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Reviewing the season
December 1, 2016 - Bob Streit
December will be here by the time you read this. Temperatures are bound to become more seasonal, meaning having a good winter jacket and internal antifreeze will be important to survival.
From a Minnesotan From here I am going to give credit to Dr. Dean Malvick of the University of Minnesota for his monthly column on the growing season, its particulars, and how it contributed to above trendline yields for both corn and beans. Remember that given the amounts of rain each region received and the concurrent soil and air temperatures in another year the combination could work to reach below-trendline yields. A few degrees difference of the presence of a new disease inoculum could make the difference. I will interject as to how I interpret the seasonal conditions and where we may have to take action if something starts to occur. My added comments are in italics. According to Malvick: Two common themes across much of Minnesota’s croplands in 2016 were rain and high yields. Conditions were also favorable for diseases in a number of areas. Sometimes there was obvious damage and yield loss, and in other cases the role that disease played in holding back yields from even higher levels was unclear. Disease was again highly variable across the state, even more inconsistent than the weather that favors disease development. Some diseases were common over wide areas, but others were much more scattered. With harvest nearly complete it is once again a good time to review some of the diseases issues that occurred across Minnesota in 2016. ? Soybeans: SDS was widely observed in southern Minnesota. Frequent rains favored this disease. Planting date also seemed to be a factor. High levels of SDS developed in research plots that were planted in early May and much less disease developed in plots planted in late May. The effect of this disease on yield can be difficult to determine because it tends to be patchy in fields and often occurs where conditions favor high yields. Our SDS field research studies once again showed the importance of using resistant soybean varieties as well as the potential value of selected seed treatments to manage SDS. There was a heightened concern for white mold in July based on the frequent rainfalls and early observation of disease development. Recall that wet weather along with relatively cool temperatures during flowering periods are what this disease prefers. However, warm temperatures during the critical July to early August period seemed to suppress this disease in most areas. Although white mold was widely observed, it appears that few areas (with some exceptions) had significant yield loss due to this disease. The key, according to Dr. Gus Lorenz from the University of Arkansas, is soil temps have to be above 74 F for the spores to be unable to infect. Thus the hot temps from the start of flowering — around June 21 — and continuing for four to six weeks was critically beneficial. In most years this amount of rainfall would have brought on the worst white mold we have seen. Rhizoctonia root rot caused significant problems in some soybean fields. The most common set of conditions that favor Rhizoctonia root rot are warm and moist soil for the first two to three weeks after planting. This most often occurs in fields after mid-May. Looking back, this disease was not a major problem in 2015, but was one of the most common soybean diseases in parts of Minnesota in 2013 and 2014. ? Corn: Goss’ leaf blight and wilt was seen at significant levels in multiple fields, especially in fields damaged by hail in July. Significant yield losses were reported from a few fields. Thus, Goss’ wilt continues to be a disease to watch for given its potential to reduce yields. In fields with confirmed risk of Goss’ wilt, especially where it was confirmed previously, highly resistant hybrids should be considered in future years when corn is planted. My take on this is different in that we have seen a new combination disease in the last eight years that does not need hail or a windstorm to infect. I look for caramel-colored lesions coming from the ground up as it flows up the vascular system and through the entire plant. This typically become visible at about V8 to V10, and is so slight at first it is easy to ignore. It always kills the plants and this year we were granted a reprieve because there were no 90-plus degree temps with a strong south wind with low humidities after Aug 15 and until Sept 6. Were we looking for those conditions to occur? Definitely and when they did we knew most of the crop would turn whitish brown in short fashion, and it did. By delaying death until Sept. 6 most early planted fields were granted an extra two to three weeks worth of grain fill versus other years. Given its prevalence last year in southern Minnesota Iowa, northern corn leaf blight was perhaps the most anticipated and watched-for corn disease. Later in the season it was not difficult to find NCLB lesions in many fields, a reminder that there is a fresh crop of inoculum of the NCLB fungal pathogen in many areas that could incite problems in 2017 if conditions favor for disease development. NCLB was very damaging in 2016 to B37 based hybrids as well as others. Drier weather slows it down. But it is likely to return in future years. Other corn diseases reported include seedling diseases, stalk and ear rots from many fields in southern and central Minnesota, and Physoderma leaf spot. Of these, stalk rots late in the season appeared to be the biggest problem in many fields. So there is one new disease to be alert to there is a new disease of corn called bacterial leaf streak, caused by the bacterial pathogen Xanthomonas vasicola pv. vasculorum, was confirmed in the U. S. and Minnesota for the first time in 2016. Following confirmation in Nebraska, surveys were initiated in a number of states and the disease has been confirmed in at least nine states, including, Minnesota, Iowa, South Dakota, Illinois, Kansas, Coloado, Texas, Oklahoma and Nebraska. Experienced crop scouts have noted and discussed this and knew something new was here, but did not have a name for it. Corn leaves with this disease have narrow tan to brown streaks that are up to several inches long, and the symptoms are similar to gray leaf spot. The potential for this disease to cause significant yield loss is unknown, but is thought to be low. More information on bacterial leaf streak can be found in these reports from the UNL, ISU, MSU and KSU. The damage and effects from corn diseases are typically cumulative. If a new fungal or bacterial disease would remove an additional amount of green tissue from the total leaf tissue amount it can have an effect. With few tools to fight it outside of medical grade pharmaceuticals, which would be financially prohibitive, having a good mineral profile in each plant would be our best course of action. If leaf streaking becomes pronounced be ready to react. In furrow at planting would take less work.
Bob Streit is an independent crop consultant and columnist for Farm News. He can be reached at (515) 709-0143 or www.CentralIowaAg.com.
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