Over the last few weeks the wheat crop has developed rapidly. Temperatures across much of the state during this period have been in the low to mid 80’s for highs and lows in the low 60’s which are, as you might imagine, well above normal. To put things in perspective, the temperatures that we have been experiencing are typically seen in for mid-May! The warm temperatures have really accelerated wheat development. I looked at a field of an early maturing variety on Feb. 28th and considered it to be just starting to joint. I looked at the same field on March 2oth, three weeks later, and it was in the boot stage. Some early maturity varieties are heading very early this year as influenced by temperature. Growth stages across the state range from flowering in some early planted, early maturing varieties in south Arkansas to Feekes growth stage 7 (2 nodes visible). Overall, I would say that wheat growth is 10-14 days ahead of normal.
Temperatures have cooled off the last day or two as a slow-moving front has dumped heavy rains across Western and Central Arkansas. I dumped 6.45 inches out of my rain gauge this evening near Conway, AR! Rainfall amounts in Eastern Arkansas are anticipated to be less. A little rain has been beneficial for some producers who needed to get late-season nitrogen applications incorporated, but we didn’t need 6 inches for sure. Temperatures are expected to cool off slightly this week but jump back into the 80’s next week. Overall, the wheat crop looks good at this point.
Unfortunately glyphosate drift reports are starting to come again this year. The symptoms I have seen so far have been mild; however, I have gotten reports of more severe damage in isolated instances. Impact on yield will likely vary on timing of glyphosate drift.
An update on the stripe rust situation from Dr. Gene Milus, U of A Professor of Plant Pathology:
Based on this week’s data from Wheat Variety Testing plots at Marianna, Progeny 117, Progeny 185, Dyna-Gro Baldwin, Pioneer 26R22, Syngenta Arcadia, Hornbeck 3266, and AGS CL-7 were susceptible to stripe rust. Other known susceptible varieties not in this year’s variety test include Croplan Genetics 554W, Roane, Croplan Genetics 514W, and Armor Renegade. Syngenta Beretta and Armor Ricochet, that had considerable stripe rust earlier in the season, appear to be expressing adult-plant resistance, as lesions on upper leaves are few and look like those in the bottom photo above. The level of adult-plant resistance in these and other varieties with adult-plant resistance likely will increase (become more resistant) as plants enter heading stage.
A field of Progeny 185 in Crittenden County had a considerable amount stripe rust with a fully susceptible reaction (similar to top photo above) even though there were no obvious hot spots where stripe rust had overwintered. Apparently the rust spores had blown into this field from other fields over the past two months and developed quickly throughout most of the field.
The weather during the past week or so has been too warm for rapid stripe rust development. Night temperatures are the most important because this is when new infections occur if there is dew on the leaves. The optimal temperature for infection is around 46°F, and no infections will occur at temperatures above 65°F. High daytime temperatures will slow the spread within infected leaves but will not eliminate the disease.
Based on observations of several fields of Ricochet and Beretta that had been sprayed with Tilt fungicide, no rust was seen on upper leaves even though rust had been widespread on lower leaves. It is not possible to separate the effects of Tilt, adult-plant resistance, and warm temperatures during the past week, but the net effect is good for the wheat and bad for stripe rust. Rust spores collected from a field that had been sprayed the previous day with Folicur fungicide were not viable, indicating that this fungicide has a rapid effect on reducing the spread of viable spores. Tilt should work similar to Folicur.
Varieties that have not been mentioned above have not been reported or observed to have stripe rust this season. This list include varieties such as 26R87 and AGS 2060 that have resistance gene Yr17 and were susceptible in 2010 and 2011. It appears that this year’s stripe rust is not virulent on Yr17. I would be very interested in hearing about stripe rust on flag leaves of any variety not listed as susceptible or very susceptible, as it likely would be a new or different race.
Advice for stripe rust management
Fields of susceptible and very susceptible varieties (listed above) likely have stripe rust should be the highest priority for scouting. These varieties have little or no adult-plant resistance, and stripe rust is likely to increase. If stripe rust is found, a fungicide should be applied sooner rather than later. If the field is at boot stage or later in maturity, one fungicide application may be enough for the rest of the season. If the field is less mature than boot stage, two applications may be needed to protect the wheat through the rest of the season. The level of stripe rust pressure and the need for fungicide will depend on the weather between now and flowering.
Fields of varieties such as Armor Ricochet and Syngenta Beretta that had a lot of stripe rust earlier in the season also need to be scouted, but it will be important to distinguish whether rust is moving to the new, upper leaves or only on old, lower leaves. If rust has not developed on the upper leaves, then there is no immediate need to apply a fungicide. If rust is on the upper leaves but the reaction type is more moderately resistant than susceptible (see above photos) , then there is no immediate need to apply a fungicide. Be aware that there may be a few off-type, susceptible plants in any given field, so don’t over-react because of a few plants with stripe rust. Fields of all varieties should be scouted diseases in addition to stripe rust, as leaf rust and leaf blotch have been found in several fields.