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Extension > Minnesota Crop News > Archives > August 2009 Archives

August 2009 Archives

By Dean Malvick, Department of Plant Pathology

Cool weather and low rainfall have held the soybean crop back in parts of Minnesota this summer. Now, significant diseases are a concern in some areas. Several diseases have started to appear recently that can significantly damage soybeans, especially sudden death syndrome (SDS), brown stem rot (BSR), and white mold. These diseases are favored by weather conditions that have occurred in large areas of Minnesota this season. The information we gather now can help to explain why yield may be low in some fields and can assist with targeting disease management where these diseases occur most often. There are no effective fungicide or other treatments that can be used this season to reduce damage from these diseases.

SDS has been reported at significant levels in Dodge, McLeod, and Waseca counties, and it is undoubtedly also appearing in other areas in Minnesota. Based on results from previous years, SDS probably occurs across much of the southern half of Minnesota, but it could occur anywhere in the state. Foliar symptoms of SDS have been developing for at least the past two weeks. The last two weeks of August and the first week of September is a good time to scout for SDS. Look for this disease first in low or compacted areas of fields and near field entry points. Initial symptoms are yellow areas on leaves, and these yellow areas turn into brown patches. Reports have indicated different levels of disease severity among some varieties. This would be a good time to look for different factors that could affect SDS severity, including soybean variety, tillage, drainage, crop rotation, and planting date. The map of the confirmed distribution for SDS in Minnesota, as well as more information and photos for SDS and other soybean diseases, can be found at the Minnesota Crop Diseases web site (www.extension.umn.edu/cropdiseases/soybean/index.html).

Foliar and stem symptoms of BSR are also starting to appear. BSR occurs in fields throughout Minnesota, including all areas where SDS occurs. When scouting fields or attempting to diagnose whether plants are infected with SDS or BSR, keep in mind that the foliar symptoms can look very similar for these two diseases. The key for BSR is to split the bottom 6" of the soybean stems and look for brown pith, especially at the nodes. Plants with BSR will have brown pith, but plants with SDS will have white pith. Keep in mind that plants with BSR always develops brown pith, but foliar symptoms (brown tissue between veins) do not always develop. The only way to know how much BSR is in a field is to split stems, and internal BSR stem infections alone can cause yield loss. BSR tends to be more severe when temperatures are cool (60- 80°F) during reproductive growth stages. When leaf symptoms develop, they can be most severe when soil has been wet at early flowering to pod fill growth stages and dry during maturation. The conditions in large areas of Minnesota appear to have been favorable for BSR this season.

White mold may also be significant in some parts of Minnesota based on growing conditions this year. We have had reports of white mold damaging and killing in several areas of Minnesota. This disease is favored by cool temperatures and rainfall from mid- July through August. All of Minnesota has had cool conditions, and the areas that received enough rain to maintain wet soil for short periods in July and early August are set to develop white mold. Look for scattered and patches of dead and dying plants, and the presence of white moldy growth and sclerotia (black to gray round to oblong structures up to 1/4") on and inside the stems. As with the other diseases, look to see if you can associate the more severe areas with factors that may affect white mold such as soybean variety, manure or fertility history, tillage, row spacing, or high plant population.

Finally, downy mildew is common in a number of areas. Although this soybean leaf disease rarely if ever reaches levels that damages yields in Minnesota, it can be fairly easy to see. Downy mildew typically only infects leaves at this time of the season, although later it can infect pods. Symptoms of downy mildew on the tops of leaves are irregular yellow to brown spots, and the bottom of the leaves under the spots have tan to gray tufts of fungal growth that can be seen easily with a hand-lens.

Recognizing Glyphosate-Resistant Weeds: A Field ID Experience

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Wednesday, September 9 ----- Plummer, MN

Meet at and depart from Plummer Co-op Creamery
(Cenex Station) 1 pm


Thursday, September 10 ----- Hawley, MN
Meet at and depart from RDO Equipment 9:30 am


Is glyphosate less effective than 10 years ago?


