White mold continues to be a "pesky" yield limiting disease in dry beans everywhere the crop is grown. Refinements to three common production practices can help reduce white mold in irrigated dry edible beans grown on sandy soils by providing a more effective integrated pest management (IPM) program.
All classes of dry beans are susceptible to this disease. Although some varieties may show some tolerance when the disease pressure is high, all varieties will succumb and suffer severe yield loss. Planting upright varieties in wide rows and allowing three to four years between bean crops or other susceptible crops, such as sunflower and canola in a rotation will help reduce fungal inoculum. Small grain and corn are ideal non-host crops and are recommended in a rotation where dry beans are included.
Figure 1. Severe white mold infection showing the characteristic bleached, chalky colored stems.
White mold is caused by the fungus Sclerotinia sclerotiorum and develops as a white cottony growth on the stem, branches and pods of bean plants. Infected tissue becomes dry and has a chalky or bleached appearance (Figure 1). The fungus also produces black, hard mats of mycelium called sclerotia near these cottony growths. The sclerotia allows the organism to survive adverse conditions such as dry or cold for long periods of time.
The disease cycle (Figure 2) starts when the soil surface is cool but moist enough for the sclerotia to germinate. Soil moisture conditions near field capacity in the top 2 to 3 inches of the soil for 10 to 14 days and the soil temperatures between 59 and 65 degrees F during this same period favor sclerotia germination.
Figure 2. Life cycle of white mold (Sclerotinia sclerotiorum) on beans.
Upon germination, small mushroom-like bodies called apothecia appear in the soil surface under the crop canopy or in adjacent fields (Figure 3). Spores are produced by the apothecia and infect wilted flowers or other dead plant tissue (Figure 4). The infection then spreads to living plant tissue.
Figure 3. Apothecia stage of white mold.
Infection kills some plants and severely reduces the yield of plants with pod infection. High humidity and air temperatures between 68 and 78 degrees F favor the spread of this disease.
Wet conditions during the period from June 1 to ten days after bloom onset can significantly increase the amount of white mold infection.
University of Minnesota research with pinto beans on Verndale sandy loam soil at Staples, Minn., has shown (Table 1) that if a field has 3 to 5 inches of rain or irrigation between June 1 and ten days after bloom onset, there will be positive benefit to a spray program only 20 percent of the time. However, if the field receives 5 to 7 inches, economical benefit occurred 67 percent of the time; and when more than 7 inches fell, fungicide spraying showed a benefit 85 percent of the time. This spray decision aid is based on the assumption of a bean market over $.15/lb. and a canopy closure of 60% or greater within 7 days after bloom onset. Data input requires daily monitoring of in-field precipitation (rain and irrigation).
Figure 4. Spores produced by Apothecia infects senescing flowers or other dead plant tissue.
Wet conditions during the pre-bloom and bloom period create conditions for infection sites that will enlarge as the growing season progresses. The amount of leaf wetness after bloom influences the rate of spread of each infection site.
Benlate and Topsin M fungicides used in the University study, are two common fungicides used to control this disease. Our research was conducted with a ground rig using 40 gallons of water per acre delivered at 75 to 125 pounds of pressure with hollow cone nozzles. Banding or broadcast applications are effective. Two applications are economical in years of high disease pressure.
Always follow the label for proper timing and the amount of product to apply. Application by chem-igation is also effective when applied with less than one-quarter of an inch of irrigation water. Information on chemigation safety, equipment and calibration is described in University of Minnesota Extension Service bulletin FO-6122.
|Table 1. Economic benefit to spraying at a bean price of $.15/lb. or greater.|
|Precipitation^ Depth - Inches||Economic Spray Benefits|
|3 to 5||2.0 out of 10 years|
|5 to 7||6.7 out of 10 years|
|Over 7||8.5 out of 10 years|
|^ Between June 1 and 10 days after bloom onset.|
A full canopy at bloom creates microclimate conditions favorable for white mold infection (Figure 5). Delaying full canopy development until after bloom will decrease the potential for damage from white mold. Nitrogen (N) rate and timing can affect early canopy development.
Split application of the required N fertilizer is a sound best management practice for the production of edible beans grown on irrigated sandy soils. For determining proper N rates for selected yield goals and soils refer to University of Minnesota Extension Service bulletin FO-6572. A typical recommendation for edible bean production on irrigated sandy soils would be 120 pounds of N per acre.
A study at Staples showed a yield increase of 250 pounds per acre in one of three years by delaying the second of two, 60 pounds of N per acre applications until two weeks after blooming. Split applications also decreased the number of plants with white mold infection. There was no change in yield in the other two years when N applications were delayed.
After applying 15 to 20 pounds per acre of N in the starter, apply half of the remainder of the recommended amount about two weeks after emergence. Apply the rest of the recommended amount as late as travel equipment in the bean field will allow. Another option for the split application is to apply the last portion of the recommended amount of N with the irrigation system if the center pivot is equipped with the necessary safety devices and permitted for chemigation by the Minnesota Department of Agriculture. This application can take place within two weeks after first bloom.
Application of N by chemigation should be done with 1/4 to 1 inch of irrigation water. Information on nitrogen chemigation, safety equipment and N injection calibration is described in University of Minnesota Extension Service bulletin FO-6118.
|Figure 5. Severe white mold in a crop with dense canopy.|
The timing of this last application will be dictated by the growth of the crop. If ground equipment is used, the application will have to be made before canopy closure eliminates the possibility of using this equipment.
Irrigation scheduling is also another practice that can significantly reduce the amount of white mold in a field. Six years of research at Staples, on Verndale sandy loam soils, showed that scheduling irrigation events when the average soil water tension in the upper ten inches of the soil reached 65 to 75 centibars (cbs) could greatly reduce the potential for white mold development. Soil water tension was monitored at 4 and 10 inches depth by Watermark soil moisture sensors.
This irrigation strategy and the spray decision-aid were applied in a field of navy beans in 1997 near Wadena, Minn., that produced a net yield of 2,350 pounds per acre under severe disease pressure. The producer irrigated his field only when the soil water sensors indicated an average water tension around 75cbs. The spray decision-aid program suggested spraying and the producer applied two fungicide applications since the total precipitation was over 7 inches by early bloom.
Determining when to irrigate is best accomplished by using several in-field soil water tension sensors and/or a soil water accounting "checkbook" method. In 1998 four demonstration fields were managed using this irrigation scheduling program and the spray decision-aid (supported by an EQIP (Environmental Quality Incentive Program) educational grant.
Disease pressure was light in 1998, hence only one grower applied a single fungicide application spray, resulting in a 28 pound yield per acre difference from the unsprayed check strip. University of Minnesota plot research at Staples over the past three years has shown that a single spray produced 587 to 924 pounds per acre over the unsprayed check.
|Figure 6. Wadena County kidney beans, 1998. Watermark sensor readings compare well to estimated soil water deficit.|
|Figure 7. Rain events and irrigation depths, in inches, in 1998 in the Wadena County kidney bean field.|
Figure 6 shows the daily average soil water tension readings of four Watermark soil moisture sensors (two locations at two depths, 4 and 10 inches) taken in 1998 by one demonstration EQIP cooperator in Wadena County, Minn. Figure 7 shows when the rain events occurred and also when the cooperator initiated irrigation to prevent the soil profile and sensors from exceeding critical soil water levels. Note how the farmer applied each irrigation only when the soil water sensors averaged 60 cbs. or higher.
Figure 6 also shows how well the daily soil sensor readings trace the predicted daily soil water deficit (SWD) as estimated by the Minnesota Checkbook method (daily SWD was corrected five times during this period based on the soil water sensors readings).
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