What’s Cropping Up? Volume 29 Number 3 – July/August 2019

White Mold of Soybean: What to expect with variable growth stages

Jaime Cummings and Ken Wise, NYS IPM

White mold
Figure 1. White mold infected soybean stem. (Photo by J. Cummings, 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.

Nozzle recommendations
Figure 2. Nozzle recommendations for white mold suppression from Michigan State University.

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.

Research results table
Figure 3. Research from North Dakota State University shows that combining wider row spacing with timely fungicide applications can decrease white mold disease severity and increase yields.

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.

Field trial results table
Figure 4. Field trial results in Michigan show that both Omega and Propulse fungicides each significantly increase soybean yields, particularly when white mold disease pressure is high.

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.

 

Fungicide trials results
Figure 5. White mold fungicide trials in Michigan demonstrate the economics of fungicide applications in fields with high and low disease pressure.
Fugicide efficacy data table
Figure 6. A table from University of Wisconsin outlines fungicide efficacy data on disease suppression and yield for various fungicides products and application timings. (Not all products evaluated in this trial are labeled for use 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).

Fungicides for white mold in soybeans in NY

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.

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What’s Cropping Up? Volume 29, Number 2 – March/April 2019

Western Bean Cutworm and Mycotoxins in Corn Silage

Joe Lawrence, Gary Bergstrom, Jaime Cummings, Elson Shields, Ken Wise, Mike Hunter

Mold and mycotoxin development in corn ears and stalks, and the resulting corn silage continues to be a major concern for dairy producers.  Mycotoxins can result in a range of problems for livestock throughout the year as they are ingested with the feed.  The presence of mold does not always have a strong correlation to mycotoxin development but it does present the chance for incidence to occur.

A number of factors influence the prevalence of molds from year to year. Conducive weather conditions for mold and mycotoxin development are outside the control of management options.  But hybrid characteristics and physical damage to the ears can be managed through the selection of hybrids and pest resistance traits in the hybrids.

WBC larvae beginning to feed on tip of corn ear prior to silage harvest. Photo by Joe Lawrence

Western Bean Cutworm (WBC) is a pest of corn (as well as dry beans) and its territory has been expanding eastward over the last 10 to 15 years with pockets of high populations now found in New York and Ontario, Canada. The moth emerges near the time of corn tasseling and lays its eggs near the ear leaf of a pollinating corn plant. When the larvae hatch they enter the corn ear, often opening a wound in the husk, and feeding on kernels. Unlike other earworms, which are cannibalistic, you can find multiple WBC larvae feeding on one ear, increasing the chances for significant ear damage.

Where WBC populations are high, the corresponding ear damage from WBC feeding can leave wounded corn ears more susceptible to pathogen development, but a clear relationship between ear damage and mycotoxin development has not been documented. A number of mold species may develop on corn ears though relatively few of these produce mycotoxins. Principal concern in New York is with the mycotoxins deoxynivalenol (DON or vomitoxin) and zearalenone (ZON), both produced by the fungus Fusarium graminearum. Infection by this fungus also occurs in roots and stalks and leads to Gibberella stalk rot and the accumulation of DON and ZON in stalk tissues. Much of the toxin loading in 2018 corn silage in New York was contributed by contaminated stalks as well as ear tissues.

While WBC damage to corn ears can be significant and may have detrimental effects on corn grain yield and quality, the economic impact on corn silage is less understood. For corn silage growers, understanding whether or not this pest significantly impacts the yield or quality of the forage is critical to their decision making for managing this pest.

Since the Cry1F protein, which has most commonly been utilized for protection against numerous corn insect pests, has been found to be ineffective against WBC, producers are left with limited management options.  Currently, the Vip3A trait in select corn hybrids in combination with a scout and spray program is the best option for WBC management in areas where the pest is prevalent.

With the increased population of WBC in NY, the Commercial Corn Silage Hybrid Evaluation program conducted by Cornell University in collaboration with the University of Vermont and the Northeast dairy industry offers a good opportunity to evaluate numerous hybrids for ear damage from WBC and mycotoxins. This was done in 2017 and 2018 with financial support from both the New York Corn Growers Association and the Northern New York Agricultural Development Program.

Each hybrid is planted (in triplicate) at two locations in NY and one location in Vermont (VT), with the locations each hybrid planted at based on hybrid relative maturity (Table 1).

Mycotoxin screening was limited to the NY locations based on funding available. In 2017, composite whole plant silage samples (3 replicates combined) were taken for each hybrid at two locations; Madrid in Northern NY and Aurora in Central NY. In 2018, a slightly different strategy was used with individual replicate samples taken on a subset of hybrids at each location.

Both seasons, each plot was scouted prior to harvest to assess WBC feeding damage to the ears. At harvest a whole plant silage sample was collected and submitted to the Dairy One forage laboratory for a mycotoxin screening package which included aflatoxins B1, B2, G1, G2, vomitoxin, 3-acetyl DON, 15-acetyl DON, zearalenone, and T2 toxin.

