Bean, Snap (Phaseolus vulgaris)-Strategies for Controlling White and Gray Mold in the Same Field

Latest revision: 
March 2023

By C. M. Ocamb and R. B. McReynolds, Oregon State University Extension

In the western valleys of the Pacific Northwest, both white and gray mold can be in the same snap bean field. Thiophanate-methyl (Topsin M WSB, Topsin 4.5FL, and T-Methyl 4.5F AG) controls white mold well but has little effect on gray mold because many gray mold strains are resistant to this active ingredient. Iprodione (Rovral 4F, Nevado 4F) controls gray mold and white mold. Gray mold strains resistant to Rovral do not survive well in the field. Endura (boscalid) is a newer material that has had good efficacy on white mold in field studies conducted by OSU. Studies on Endura done at Cornell have shown good control of both white and gray mold. Switch 62.5WG (fludioxonil plus cyprodinil) is also registered and has shown good control in a 2-spray program of white mold in studies by OSU. Omega 500F (fluazinam) is also registered and limited studies by OSU show that it controls white and gray mold when used at higher rates in a 2-spray program. Chlorothalonil products (Bravo Ultrex, Echo 720) also are registered but do not control gray mold as well as dicarboximide fungicides (Rovral) and are ineffective against white mold; however, they may be useful if resistance to other fungicides is a problem. Botran 75W (dichloran) is registered for white mold control on snap bean but use in the past has shown poor efficacy.

A biological product, Contans WG, is labeled for snap bean and can also be used in organic production. The microbe in Contans parasitizes sclerotia of Sclerotinia sclerotiorum but efficacy requires physical contact between Contans and sclerotia as Contans will not move or grow toward sclerotia. Contans requires 2 to 3 months with soil temperatures between 40° and 77°F to be effective at parasitizing sclerotia. Published reports indicate that Contans can reduce the number of sclerotia in soil, but protective fungicides are still needed on snap bean. If a field develops an epidemic of white mold, an application of Contans postharvest but before soil incorporation can aid in reducing the viable sclerotia returning to the soil.

There are several possible fungicides for mold control, each of which has advantages and disadvantages.

  • Boscalid (Endura) at 8 to 11 oz/A with a nonionic surfactant or oil-based surfactant gives good control of white and gray mold. A single field season study by OSU showed that JMS Stylet Oil at 0.5 gal/A enhanced the performance of boscalid over the use of a crop oil surfacant. Apply Endura at the beginning of flowering and again at full bloom if conditions are favorable for disease or disease is present. Endura is prone to the development of fungicide resistant pathogen populations and should be used as a tank-mix or rotated with another fungicide. Endura has a 7-day preharvest interval.
  • Cyprodinil plus fludioxonil (Switch 62.5WG) at 11 to 14 oz/A. Apply when 10% to 20% of plants have at least one open bloom and again 7 days later. Switch has a 7-day preharvest interval.
  • Fluazinam (Omega 500F) is registered at 0.5 to 0.85 pint/A. Apply when 10% to 30% of the plants have at least one (1) open blossom and if needed again, 7 to 10 days later. Omega has a 14-day preharvest interval in snap bean, and the shorter cycle of midseason plantings may preclude a second application with this product.
  • Iprodione formulations (Rovral 4F, Nevado 4F) at 1.5 to 2 pints/A should give good control of both gray and white mold; the lower rate should give good control when disease pressure isn't high. On snap bean, apply at 1 to 10 percent bloom and again 5 to 7 days later or up to peak bloom if conditions are favorable for disease. Use of a nonionic surfactant can improve plant coverage.
  • Thiophanate-methyl formulations; T-Methyl 4.5F AG (30 to 40 fl oz/A), Topsin M WSB (1 to 2 lb/A), or Topsin 4.5 FL (20 to 30 fl oz/A) as two applications should give good control of white mold and the lower rates should give good control when disease pressure isn't severe. Make the first application when 10 to 30 percent of the plants have at least one bloom open and repeat 4 to 7 days later. If there is an extended period of cool, moist weather during the season, then gray mold could cause large losses because of resistance in the gray mold pathogen population to Topsin. Therefore, a tank-mix or rotation with another fungicide is recommended. Though a surfactant is not necessary for Topsin, a nonionic or oil-based adjuvant should not be a problem. Topsin has a 14-day preharvest interval in snap bean, and the shorter cycle of midseason plantings may preclude a second application with this product.

