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Scouting and Managing the "D Word"
June 21, 2007

Was it just last summer that we were talking about the dreaded D word  drought?  For many in the Northarvest growing area, rain this spring has resulted in opposite D words  drizzle, dampness, downpours, delays, and a few D-lettered cuss words we wont get into here.

We dont know what the rest of the growing season will bring, but the way things are starting off, well need to keep an eye on what may be the most dreaded D word in dry edible bean production  disease.

Keep in mind that seed treatments will help protect the plant from root rots during germination  and early growth, but generally do not offer protection against foliar diseases.   As well, be extra vigilant about scouting for diseases when planting bin-run seed.  Using bin-run/saved seed may increase your risk of seed-borne diseases, and may leave a crop more vulnerable to key diseases such as anthracnose.

Heres a run-down of key diseases that can affect dry beans:
Anthracnose

Anthracnose can cause symptoms on the foliage, pods, and seeds. Symptoms on leaves appear as reddish-brown lesions that occur on the leaf veins on the underside of the leaf. Pod symptoms appear as sunken tan lesions with dark borders. Avoid cultivating plants when wet; this helps prevent spread of pathogens, especially bacterial pathogens and anthracnose.

Fungicides labeled to control anthracnose include Bravo, Echo, Amistar, Quadris, Quadris Opti, Headline, and Thiophanate methyl products (Topsin).

Common blight

Common blight can be spurred in part by hail and damaging winds. Because common blight is caused by a bacteria, only the copper-hydroxide bacteriacides (Basicop, Champ, Kocide) will provide some control of this disease. In regular production fields, protection against common blight may not be needed; however, in seed production fields, an application of a copper-hydroxide compound may help reduce the number of pods with common blight lesions.

Common bean rust

Common bean rust can be a problem on susceptible varieties if conditions are right for the disease.  Because new races can be introduced or can evolve that might be able to overcome these resistant genes, it is a good idea to keep an eye out for rust even in fields planted to resistant varieties. Many resistant varieties are available in common market classes such as pinto and navy, however, few to none of the varieties in special market classes such as small red and pink are resistant.

A fungicide may be needed to control common bean rust on susceptible varieties. All fungicides registered for rust control on dry bean should be applied prior to onset of disease for maximum efficacy.  Many of the same products labeled for control of anthracnose are labeled for rust control as well, with the exception of thiophanate methyl products.

The same fungicides used to control common bean rust are also effective against Asian soybean rust.  Asian soybean rust is primarily a soybean disease; however, dry bean is also a known host, although preliminary USDA research indicates dry beans as a whole appear to be less susceptible than soybeans.  Asian soybean rust causes much smaller lesions and pustules compared to common bean rust.

White Mold

Weather is the key factor that determines how bad white mold will be in a given year. Soil moisture is needed for the small mushroom structures, known as apothecia, to emerge from the soil and release ascospores. Wet foliage provides a conducive environment for disease progression to occur more rapidly. If conditions around the time dry bean plants are flowering are cool and wet, then the potential for white mold problems is increased.

Keeping an eye on the weather and scouting just prior to and during bloom will help with spray decisions. The apothecia that release ascospores grow from sclerotia in the soil after the top 3 to 4 inches of soil have remained moist for 10 to 14 consecutive days.

Fungicides can help manage white mold, but timing is critical  protect the flowers where infection occurs.  Research conducted at the
University of Minnesota to determine the potential for a fungicide to be profitable on dry bean when total moisture (rainfall and irrigation) from June 1 until 10 days into bloom was recorded showed 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

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 for the best coverage and control.  See labels and/or consult with an agronomist for more specific application details.

Fungicides registered for control of white mold on dry bean include:

Endura  Apply at the beginning of flowering, prior to disease onset.  Use higher rate for extended protection.  Make a second application at full bloom if conditions continue to be favorable for disease development.

Switch 62.5 WG  Make first application at 10-20% bloom.  A 2(ee) label allows Switch to be applied in tank mix with Thiophonate-methyl for improved white mold control.

Rovral 4F  Apply at first bloom (10% of plants with 1 open blossom) and again at peak bloom, if needed.  Do not apply after full bloom. Use 50-100 psi and 3 nozzles, 1 over the row and one on each side.  If pH of spray water exceeds 7.0, buffer it to phH 5.0-7.0.

Thiophanate-methyl products (such as Topsin M)  Apply 1.5-2 lbs once when 70-100% of the plants have at least one open blossom.  Or apply 1-1.5 lb twice with the first application when 10-30% of the plants have at least one open blossom and the second application 4-7 days later.

Proline 480 SC  Apply at first sign of disease.  Use higher rate when conditions are favorable for severe disease pressure and/or when growing less disease resistant varieties.

The Effect of Headline on Dry Bean Plant Health

There have been some observations that Headline fungicide (pyraclostrobin) may offer improved plant health benefits that go beyond disease control. BASF, maker of the fungicide, explains in its product literature for soybeans that the product improves plant health by:

Improving plant growth efficiency  more efficient use of carbon and nitrogen within the plant.

Increased tolerance to stress  helps the plant protect itself from stress, which can reduce yield by triggering a survival mechanism in which plants respond by concentrating on reproduction at all costs  resulting in fewer and smaller seeds. Headline suppresses ethylene, a plant hormone responsible for early leaf drop and accelerated maturity.

Disease control  helps prevent energy loss from disease.

BASF has conducted on-farm trials which indicate that Headline has a variable  but overall positive effect on plant health and yield in dry beans (see graphic below).

BASF research on the effect of Headline on plant health continues this summer. NDSU extension plant pathologist Sam Markell cautions against applying Headline or any other strobilurin fungicide for the sole purpose of attempting to improve plant health  the intention of the product is to prevent disease, and in the absence of disease, the expense of a fungicide may not be justified.  Markell says little data on fungicide applications for plant health on dry bean exist, and replicated trials with Headline and/or other strobilurins would have to be conducted before such an application could be recommended.

The
Northarvest Bean Grower magazine is interested in hearing from bean growers who might have observations this summer on this issue.  Take a digital photo or two if you can, and jot a few notes down about application timing, rate, and plant stage.  Email us at nhbean@loretel.net  well do a follow-up feature in a future issue.


 

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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