ISSUE 12 July 29, 2010
NDSU IPM SURVEY RESULTS, JULY 23
NDSU IPM scouts looked at 83 wheat fields during the week of July 19-23, with the average growth stage of these crops at early milk. Field scouts in the east and south central areas will be concentrating on soybean and sunflower surveys in the next few weeks, as wheat in their areas has matured.
In wheat, almost all fields had tan spot infection, with an average tan spot severity of 5.2%. Leaf rust was observed more frequently than in previous weeks, with 7.2% of the scouted fields showing some common leaf rust, with an average severity of 3%. Stripe rust also was observed in 7.2% of the surveyed fields In field tours at Research Extension Centers in Carrington, Minot and Langdon last week, I observed quite a bit of stripe rust in some varieties in the spring wheat drill strips. Stripe rust had developed in winter wheat, and now, late planted spring wheat variety plots in the cooler parts of the state this year are showing more of this disease. Most commercial fields surveyed are not showing stripe rust, in part, possibly due to fungicide use.
Twenty percent of the scouted wheat fields showed some symptoms of head scab, with an average severity of 2.5% in those symptomatic fields. Another disease observed more frequently this past week was loose smut, in 9.6% of the fields, with an average of 8% of the tillers in those fields showing smutted heads.
Seven barley fields again were surveyed and the average growth stage in these surveyed fields was early soft dough stage. Fungal leaf spot diseases are commonly observed in barley, with an average severity on the flag leaves of 13.6% in those surveyed fields.
WHEAT SCAB RISK AGAIN FOR LATE PLANTED CROPS
Recent rains have raised the
risk of head scab again. The wheat
scab forecasting web sites on July 27th showed moderate to high risk
of Fusarium head blight (scab) infection in moderately susceptible to
susceptible cultivars, along the northern tier of ND counties.
Most wheat is well beyond fungicide application stage, but there are
still some very late planted fields that may need a fungicide application at
flowering, to reduce the risk of head scab.
Extension Plant Pathologist
SOYBEAN RUST A NON-ISSUE
I have been periodically getting calls about soybean rust, and I am happy to report that the disease will be a no-show again. The pathogen can only overwinter along the gulf coast and although it did survive the winter, it has not progressed north and will not be an issue again this year (Figure 1).
Figure 1. Soybean rust occurrences in the United States. Red colored
counties are positive for soybean rust, green are negative.
The graphic is taken from the www.sbrusa.net website. In addition to disease monitoring, information on disease identification, management, and other educational information is available.
FUNGICIDES AND SOYBEANS
I generally get three fungicide usage questions on soybeans.
Foliar disease pressure on soybeans is pretty limited in North Dakota. The most common pathogen we found in a two year survey (2008 and 2009) was caused by bacteria, which is not managed by fungicide applications and not thought to be very yield limiting. Fungal disease pressure (Septoria Brown Spot, Downy Mildew, etc..) was very low. Based on this information, fungicide application for foliar blights would likely not be economically viable in the broad sense. Exceptions are possible if an outbreak of a disease occurs, but that is generally unlikely.
White mold is a problem on soybeans, and fungicides can manage this disease to some degree. Last year we had a perfect environment for white mold and some soybeans did take a hit. White mold pressure is going to be less than last year, but the environment is still somewhat favorable. Just keep in mind that soybeans can take some white mold, and not be yield compromised. If considering a fungicide application on soybeans, the most important factor is not necessarily which product to use, but when to use it. Fungicides are most efficacious when applied at the early bloom stages. In our trials, R1-R2 appears to be the best time to get the chemical on. By R3 your efficacy is reduced. Although an R3 application may clean up some new infections, the most important infections (lower stem, earliest infections) have already taken place.
Fungicides on soybeans in the absence of disease are sometimes applied for physiological yield bumps. In 17 North Dakota trials (2004-2009, Headline data only) we have only observed a statistically significant yield increase in two. Two additional trials were close to statistical significance (significant at P=0.1). Either way, two of 17 or four of 17 is not good odds for yield increases. Although we do see these unexplained yield bumps periodically, the odds of receiving a significant yield increase, and the inability to predict the occurrence suggest that a fungicide application in the absence of disease pressure may have more potential cost risk than potential reward. We are currently conducting additional trials on soybeans to try and obtain more information about fungicide in non-disease environments.
Extension Plant Pathologist