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Disease Control Tips
June 01, 2001

White mold: Which fungicides should I use?
Benlate and Topsin M are registered for suppression of white mold. Rovral is also registered, but is pH sensitive, and the pH of spray water should be acidified if the pH of the water is above 7, which is the case with much of the spray water in North Dakota. Fungicides can be used at the full rate, as indicated on the label, or if applied in a banded application (directed spray), can be applied at the full rate in the band. In the case of banded applications, if a fungicide is applied in a 15" band on 30" row centers, the rate per planted acre would be half that per treated acre.

When should I apply the fungicides?
Over the years I have stressed application at early bloom. Data developed by Dr. Dick Meronuck, University of Minnesota and Dr. Howard Schwartz, Colorado State University, indicate that the best time to apply Benlate or Topsin M is 4-10 days after the onset of bloom. I would guess that some blossoms would be wilting and dying about 10 days after the onset of bloom. Since most infections start on the dead blossoms, this should be the time that infection starts. Thus, infection seems unlikely prior to this stage, which would explain why application at or prior to 10 days after the onset of bloom would be the most effective.

How should I apply the fungicides?
Both ground and aerial application has been used successfully for white mold suppression. For ground application, pressures of 100 psi or slightly greater have provided better white mold suppression and better yield response than pressures of 40-60 psi, although the lower pressures have provided some disease suppression and yield increase. The use of drop nozzles between the rows has provided the most efficient white mold management, along with the use of air assist sprayers. For aerial application, many applicators use 7-10 gal/A rather than 5, and the higher rates have provided good white mold suppression for growers. The higher rates also were more effective in a strip trial comparing 5, 7-½ and 10 gal/A.

How do I know if a fungicide is needed?
Rainfall and/or irrigation are the triggers for white mold. The top inch of soil must be at or near saturation for 10-14 days to stimulate germination of the hard black bodies, the sclerotia. When the sclerotia germinate they produce tiny mushroom bodies resembling golf tees. These mushroom bodies, called apothecia, liberate millions of airborne spores than can blow about within the field and from field to field. The spores do not infect green plant tissues, but must first establish growth on a dead food source; the most favorable food source are the dead blossoms. Wet conditions down in the canopy are needed for growth to become established on the dead blossoms, and for the fungus to then infect the plant.

Dr. Dick Meronuck, University of Minnesota, conducted trials at Staples, MN. He determined the potential for a fungicide to be profitable when total water (rainfall plus irrigation) from June 1 until 10 days into bloom was recorded:

  • 3-5 inches  profitable 2 out of 10 years
  • 5-7 inches  profitable 6.7 out of 10 years
  • Over 7 inches  profitable 8.5 out of 10 years

    These calculations were based on an assumption of a bean price of $0.15/lb. Other factors to consider are a good yield potential, which usually occurs when there is adequate rainfall for white mold, and the presence of white mold in the area in recent years. Many bean growing areas have had a history of severe white mold in past years. The presence of severe outbreaks of Sclerotinia (white mold) in other crops in an area also should be considered. All of the above may be a bit tricky to determine in some areas where the plants are flowering, but the rows have not closed due to cool weather earlier in the season. This may result in plants with a somewhat reduced yield potential that also may dry out faster than those that formed a canopy.

    What about applying calcium?
    Trials over the past several years by NDSU bean pathologist Dr. Jim Venette have shown that calcium compounds, although having no fungicidal activity in the laboratory, do enhance the plants defense mechanisms, resulting in reduced white mold and increased yields. The calcium compounds are sprayed on at blossom time. In general, calcium, used alone or in combination with 0.5 lb/A of fungicide has resulted in a yield increase of several hundred pounds per acre, compared to 500-700 lb/A for a full rate of either Benlate or Topsin M.

    Several calcium compounds are available. A calcium sulfate powder can be used at 3.5 lb/A. A 15% calcium liquid can be used at 2 qt/A. This may help to provide guidelines to the rates for other calcium compounds that may be available.

    Rust: When should I begin monitoring?
    Dry bean rust often begins to show up around the end of June or early July. All pinks and most small reds are susceptible. Some light red kidneys also are susceptible. Most older pintos are susceptible, but many new varieties are resistant to rust races currently present in North Dakota. Rust resistant pintos include Apache, Burke, Buster, Chase, Elizabeth, Focus, Frontier, Kodiak, Maverick, Montrose, Remington, UI-320 and Winchester. These varieties have been released within the past few years, and represent an exciting advance in pinto variety improvement. Rust races are constantly changing and the number of races present in North Dakota has increased in recent years. This underscores the need to monitor not only fields of susceptible varieties, but all varieties.

    When is spraying warranted?
    Susceptible varieties should be sprayed with a rust fungicide once there are 2-3 pustules per leaf and the pods are not yet striping. If there are one or two hot spots in a field, it should be sprayed. If rust is present in other fields in an area and the crop is in the flat pod stage or earlier, it should be sprayed. If rust begins to build up in a variety that is supposed to be resistant, a fungicide might be required.

    Bravo and maneb are registered for rust control. Bravo gives 7-10 days of protection and maneb gives 5-7 days of protection. Both are effective as protectant fungicides but will not stop established infections. Tilt has a Section 18 emergency exemption in both North Dakota and Minnesota. Tilt is locally systemic, provide 14 days of protection and can kill infections up to four days old.


     

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    Northarvest Bean Growers Association | 50072 East Lake Seven Road | Frazee, MN 56544
    Ph: 218-334-6351 | Fax: 218-334-6360 | Email: nhbean@loretel.net