White mold was first seen in the United States in 1924 and it the disease first appeared in the Midwest in the mid 1940’s. Sclerotinia sclerotiorum can now be found throughout most of the world. Yield loss between the years of 1996 through 2009 were estimated to have been greater than 10 million bushels in 7 of these 14 years with the highest yield loss reported in 1997 (35 million), 2004 (60 million), and 2009 (59 million). Yield loss is caused by the reduction in seed quantity and seed weight and it is estimated that every 10% increment of the plant population that is infected can cause a 2-5-bushel decrease in yield. Several years since 2009 have had production issues with white mold in soybeans and 2017 was not an exception.
Environment has the greatest influence on the severity of the development of this pathogen. When soil temperatures are in the 40 to 60-degree range, moist, and shaded, Apothecia will germinate from sclerotia in the top two inches of the soil profile. Apothecia are small, tan, cupped mushrooms that produce millions of ascospores that colonize senescing flowers. These flowers are a food source for the ascospores until they infect the plant through the nodes of the stem. This can occur any time in the season if conditions are conducive for development and senescing flowers are present (R1 to R3). Depending on the area, some inoculation occurred later in the season in 2017 because of the upper portions of the plants were affected in areas that were hot and dry through July and the weather then turned cooler and wetter in August.
The infection of the plant is favored by daily air temperatures that are 85 degrees or cooler and moisture from rain, fog, dews, or high relative humidity. Free water that persists within the canopy for a minimum of 12 hours leads to the highest occurrence of inoculation. A dense canopy that results from early planting, reduced row widths, high populations or high fertility during the R1 to the R3 stage will increase the odds of infection.
Management of white mold begins with scouting and keeping track of the infestation levels in individual fields based on the percent of infected plants. If infection levels are above the 10 percent level, management decisions need to be made to keep the infection levels and yield loss potential from increasing.
There are several cultural practices that can be utilized to reduce the development of white mold of soybeans. There is not one practice alone that can eliminate this disease and environment still determines the overall success.
These cultural practices include:
- Crop Rotation. This is one of the best practices to utilize when dealing with white mold. Corn or any of the other grass crops are not host for Sclerotinia sclerotiorum. Fields with high pressure from white mold are recommended to be rotated with non-host crops for 3 years to reduce the amount of inoculum. Alfalfa and other forage legumes can be an alternative host but the occurrence of infection is lower compared to soybeans.
- Tillage. The first reaction when a field is affected by white mold is to till the sclerotia into the soil. Although data have been variable on the effects of tillage on the development of this pathogen, studies have shown that fewer apothecia are formed under no-till. It is theorized that the sclerotia are more prone to degradation in no-till situations. Tilling the sclerotia into the soil can extend the inoculum in the field because they can survive up to 10 years in the soil and subsequent tillage can bring sclerotia back to the upper 2 inches and germinate.
- Canopy Management. This has been an area that recommendations have focused on in the past. Several management decisions can have an influence on the density of the soybean canopy. Planting rates should be kept at the minimum recommendations that will obtain maximum yields and high populations should be avoided. Row widths can also be used to help reduce density of the canopy. It is recommended that row widths of 20 inches or more be utilized although these row widths may impact the ability to obtain high yields. High yield recommendations from universities have recommended planting soybeans earlier in the season. Producers struggling with white mold should plant fields with higher white mold infestation levels later. The type of plant canopy and the maturity of the soybean variety are the final factors that can be utilized to manage the density of the canopy. Late maturing varieties and bushy varieties tend to have closed canopies and they are more prone to issues with Sclerotinia white mold.
- Fertility. High fertility fields or fields that are following nitrogen rich manure or fertilizer have a higher potential for disease issues. If these fertilizers have been applied to fields with a history of white mold, the fields should be rotated to a non-host crop species that were discussed earlier.
- Weed control. Several common weeds that producers have in their fields are alternative hosts and these need to be controlled for effective white mold management. These include; Common Ragweed, Cocklebur, Sunflower, Field pennycress, Lambsquarters, Catchweed bedstraw, Common burdock, Common chickweed, Canada thistle, Common vetch, Curly dock, Dandelion, Henbit, Hemp, Jimsonweed, Prickly lettuce, Jerusalem artichoke, Velvetleaf, Venice mallow, Wild carrot, Wild mustard, Wild parsnip, Toothed spurge, Sow thistle, Shepard’s purse, Common purslane, and Redroot pigweed.
