2003 disease watch
June 25, 2003
Plant diseases always threaten the profitability of dry beans. But the diseases can be managed.
Fig. 1. White mold on dry bean. Fig. 2. Apothecia emerging from a
Photo: NDSU Extension Service sclerotium. Photo: Dr. James Venette, NDSU
White mold (Figure 1) may be a problem on dry bean if wet and cool weather occur around flowering time, but can be kept below economic levels with the use of several management options listed below.
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. Spores of the white mold fungus can be blown in from adjacent fields, infect plants, and cause disease.
The only two fungicide compounds that are registered on dry bean for control of white mold are thiophanate-methyl (Topsin M, T-Methyl) and iprodione (Rovral). 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 (Figure 2) 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
Fig. 3. Dry bean rust pustules on
leaves. Photo: NDSU Extension
Bean rust (Figure 3) 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 include: Apache, Burke, Buster, Chase, Focus, Frontier, Maverick, Montrose, Remington, UI-320, and Winchester. Resistant varieties are available for most bean classes except small red 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, and Headline 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, and Headline will give 10 to 14 days protection.
Fig. 4. Anthracnose lesions following
vein patterns on lowers side of leaf.
Photo: Dr. Luis del Rio, NDSU
Anthracnose (Figure 4) appeared in North Dakota for the first time in 2001, and was observed in very few fields in 2002. Most varieties are susceptible to the race that was found in 2001; 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 would be required for effective control.
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