Examining Risk Factors Associated with White Mold in Soybean

I just spent the past three days participating in various field days in the western part of the state. These travels really provided an excellent opportunity to examine stand quality across the different field crops, in particular the soybean crop. Soybean stands were pretty variable across these areas, ranging from some really good looking soybean to areas where the stands were struggling quite a bit. Given that many of these fields were last in soybean during 2009, when we had excellent conditions for white mold to develop, this was a topic that was on the minds of many of the producers and consultants.

With soybean moving into the early flowering period, now is a good time to think about several risk factors that can be integrated to determine if using a management tactic like foliar fungicides is warranted.

(1) What has been the previous history of white mold in my fields? In several of the meetings during this past week, multiple growers indicated that white mold was their main disease affecting soybean yield. During these discussions, it was also clear that they recognized that they either needed to, or had already changed several of their production practices to better manage this disease. Recognizing the risk to white mold is critical for constructing both a short- and long-term management plan for reducing the risk of this disease it is important to maintain good records for previous occurrences of white mold as well as field history information like the crop rotation, soybean variety, row spacing, plant population, and general weather conditions.

(2) How does planting date and relative maturity affect risk? Early planting, late-maturing, and varieties with a bushy architecture can all contribute to increased close canopies. This year, from our discussions with various growers, the planting date is much more variable than in 2010 so it remains to be seen how much that affects differences in flowering dates around the state. Many of the fields we visited were planted weeks later than last year.

(3) What variety have I planted and what level of partial resistance does this variety have for white mold? During the discussions, there were several questions about “what soybean variety” should I consider planting. Keep in mind that there are no varieties with complete resistance to white mold but there are partially resistant varieties available. What this means is that in years where white mold occurs, the severity of the disease will be less than in a susceptible variety. If you are not sure what the level of resistance is to white mold in the variety you are currently using, asking your local extension agent or seed dealer to help you find that information is important. Also, do not be shy to ask how that rating was determined.

(4) What was the plant population I planted? When I asked many of the growers what their current plant populations were, the common answer ranged from something like “140,000 to 160,000 plants per acre” to “one bag, one acre”. Previous research has shown that when the plant population is greater than 175,000 plants per acre, the risk of white mold can increase. It is advisable to consider to taking stand assessments across your fields with a history of white mold to accurately determine the average plant population in those fields.

(5) What is the row width of my soybean? We saw a wide range of row widths in our travels from 7.5″ to 30″ as well as twin rows. Narrower row spacings can lead to faster and more complete canopy closure, which can increase the risk of white mold. Several growers commented how they had moved to wider row spacing to improve air flow through the canopy especially in fields where they had a history of white mold.

(6) What conditions favor the development of white mold? There are several environmental conditions that need to occur to increase the risk of white mold. Sclerotia (the overwintering structure of the pathogen) in the top two inches of soil can germinate to produce apothecia under high moisture and cool soil conditions (40 to 60 F). Apothecia can produce millions of spores called ascospores and these can infect the plant through senescing flowers. Infection is favored by a dense canopy during the flowering period and rain, fog, or dew which result in a shaded and moist microclimate within the canopy. Temperature is key driver in this process. Cool maximum daily temperatures (< 85F) are more favorable conditions for disease development.

After considering all of these factors, in-season management of white mold is focused on determining if there is a need for a foliar fungicide application. The use of a foliar fungicide should be part of an overall integrated program that is based on all of the risk factors previously discussed. Over the past two years, results from our white mold fungicide trials in Wisconsin have not shown a consistent result meaning that, while there has been some reductions in white mold in some of the trials, this has not always translated in increased yields. We have also not seen a consistent response by specific fungicide products meaning results differed between the years. Over the years, the best results across University studies have shown reductions as high as 60%, although several factors can influence this response. In particular, the best timing remains in the early reproductive developmental stages like R1 (initial flowering). Also, adequate plant coverage is needed to get the fungicide to the site of where infections occur (the flowers). Flat-fan spray nozzles that produce a fine to medium droplet size (200 to 400 microns) provide the best fungicide coverage. Canopy density also needs to be considered since this can affect the necessary volume of water needed to achieve optimal coverage, as a greater volume is needed when there is a heavy canopy density.

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