From Ned Tisserat, Retired Plant Pathologist, Bioagricultural Sciences and Pest Management
I visited field plots at Akron last Thursday. I don’t want to make too much from a single site visit because conditions may vary widely in the state, but I think these plots may offer some insight on what is happening in many areas. The Akron plots were established to demonstrate how planting dates and varieties influence mite-transmitted virus diseases. Varieties susceptible and resistant to WSMV were planted in August, September and October. I was stunned at the damage on the susceptible varieties caused by wheat streak mosaic virus (putative) in the August, and to some extent the September, planting dates. As expected the early October planting date showed much less infection. The susceptible varieties at the early planting date are so stunted we are going to have to set the combine at 3-4 inches just to harvest. Ron Meyer, extension agronomist, told me that the mite transmitted viruses are as bad as he has seen around Burlington. If you planted early with a susceptible variety, then it is likely showing WSMV or other mite transmitted virus symptoms. Ron surmises the heavy virus outbreak is a result of a relatively mild fall with probable movement of mites from other grass hosts or late maturing corn. I concur. Because of the heavy virus infection you should carefully scout yellow-looking fields and not simply assume the damage is completely from stripe rust. As I have stated several times, no sense putting additional inputs (i.e. fungicides) into these fields.
But there is more to glean from the Akron field plots. Even the WSMV resistant varieties were showing some yellowing and stunting from the early planting dates. My guess is that this damage is from barley yellow dwarf virus, but testing needs to be done. Again, the relatively mild fall may have favored aphid infection with BYDV.
To complicate matters, almost all of the varieties in the plots were showing some level of stripe rust infection. In many cases there were yellow streaks apparent on the flag leaf, but these lesions had not yet burst open to produce the yellow pustules containing the spores. I think these lesions will mature this week further exacerbating damage. But I am hopeful the weather this week will shut down any future infections.
Purple heads.
There is also concern about the purpling of heads on many of varieties throughout the eastern part of the state. Possible causes range from genetic and physiological effects as a result of the weird weather, Stagnospora glume blotch, or bacterial black chaff. When I was at Akron, I noted several varieties exhibited mild purpling of the heads. After close examination I concluded that, at least in these plots, the coloration was not the result of Stagonospora or black chaff. My best guess is the cool May weather caused a physiological purpling, as Dr. Scott Haley had speculated in last week’s email. My counterpart in Oklahoma Dr. Bob Hunger (I guess I should say my former counterpart since I am no longer working-old plant pathologists don’t die, they just rust!) noticed a similar purpling in the panhandle region and he has tentatively concluded it is also physiological.
Finally Ron Meyer noted some minor Russian wheat aphid at the Akron plots. Dr. Gebre Amlak has noted RWA in Weld county and has offered the following information.
Assefa Gebre-Amlak Extension Specialist, Colorado State University Extension
Russian wheat aphid (RWA) infestations have been observed in the Front Range as well as northeastern counties. Some fields in Weld County showed economic infestations of the insect and insecticide application will be justified.
RWA can be found in winter wheat, usually on the younger leaves, from emergence in the fall to grain ripening. Aphid feeding prevents young leaves from unrolling. RWA colonies are found within the tubes formed by these tightly curled leaves. This not only makes it difficult to achieve good insecticide coverage, but also interferes with the ability of predaceous insects to reach and attack aphids. Leaves infested by RWA have long white, purple or yellowish streaks. Under some conditions, infested wheat tillers have a purplish color. Heavily infested plants are stunted and some may appear prostrate or flattened.
After flowering, some heads are twisted or distorted and have a bleached appearance. Heads often have a “fish hook” shape caused by awns trapped by tightly curled flag leaves. At this time most RWA are found feeding on the stem within the flag leaf sheath or on developing kernels. There may be poorly formed or blank grains and the entire head sometimes is killed.
Ned Tisserat
Retired Plant Pathologist
