Jaime Cummings and Ken Wise, NYS IPM
It’s that time of year where we typically consider fungicide applications for white mold protection in our soybeans. However, this year is a little different. Soybeans across NY range from V4 to R4 this week, making it a challenging decision regarding whether or when to spray. As you know, white mold (Sclerotinia stem rot) is our most challenging and undermanaged disease of soybeans across the state (Fig. 1). It typically rears its ugly head when the rows and canopies close between growth stages R3-R6. We have no silver bullet for this disease, and therefore rely on an integrated management approach for the best results.
The pathogen produces sclerotia, which are the hard, black survival structures that can easily survive in the soil for at least 10 years, with some reports of up to 20 years. These long-lived sclerotia, and the wide host-range of this pathogen, make crop rotation as a management strategy difficult, if not impossible. Resistance to this devastating disease is moderate, at best, in some elite commercial varieties, but none are immune or strongly resistant. Canopy management is a goal of some growers who struggle with white mold, and efforts include reduced seeding rates and wider rows. There is plenty of evidence that increased airflow in the crop rows can reduce white mold infection, because the disease is favored by the humid conditions of a dense and closed canopy. Research on biological control with a product called Contans WG has shown limited or variable efficacy in New York and Michigan, and requires a multi-year commitment for applications for the best results. However, some NY soybean growers have been successful at reducing white mold incidence and severity in their fields treated with Contans WG, and consider the results well-worth the $35 per acre cost. Recent research at Cornell by Dr. Sarah Pethybridge (vegetable pathologist) has shown that planting soybeans into roller-crimped rye cover crops can significantly reduce the sporulation of the white mold fungus, resulting in significantly less disease. Paying attention to the expected weather patterns and forecasting models, such as Sporecaster, are also critical in making white mold management decisions, because this disease can be particularly devastating in times of high precipitation or humidity during temperatures below 85°F. Though, we have seen fairly severe epidemics in some fields even in hot, dry years.
Timely foliar fungicide applications with appropriate nozzles for canopy penetration (Fig. 2), in combination with crop rotation, genetic resistance, canopy management, and biological control remains our best approach for managing white mold in soybean fields. The main goal for your fungicide applications should be to get them applied BEFORE you have a major outbreak of white mold in your field. If you have soybeans in a field with a history of the disease, and if the weather conditions are forecasted to be favorable for disease, it’s recommended to get a protective fungicide on between the R1 and R3 growth stages. Fungicide applications can be a waste of money after R4. It’s important to note that once you have an epidemic in a field, no amount of fungicide will stop or cure the spread.
A number of foliar fungicides are labeled in NY for white mold protection on soybean that are rated ‘Good’ to ‘Very Good’ in the Cornell Guide for Integrated Field Crop Management, based on national replicated field trials. These include Aproach, Endura, and Omega. Other fungicides are rated as ‘Fair’, including Topguard, Proline, Domark, and Topsin-M. It’s important to follow all label recommendations, and note that some products, such as Aproach, recommend two applications when other products may only require a single application.
There have been a lot of soybean white mold fungicide efficacy trials in other states that have similar weather patterns and epidemics to ours, including Michigan, Wisconsin and N. Dakota. Dr. Michael Wunsch of N. Dakota State University demonstrated that increased row spacing in combination with timely application of Endura fungicide resulted in significantly lower disease incidence and higher yields compared to narrow rows and the non-treated control (Fig. 3). Mike Staton of Michigan State University demonstrated that a comparison of the fungicides Omega and Propulse showed that they both significantly increased yields compared to the non-treated control, especially in trials with high disease pressure, but that Propulse was a much more cost-effective option (Figs. 4 and 5). Dr. Damon Smith of University of Wisconsin evaluated the effect of various fungicide combinations and application timings on disease incidence and yield, and found significant improvements in yields from applications of Propulse + Delaro, Proline + Stratego, and a double application of Delaro (Fig. 6). All registered products evaluated in these trials are labeled for use against white mold of soybeans in NY.
Though we have a number of fungicides labeled for use on white mold in NY, not all field crop dealers carry all products, and pricing may vary by location. When opting to utilize a fungicide application as part of your integrated management strategy for white mold, keep in mind that there are wide ranges in efficacy and cost among products. A quick inquiry with only two sources provided prices or price ranges per acre of some of the products you may consider using, as outlined in Table 1 (in alphabetical order, at the highest labeled rates).
Considering the abnormally wide range in growth stages and canopy closure as we experience or approach flowering in our soybean fields, I think we can expect some difficulty in managing white mold in some locations this year. One of the most perpetuated fallacies I hear is that white mold requires soybean flowers for infection. Even though this is consistently mentioned in fact sheets and other resources, it is not entirely true. A soybean plant at any growth stage can succumb to infection by the white mold fungus if the conditions are favorable and if the spores are in the air. However, soybean flowering usually coincides with canopy closure, and this canopy closure encourages a humid environment within the rows which does enhance disease initiation and progression. And, shed flower petals do provide a nice food source for germinating spores. But, again, the flowers are not required for infection.
Although I have seen some nice soybean fields this year that were planted on time, either before or between all of the spring rain events we experienced, much of the soybean planting across the state was delayed this year due to wet conditions. That means there may be closed canopies with flowering soybeans across the road from fields with much younger or smaller plants. If the weather favors white mold with moderate temperatures, precipitation and humidity, the disease may initiate in one dense, flowering field and spread among many others. Or, it may initiate in a field where the plants are stunted with a fairly open canopy, if it’s a field with a history of this disease and favorable weather conditions. It’s anyone’s guess at when and where a white mold epidemic may happen this year given the variable growth stages and ranges in canopy closure.
Don’t despair, there’s still hope. I haven’t heard many reports of white mold yet from across the state, which means you still have time to make management decisions. Get out in your fields to scout, and pay attention to the weather. Know what growth stages your soybeans are at and how your canopies are looking, and what your neighbors’ beans are doing. If you have a history of white mold in your fields, and you know that the weather forecast for your area is favorable for the disease, then consider the most cost effective fungicide application to protect your crop. If you expect dry conditions in a field without a history of white mold, then you can probably skip the fungicide application. You know your fields best. But, be aware that the variable growth stages this year may add an unexpected layer of complication to white mold management and timing of fungicide applications.
Thank you to Jeff Miller and Josh Putman of CCE, and Danny DiGiacomandrea of Bayer for assistance with fungicide price ranges.