Gray and White Mold in Snap Beans

A number of growers and agricultural professionals have asked me whether we are seeing resistance to mold in snap beans to our currently registered fungicides. The short answer is, not to my knowledge. In a normal year, the primarily threat to snap beans is white mold (Sclerotinia sclerotiorum). When we have a cool, wet spring, we see both white mold and gray mold (Botrytis cinerea). Gray mold is a game changer. Gray mold control requires a tank mix, early timing, and a two spray program.

• Both gray and white mold for durable sclerotia that overwinter in the soil. Both gray and white mold will tend to be more severe when beans follow beans. One has to rotate to cereals or corn for at least 2 years to reduce sclerotia density.

• If possible, avoid planting adjacent to or overlapping a field where a susceptible host had mold problems the previous season. Spores can blow in from an adjacent field.

Botrytis cinerea (gray mold) has a wide host range. You may see evidence of gray mold on blackberry fruit at the edges of the field (Figure One: Gray Mold Infected Black Berries). Botrytis is an opportunistic pathogen that invades weak, damaged, or senescing tissue. You may see evidence of gray mold on the cotyledons of recently emerged bean seedlings (Figure Two: Gray Mold Infected Bean Seedlings). If you see either of these phenomena prior to the flowering of a bean planting, you have an early warning. In a wet spring, you need to use a tank mix (Figure Three: Topsin Versus Endura). You need to seriously consider a two spray program, applying both sprays early at 10% bloom and 4-5 days later.


         Figure One                                                                                                     Figure Two

                                Figure Three

                        Figure Four

• Thiophanatemethyl (Topsin) is highly effective at controlling white mold but has little effect on gray mold. Many gray mold strains are resistant to Topsin.  Iprodione (Rovral) and boscalid (Endura) have shown good control of gray mold, but only fair control of white mold.

Cool temperatures (68°F is optimal), high humidity, and free water on plant surfaces favor gray mold. The critical weather occurs prior to first bloom. If the weather is cool and wet prior to the flowering of the beans, all of the other host plants give rise to a “snow storm” of gray mold spores. They fill the air with gray mold spores right when the bean plants are the most vulnerable. The senescing blossom tissues are a perfect medium to start a gray mold infection.

• 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.

Unlike Ronilan, the currently registered fungicides are highly sensitive to timing because they do not cure mold. To be effective, these fungicides must be applied to blossoms before the mold develops on those blossoms. Currently available fungicides protect the blossoms that are open when the fungicide is applied. The bean flower is indeterminate; blossoms keep unfolding. Blossoms that open and unfold after the fungicide application remain unprotected.  This phenomenon is reflected in the data on spray timing comparing Ronilan to Topsin (Figure Three: Spray Timing With Ronilan Versus Topsin).

In this experiment, the infection timing was controlled. White mold spores were spray on snap beans that were just beginning to bloom on day zero. The fungicides were applied either on day zero (the moment of mold infection) or two, four, or six days after mold infection. Ronilan applied even six day after the moment of infection was still good. In other words, Ronilan cleaned up the mold infections that were initiated six days earlier. Topsin, on the other hand, failed to control the mold initiationed six days earlier. Ronilan applied on day six cured infections that were initiated on day zero. Topsin is only good at protecting blossoms that are open at the moment the Topsin is applied.

If cool wet weather occurs prior to first bloom and persists following first bloom, a two spray program is justified. The first application should be made when 10% of the plants have at least one open blossom. This is earlier that you might think. Pull ten plants. One out of the ten plants has an open blossom. A second fungicide application should be made 4-5 days later. 

• There are about 21 days between first bloom and harvest.

• Topsin, the best material for controlling white mold, has a fourteen day pre harvest interval.

• Applying a second fungicide application is very difficult after the canopy is closed. 

• Apply two fungicide sprays back to back. Start early at (10% bloom) and chase it with a second spray 4-5 days later.

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 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.

Regardless of your fungicide program for bean mold management, the fungicides are only effective with good spray coverage. 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).

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