Can you recognize the presence of glyphosate-resistant weeds in a field?


Irrigated Corn Silage Plot Tour

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Friday, August 28, 2009
10:30 a.m.
Dan Dreyer Farm - Ottertail City

Tour Agenda:
Forage Production and Management Update
Forage Insects Past, Present, and Future
Alfalfa/Grass Stand Management
Hybrid Evaluation and Industry Update From Seed Companies

HRSW Varieties with a Higher Risk of Preharvest Sprouting

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The continued wet weather and harvest delays are increasing the potential for preharvest sprouting. Once the dormancy of the seed is broken and sprouting is initiated the quality of the grain deteriorates, grain elevators will check for this decline in quality using the Hagberg Falling Numbers test. The HRSW that are ranked moderately susceptible to pre-harvest sprouting are listed in Table 1. Understand that the potential for preharvest sprouting increases if you swath the grain or if you leave it stand too long while waiting for the grain to reach 13% moisture, all the while rain and heavy dews are forecasted. Rather, harvest the grain as quickly as possible and as soon as moisture content approaches 15% as HRSW can be readily stored up to three months at that moisture content.

Table 1 - HRSW varieties with a higher risk of preharvest sprouting

Variety Preharvest Sprouting Rating*

Bigg Red 4
Blade 5
Granger 4
Hat Trick 4
Sabin 4
Samson 4
Traverse 4

* 1=best, 9=worst

Soybean Growth Stages for Pest Management Decisions

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by Phillip Glogoza, Extension Educator, Crops

Management decisions on whether to treat soybean aphids will be affected by the soybean growth stage in a field during the next two weeks. As plants progress to the later reproductive stages (e.g., R5, R6, R7, etc.) risk of yield loss from aphids declines. Currently, the soybean crop ranges from R3 to R5. Insecticide treatments for R5 stage soybeans may respond positively to soybean aphid treatments when populations exceed threshold, however the level of the yield response is less predictable. Early R5 treatments are more likely to realize a positive response than late R5 treatments. Treatments for aphids are generally not recommended beyond the R6 growth stage.

Preharvest Management Options for Wheat

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There are two methods of pre-harvest management for wheat that can speed up harvest. Swathing or windrowing is one method. An application of glyphosate is a second option. Several brands of glyphosate are labeled for preharvest weed control. Research has shown that glyphosate can also quicken the dry down of the wheat crop if conditions for dry down are adverse. The preharvest interval for preharvest glyphosate is seven days and expects only to gain a couple of days at the most. More time can be gained with swathing.

The optimum time for either pre-harvest management tool is right at or just after physiological maturity of the crop. At physiological maturity, the crop has the maximum kernel dry weight and no additional dry matter will be deposited in the grain. The kernel moisture percentage at physiological maturity is relatively high and can vary from 20 to 40%. Research has shown that swathing just before physiological maturity does not harm the grain yield or quality. This practice, however, is not recommended when using glyphosate as a pre-harvest tool.

There are two visual indicators that can be used to determine whether the crop has reached physiological maturity. The first indicator is the loss of green in the kernel and the appearance of a dark layer of cells or pigment strand along the crease of the wheat kernel (Photos 1). Kernels in the same spike will reach physiological maturity at different times with the middle of the head maturing first.

pigment_strand.jpg Photo 1 - Wheat kernels before (above) and at (below) physiological maturity.


Another visual indicator is the loss of green from the peduncle and glumes. If the peduncle just below the head becomes straw-colored, transportation of water and nutrients to the head has been cut off and the crop has reached physiological maturity (Photo 2). The advantages and disadvantages of pre-harvest glyphosate and swathing are listed in Table 1.

Physiological Maturity.JPG Photo 2 - Wheat spikes before (left) and at (right) physiological maturity

Table 1 - Advantages and disadvantages of different methods of pre-harvest management.

09 Table 1 Preharvest Management.jpg

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