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The What, Where, Why of White Mold

09 June 2011

US - Caused by the fungus Sclerotinia sclerotiorum, white mold is an important yield limiting disease of soybeans in the North Central United States.

White mold also impacts a broad range of other broadleaf (dicot) crops including dry beans, sunflowers, canola, potatoes and forage legumes. A chronic problem in Michigan, Minnesota and Wisconsin since the 1970's, beginning in 1990; the occurrence of white mold started to become widespread in other Great Lakes states, and by 1992, white mold was prevalent throughout North Central states.

Speculation has many pointing at the increased focus on soybean yields and associated practices. These practices typically result in higher plant populations, narrow row spacing, heavier crop canopy, irrigation and early planting, all of which can increase the risk of disease pressure. The unfortunate reality is, many times soybean producers who manage for maximum yield are more likely to experience white mold disease in soybeans.

Practices to Reduce White Mold Impact

If a field had a white mold outbreak in the last few years and soybeans will be planted in 2011, there are a few practices which will reduce the impact of the disease. These include the use of soybean varieties which have been selected as being more white mold tolerant.

Keep in mind these varieties are not immune to white mold, but have been shown to be less impacted by disease. Also, stick with the basics to ensure that you use clean seed, which is free of sclerotia. When harvesting known problem fields, do not forget to clean your combines.

Planting and tillage equipment should also be cleaned as you move from field to field. Preventing the problem in the first place is one of the strongest defenses against white mold.

Soybean plant spacing has also been shown to have impact, as narrow rows cover the soil and create less air flow and better conditions for the disease to produce the mushrooms which spread the spores to the flowers, thereby infecting the soybean plant.

When possible, avoid planting soybeans with narrow rows (15 inches or less) in fields that had white mold in the past. Higher plant populations (more than 200,000 plants/acre), irrigation, high soil fertility, and early planting increase the risk of more disease pressure.

Incognito for Protection Against White Mold

Environment plays a major role in determining white mold risk, with favorable conditions being cool temperatures, high humidity and a full crop canopy, particularly as plants are flowering. When such conditions are present, MANA offers the proven technology of Incognito (thiophanate-methyl) fungicide.

One to two applications targeted in the R1-R3 stage will effectively protect soybeans when applied correctly. In addition to white mold, other diseases such as Anthracnose (Colletotrichum), Brown Spot (Septoria), Frogeye Leaf Spot (Cercospora) Stem and Pod Blight (Diaporthe, and the imperfect stage Phomopsis), Purple Seed Stain (Cercospora) and Aerial Blight suppression can be obtained, though timing must be considered.

Critical Factors for Effective Control of White Mold with Incognito Fungicide:

  • Proper Scouting: Pay particular attention at canopy closure for signs of the apothecia mushrooms on the soil surface. Canopy closure generally occurs the last week of June and first week of July, but is determined by fertility, planting timing, row spacing and environment. Areas most susceptible are those where moisture collects due to fogs, extended dew periods, and areas of poor air drainage.
  • Timing: Incognito is targeted for the R1 to R2 stage of soybean growth. It is critical that Incognito is applied prior to the disease infecting the plant, as this is a protectant fungicide. Look also for mushrooms to begin emerging to gauge the risk of infection. Apply a second application 7-14 days later if conditions are favorable for continued disease pressure to protect new soybean plant growth.
  • Coverage: Coverage of the flowers is critical, thorough coverage of the flowers, stems, and branches is essential for disease control. A few techniques to improve coverage will improve overall performance, considerations include:
    • Water Volume: The best coverage is managed by increasing spray volumes for fungicides to 10, even 20 GPA. If applying by air, use a minimum of 5 gallons of water per acre.
    • Spray Pressure: Consider increasing the pressure to reduce the spray droplet size, this combined with the correct nozzle (flat fan, twin even flat jet or hollow cone spray).
    • Application Speed: Slow down for better overall coverage and increase your ability to drive where you want to in larger soybeans.
    • Use an Adjuvant: Disposition and canopy penetration are key, and when combined with proper sprayer application, will get the maximum return for your dollars spent. Good quality adjuvants reduce surface angles, reduce droplet bounce off the leaves, reduce evaporation and control droplet size.

Incognito is one of a very limited number of products which have activity on white mold. Not all fungicides are effective for white mold. Always read and follow the label when you making your plans for management.

TheCropSite News Desk

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