Never take a good dry bean crop for granted. Plant pathogens are always on the lurk, ready to cause diseases when the conditions are right.
Even when you think a disease problem has been solved, new variants or "races" can appear, and new diseases can be introduced. Knowing what to look for and how to manage the diseases are two steps that can be taken in preparation for this year's growing season. Keeping an eye on the crop through scouting may help in recognition of a problem more quickly. This may allow for more timely treatment, which often leads to better control. The purpose of this article is to provide some details on dry bean diseases that have a history or potential of causing damage to dry bean crops in the Northarvest region.
White mold, caused by the fungus Sclerotinia sclerotiorum, has a history of causing problems on dry bean fields in the Northarvest region. The white mold pathogen is able to survive in the soil of a field for several years as small dark hardened structures known as sclerotia. The sclerotia levels in some fields may have been drastically increased last year, as white mold was a big problem in many crops, especially dry bean and sunflower.
Weather is the primary factor that determines how bad white mold will be in a given year. If conditions around the time dry bean plants are flowering are cool and wet, then the potential for white mold problems is intensified.
Rotating with non-susceptible crops such as small grains and corn will prevent sclerotia levels from increasing in the soil. Although good crop rotation practice is a very important piece of a white mold management program, it may not be effective alone. Fungicides are very effective in managing white mold, if they are applied timely and in the best manner. Fungicide compounds that are registered for control of white mold on dry bean include boscalid (Endura), iprodione (Rovral), and thiophanate-methyl (Topsin M, T-methyl, and others). These fungicides should be applied at the onset of bloom to 7 days after. Rovral is pH sensitive; therefore, the water should be buffered to a pH of 5 to 7. 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.
Keeping an eye on the weather and scouting for the small mushroom structures known as apothecia (left) just prior to and during bloom, may help with spray decisions. These apothecia grow from the sclerotia in the soil after the top 3 to 4 inches of soil have remained moist 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 showed that:
* inches = fungicide profitable 20% of the time
inches = fungicide profitable 67% of the time
* 7+ inches = fungicide profitable 85% of the time
Common bean rust
Common bean rust (below), caused by the fungus Uromyces appendiculatus, can be a problem on susceptible varieties if conditions are right. Last year, very little common bean rust was observed in the region, most likely due to unfavorable weather. In the last few years common rust has been kept at bay with the use of superior resistant genes in some varieties. Because new races can be introduced or can evolve that might be able to overcome these resistant genes, it is still 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. Products registered for rust control include Amistar, Quadris, Quadris Opti, Headline, chlorothalonil products (Bravo, Echo, etc.), and maneb products (Maneb, Manex).
Asian soybean rust
Asian soybean rust, caused by the fungus Phakopsora pachyrhizi, is primarily a soybean disease; however, dry bean is also a known host. Asian soybean rust was detected for the first time in the southeastern U.S. late last year on soybean and a weed known as kudzu. It has already been detected in Florida and Georgia this spring, but has not been spreading rapidly at the time this article was written. Asian soybean rust causes much smaller lesions and pustules compared to common bean rust (above). From preliminary research lead by Dr. Marcial Pastor Corrales (USDA-ARS, Beltsville, MD), dry bean as a whole appears to be less susceptible than soybean.
Differences in susceptibility levels do exist among dry bean cultivars, however. Although fungicides may not be needed to control Asian soybean rust on dry bean, the same fungicides used to control common bean rust are also effective against Asian soybean rust.
Anthracnose, caused by the fungus Colletotrichum lindemuthanium, is a relatively new disease to the region. This disease 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 (Top, right). Pod symptoms appear as sunken tan lesions with dark borders (Bottom, right). Bean anthracnose was first detected in North Dakota in 2001, and has made sporadic appearances since then. The anthracnose pathogen is most likely not established in our region; rather, each occurrence in 2001 and since is believed to have been due to seedborne infections. This stresses the importance of planting disease-free seed that has been certified by a laboratory. Different races of anthracnose exist throughout the U.S. and Canada; however, only race 73 has been observed in the Northarvest region thus far. Varieties resistant to race 73 are available in some market classes. Fungicides are also available to control anthracnose. Those labeled include Amistar, Quadris, Quadris Opti, Headline, chlorothalonil products (Bravo, Echo, etc.), and thiophanate-methyl products (Topsin M, T-methyl, etc.).