- Cover crops. Many farmers are utilizing cover crops to help minimize nutrient loss and minimize soil erosion in the “off season”. These cover crops can also help reduce white mold by creating an environment that promote the germination of apothecia earlier in the season when compared to soybeans grown alone. The cover crop should consist of grass species crops and dicotyledonous species should be avoided because they can act as alternative hosts.
- Fungicides. Fungicides can provide good control of white mold but one treatment may not be enough to provide season long control. The first application needs to be applied at the R1 stage and sequential applications may need to be applied at the R3 stage and some even report seeing benefits up to the R5 stage. If plants are showing symptoms of white mold, the fungicide will not provide any control. Coverage is also important for fungicides to work and the product must get down in the canopy for adequate control. To achieve this, the product needs to be applied with 15 to 20 gallons of water, groundspeeds need to be less than 10 miles per hour, spray pressure needs to be maintained around 40 psi on flat fan nozzles producing medium size droplets, and the boom height needs to 20 inches from the target of the application which in this case is the senescing flowers. Fungicides that are labeled for control of white mold are Endura®, Proline®, Topsin®, Domark® and Acropolis® (a mix of Topsin and Domark).
- Lactofen. Cobra® and Phoenix® have suppression of white mold on their label. Some university data have shown that lactofen can provide higher yield benefits when compared to fungicides occurs because of the potential need of multiple applications of fungicides. The exact reason for suppression of white mold isn’t known except for the effects on the plants themselves. Cobraâ burns the plants, reducing the canopy and delaying or reducing flower formation. It is also theorized that the plant produces phytoalexins after the herbicide is applied which are anti -microbial compounds that can inhibit growth of Sclerotinia sclerotiorum.
- Contans®. Contans® contain the fungus Coniothyrium minitans which is a pathogen to Sclerotinia sclerotiorum. Contans® has been applied to fields in areas where the populations are high. It should be incorporated into the top two inches of soil and it needs to be applied a minimum of three months before white mold begins to develop in the field. The more time between application and the occurrence of apothecia will provide better control because this allows the fungus more time to colonize and degrade the sclerotia, producing less apothecia. Limited research has shown the Contans® reduced sclerotia numbers by as much as 95% and the occurrence of white mold of soybeans was reduced from 10 to 70%. It is important that no additional tillage is performed to keep from bringing more sclerotia up from deeper in the soil profile.
- Plant Resistance. Plant resistance is the best option for complete control of white mold. Researchers have been in the process of identifying lines that are completely resistant but techniques have been slow and expensive. Screening material in the field is difficult because of the infrequency of white mold in plots. Some lines have been identified but the resistance is a quantitative trait with multiple genes and linkage groups making it difficult to breed new lines. The resistance is thought to be from the plant’s production of phytoalexins. Some lines are currently available that offer partial resistance. Several new lines are coming forward out of the Genuity® Roundup Ready 2 Xtend® breeding programs that have the highest tolerance to white mold that we have seen. Currently LG Seeds C1838RX is available for producers to plant in 2017 and two new lines LG Seeds LGS1635RX and LG Seeds LGS2007RX will be available in 2018.
Sources and additional information:
Note: The information in this issue is based upon field observations and third party information. Since variations in local conditions may affect the information and suggestions contained in this issue, LG Seeds disclaims legal responsibility therefore. Always read and follow label instructions.
Proline® and Contans® are registered trademarks of Bayer Crop Science. Cobra® is a registered trademark of Valent Corporation. Phoenix® is a registered trademark of Adama Agricultural Solutions. Endura® is a registered trademark of BASF Corporation. Topsin® is a registered trademark of Nippon Soda Company, Ltd. Domark® is a registered trademark of Isagro SpA. Acropolis® Is a registered trademark of AMVAC Chemical Corporation. Genuity® andRoundup Ready 2 Xtend® are registered trademarks of Monsanto Technology LLC. LG Seeds and design are trademarks of SCA Limagrain.
Download a copy of this Technical Bulletin: Tech_364 - Controlling White Mold in Soybeans