, Tripp, Gregory, and Douglas counties of South Dakota last week.
"Although stripe rust is just beginning to develop, it could quickly develop to reach yield reducing levels," said Emmanuel Byamukama, SDSU Extension plant pathologist.
Byamukama added that reports from southern Nebraska indicate a severe stripe rust outbreak, indicating that there is a high inoculum of this rust in our area.
Stripe rust is favored by wet and cooler weather conditions—at least less than 70 degrees Fahrenheit. Most of the cultivars grown in South Dakota are susceptible to stripe rust; therefore, Byamukama advised planning a fungicide application should the disease pressure keep increasing.
"The ideal time of applying fungicide, considering how far wheat has come along, would be at blooming time," he said. "This timing would control Fusarium head blight and foliar diseases such as stripe rust and leaf spots."
Wheat streak mosaic virus (WSMV) was severe in a few fields in Tripp and Lyman counties. In other fields, WSMV was less severe and infected plants were scattered in the field, Byamukama said. "More severe symptoms are likely to be seen in fields that currently have mild symptoms as warmer temperatures set in."
WSMV is spread by wheat curl mites (WCM). WCM are blown by wind from nearby, previously infected, volunteer wheat or, can come from partially killed volunteer wheat when wheat is planted into wheat stubble before volunteer wheat is completely killed. Fall infections are the most severe and can cause total yield loss.
Barley yellow dwarf (BYD) was rarely found in the scouted fields however one field in Pennington county and one field in Tripp county had moderate levels of BYD. The typical symptom of BYD is the purple-reddish discoloration of the flag leaf, explained Connie Strunk, SDSU Extension plant pathology field specialist.
"Sometimes the virus may kill the leaf starting from the leaf tip. Early infected plants will be stunted," she said. "Barley yellow dwarf is spread by aphids mainly the bird cherry oat aphids." As with WSMV, Strunk said fall BYDV infections cause the most significant yield losses.
"Unlike fungal diseases where a fungicide can be used to prevent further infection and sometimes cure the infection, once plants are infected by a virus, there is little that can be done to control the disease," she said.
Management of plant viral diseases is through prevention of infection. For WSMV, effective management can be achieved through elimination of volunteer wheat and grassy weeds at least 14 days before planting.
Strunk said that delaying planting of winter wheat in areas where WSMV outbreaks are common can lessen chances of early infection. "BYDV is very sporadic and will develop in wheat under mild fall weather condition and where winter wheat is planted early in fall," she said.
She added that insecticide seed treatments may be helpful in limiting fall BYDV infections.
Other wheat diseases
Other common diseases noted were tan spot, Stagonospora blotch and powdery mildew.
"These were mainly in the lower canopy and were at low severity except in wheat on wheat and no till fields where leaf spot diseases severity was at moderate levels," Strunk said.