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Disease Control
June 17, 2004

By Carl Bradley
NDSU extension plant pathologist

There are several new things in the area of disease control in dry bean disease that are worth considering.

For example, the University of Minnesota has a good way to determine the odds that a fungicide application will pay for itself.

Also, there are several new fungicides available that will control rust. Do you know how to use them?

And there are some important guidelines for rotating the types of fungicides you use if you must treat dry beans more than once.

Here's a run down on the basics and what's new in controlling white mold, rust and anthracnose in dry beans.

White mold
White mold may be a problem on dry bean if wet and cool weather occur around flowering time, but can be kept below economic damage levels with the use of several management options including:
" Crop Rotation. Rotation with non-susceptible crops such as small grains, corn, and sugarbeet will keep sclerotia levels from building up in the soil. Although crop rotation is a very important piece of a white mold management program, it may not be effective alone. Sclerotia, the survival structure of the white mold fungus, can remain viable in the soil for several years.

" Fungicides. Fungicide compounds that are registered on dry bean for control of white mold are thiophanate-methyl (Topsin M, T-Methyl), iprodione (Rovral), and boscalid (Endura). These products should be applied at the onset of bloom to 7 days after the onset of bloom. Rovral is pH sensitive; therefore, the water should be buffered to a pH of 5.0 to 7.0. Fungicides can be applied by air, ground, or fungigation. For ground application, a pressure of 100 psi along with the use of drop nozzles may allow for better coverage and control. For application by air, spray volumes of 7 to 10 gallons per acre may provide the best coverage and control.

Keeping an eye on the weather and scouting for apothecia (photo left) just prior to and during bloom may help with the decision to spray. Apothecia are the small mushroom-like structures that produce airborne spores. The apothecia grow from the sclerotia (dark, survival structures) after the top 3 to 4 inches of soil have been at or near saturation for 10 to 14 consecutive days.

Research conducted at the University of Minnesota to determine the potential for a fungicide to be profitable when total water (rainfall and irrigation) from June 1 until 10 days into bloom was recorded shows that:
" 3 to 5 inches = fungicide profitable 20% of the time.
" 5 to 7 inches = fungicide profitable 67% of the time.
" 7+ inches = fungicide profitable 85% of the time.

Bean rust can begin to appear late June to early July. The main control options are resistant varieties and fungicides.

Many pinto varieties are resistant to the rust races present in North Dakota and Minnesota. They include Apache, Burke, Buster, Chase, Focus, Frontier, Maverick, Montrose, Remington, UI-320 and Winchester. Resistant varieties are available for most bean classes except small reds and pink.

For susceptible varieties, the use of a fungicide may be required. Susceptible varieties should be sprayed if: 1) there are 2 to 3 pustules per leaf and pods are not yet striping; 2) rust is present in nearby fields and the crop is in the flat pod stage or earlier. A resistant variety may need to be sprayed if rust begins to build up on it. Chlorothalonil products (Bravo, Echo, etc.), maneb products (Maneb, Manex), Quadris, Headline, and Endura are registered for control of bean rust. The chlorothalonil products will give 7 to 10 days protection; the maneb products will give 5 to 7 days protection; Quadris will give 7 to 14 days protection; Headline will give 10 to 14 days protection; and Endura will give 7 to 10 days protection.

Anthracnose appeared in North Dakota for the first time in 2001, and was observed in a few fields in 2002 and 2003. Most varieties are susceptible to the race that was found in 2001 (race 73); therefore, anthracnose could be a potentially devastating disease if precautionary control measures are not used.

Anthracnose control begins with the use of certified disease-free seed. Seed treatments are not effective in eradicating seed-borne anthracnose.

Chlorothalonil products (Bravo, Echo, etc.), thiophanate-methyl products (Topsin M, T-Methyl), Quadris, and Headline are registered for control of anthracnose on dry bean. Data from North Dakota on the efficacy of these products are not available. In disease-favorable weather and under high disease-pressure, multiple applications may be required for effective control.

Fungicide considerations
Fungicides are an important component of bean disease management, and taking steps to ensure that they are providing the maximum amount of protection is also important. Fungicide application timing is critical for disease control. Fungicides should be applied before or at the onset of disease. All of the fungicides registered for use on dry bean should be treated as "protectants" rather than "eradicants." Protectant fungicides will protect the plant from disease infections, but will not eradicate a disease or provide a "kickback" once an infection has been established. Rotation among fungicide chemistry classes is very important in limiting the development of fungicide-resistant pathogen populations. If more than one application of a fungicide in the same season is required, use a different fungicide chemistry for each application if possible.

White mold on dry bean. Photo by NDSU Extension Service.




Apothecia emerging from a sclerotium. Photo by Dr. James Venette, NDSU.









Rust on dry beans. Photo: NDSU Extension Service

Anthracnose lesions following vein patterns on lower side of leaf. Photo by Dr. Carl Bradley, NDSU.

Sunken pod lesion symptoms of Anthracnose. Photo by Dr. Carl Bradley, NDSU.



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