Through the New York State Integrated Pest Management (NYS IPM) WBC Pheromone Trapping Network, WBC populations were monitored at each location.  Though it should be noted that as the traps only attract male moths, they help in understanding geographic differences in WBC population but may not be representative of the population of egg laying females.

The results of the WBC and mycotoxin screening project revealed large differences in the pheromone trap counts and the number of plots damaged by WBC (Tables 2a and 2b). There was also wide variation in the prevalence of samples testing positive for mycotoxins, particularly in 2018.  However, there was a lack of correlation between WBC damage and incidence of mycotoxins in both years (Table 2a and 2b).

Additionally, despite the damage to corn kernels inflicted by WBC, in plots with up to 60% of ears showing some level of WBC damage, the WBC feeding did not correlate to any negative impact on silage yield or forage starch content in this study.

The most prevalent species of mycotoxin-producing mold found in the screening was Fusarium graminearum.  This fungal pathogen can also infect corn ears through the silk channels at the time of pollination during favorable weather conditions and result in contamination of the grain and silage with the mycotoxins DON, 3-ADON, 15-ADON, or zearalenone. A review of the weather data from both years (despite very different overall weather patterns) showed wet conditions at silking conducive to this type of infection. As expected for New York, no aflatoxins were detected.

While there aren’t many in-field management options to reduce the chances of mycotoxin development (note that controlling plant diseases and mycotoxins are not the same thing), harvesting corn silage at the proper whole plant dry matter is helpful. Based on numerous field observations, and notable at the 2018 Aurora location in this study, a whole plant dry matter in the high 30’s or above appears to increase the risk of mycotoxin development.

While there are numerous ways in which molds can establish themselves in forages, this study reflects a common challenge researchers face while attempting to document the conditions where mycotoxin development is likely. These results, over two growing seasons, provide no evidence that WBC damage is an added risk factor for corn silage growers who are worried about deoxynivalenol and zearalenone in their silage. In areas of the country where other toxins are more prevalent the impact of WBC and other insect pest may differ. It is important to note that these results do not reflect what may occur in corn harvested for grain because the time between silage harvest and grain harvest offers additional opportunities for infection and growth.

Growers should continue to scout for this pest and weigh the cost of control with the potential for damage.  However, it does not appear that controlling WBC should be viewed as a significant management consideration for reducing the risk of mycotoxin development in corn for silage.

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What’s Cropping Up? Volume 28, Number 4 – September/October 2018

Soybean White Mold Variety Trial – Genesee County, 2018

Jaime Cummings, NYS Integrated Pest Management Program

Figure 1. White mold infection on soybean in the variety trial (photo by Jaime Cummings)

White mold, or Sclerotinia stem rot, caused by the fungus Sclerotinia sclerotiorum is the most economically important and difficult to manage disease of soybeans across NY State (Figure 1).  This disease is so undermanaged because the pathogen survives for a long time (>10 years) in the soil, making crop rotations a challenging management option.  Fungicide trials in other states have shown great promise for a number of products (https://www.ag.ndsu.edu/Carringtonrec/plant-pathology/fungicide-efficacy-testing-results-2013-soybeans), but application timing and canopy penetration is critical and may require multiple applications during a highly conducive season, which may not be economical.  Genetic resistance to this devastating disease should be a viable option, but many commercial varieties lack even modest levels of resistance.

A large-scale, non-replicated strip field trial was established in Genesee County to evaluate 24 soybean varieties for resistance white mold.  The trial was organized by WNYCMA and planted on 5/1/18 in a field with a long history of white mold infection.  The varieties evaluated in this trial included entries from five seed companies, and were representative of maturity groups 0.7 – 2.8.  The trial was rated for white mold severity on 9/5/18 by Jaime Cummings of the NYS IPM Program and Dr. Gary Bergstrom of Cornell’s field crops pathology program using a 1 to 9 rating scale, where 1 = resistant, and 9 = susceptible.  The disease was well established consistently across all strip plots at the time of rating, despite it being rotated out of soybeans since 2014.  The disease ratings are summarized in Figure 2.

Figure 2. White mold disease severity ratings of 24 soybean varieties in a non-replicated field strip trial, rated on a 1-9 scale, where 1 = resistant and 9 = susceptible.

The ratings for all varieties ranged between 4 and 7, meaning that all varieties were classified as moderately resistant (3.6 – 5.9) or moderately susceptible (6.0 – 7.5) at the time of the rating.  However, the disease would most likely progress in these plots over time, which would likely add one or two points to each rating, pushing many of them into the susceptible category (7.6 – 9).  Even though none of the varieties evaluated showed strong resistance, it is good to note that there are noticeable differences among varieties.

New York soybean growers do have options for selecting varieties with some moderate levels of tolerance to this disease, and should know to avoid planting the most susceptible varieties in fields with a history of the disease.  An integrated management plan which includes crop rotation, canopy management, foliar fungicides and planting tolerant varieties is the best approach to managing white mold in NY.

 

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