For resistance management and good control of both white and gray mold, the following tank-mixes or fungicide rotations are recommended.

  • Tank-mix or apply consecutively a formulation of thiophanate-methyl with iprodione. The top labeled rate of both fungicides should give good control when used in rotation while a tank-mix at a lower rate such as Rovral 4F or Nevado 4F at 1.5 pints/A with Topsin 4.5 FL or T-Methyl 4.5F AG at 20 to 30 oz/A should give good control when used as a combination in a two-spray program.
  • Endura or Switch 62.5WG after an application of Topsin 4.5 FL + Rovral 4F or Omega 500F or tank-mix Endura with Topsin 4.5 FL (20 to 30 oz/A).

In any of the above fungicide programs, control can be increased by reducing irrigation during and after bloom and by timing irrigation so plants are wet less than 12 hours.

Bean mold sprays are often made with 20 to 22 gal water/A, but depending upon sprayer equipment, applications using a larger volume of water (up to 40 gal/A) may improve plant coverage. The pH of water used in the spray tank can have a critical effect on pesticides, including some fungicides, with generally negative effects from alkaline water. If the spray water pH is buffered to between 6 and 7, the efficacy of Topsin and Rovral should not be comprised with timely applications.

Spray timing is a critical factor for effective mold management. The fungicides labeled for use on snap bean mold should be applied at the beginning of flowering, prior to disease onset, regardless of curative or systemic activity. Mold spores infect senescing tissues, especially blossoms. It is recommended that the first spray be made at 10 percent bloom (when 10 percent of the plants have one open bloom). A second application should be made approximately 7 days later when growing highly susceptible bean varieties, there is greater disease pressure, or environmental conditions are conducive for mold development. Limited studies by OSU show a 2-spray program with a tank-mix (thiophanate-methyl plus iprodione) results in better control than a 1-spray program, and when a 1-spray program was examined for timing, spraying at 10% to 30% bloom was superior to a single spray one week later (petal fall). A 1-spray program runs the risk of greater pod infection if conditions are conducive for disease and sclerotia are present in the field or in a neighboring field. Avoid planting adjacent to or overlapping a field where a susceptible host had mold problems the previous season.

Thorough coverage of plants is essential for the protection of blossoms, pods, and foliage from mold infection. Sprayer droplet size will affect the distribution of fungicides within the canopy of snap bean plants. Nozzle droplet size is a function of the nozzle design, the size of the opening, and the operating pressure. Cone or hollow-cone nozzles produce small droplets, which increases the potential for spray drift. These nozzles are commonly used for applying pesticides where penetration and coverage are critical, but they can create a great deal of very small droplets that will not penetrate dense plant canopies. Extended-range flat-fan nozzles create smaller droplets in the higher end of their pressure operating range, but applicators should be sure not to exceed the upper pressure limit. Twin spray nozzles produce two flat-fan patterns, one angled forward and the other angled backward. Two designs are available: single tips with two openings, and modified caps that hold two individual nozzle tips. Air-induction nozzles are designed to work at higher pressure than other flat-fan nozzle designs. In the nozzle systems described, applications are made from above, but the whole surface of the bean plant (blossoms, pods, stems, and leaves) needs a coating of protective fungicide(s). Dropleg nozzles can get the spray down into the canopy.

Replace worn nozzles, calibrate sprayers, and go slowly enough with sufficient water for good spray coverage of your bean plants.

A fungicide program for bean mold management is effective only with good spray coverage.