CENTREVILLE, MI -- As soybean plants start to flower irrigators need to be aware of white mold disease. Producers have identified white mold as a major factor limiting irrigated soybean yields. White mold is favored by the crop and environmental conditions occurring when soybeans are produced under irrigation. More specifically, cool temperatures (<80F), free moisture on the foliage, high humidity and moist soil surface beneath a closed canopy are ideal conditions for the white mold fungus to produce mushrooms (apothecia) and release spores for plant infection.
Producers can reduce the potential for white mold development by managing their irrigation water applications carefully. The producer’s goal should be to wet the soil and the crop as few times as possible yet maintain at least 40 to 50% of available soil water holding capacity for soybeans through flowering and into pod fill.
If white mold is a concern, producers can minimize the number of wetting periods by keeping irrigation applications to as high as an inch to allow as much soil surface and plant drying as possible between applications. While larger applications are more water efficient, the trend for many producers has been to do several small applications each week rather than one big application. Larger 1.0” application avoids two crop wetting periods that the three small 1/3” applications creates, reducing disease potential. A producer making three 1/3” applications also has three times more water loss due to soil surface and crop foliage drying as the irrigator making one 1” application in the same time period. Some producers apply enough irrigation water to nearly fill the rooting zone prior to blossom as a way to reduce the need to apply water during flowering and avoid drought stress.
Soybean Evapotranspiration rate (E.T.) in the R1 (beginning flower) stage is nearing that of 6” well-watered grass (rET.), the reference crop standard used by most irrigation checkbook systems. For most of Northern Indiana and the soybean growing areas of Michigan this is from 0.15” to 0.2” per day (one inch every 5 or 6 days) depending on the weather as beans water use will peak by R3 (beginning pod) or just after full bloom at 120% of the rET. 0.20” to 0.25” per day (one inch every 4 or 5 days) depending on weather.
Use of a foliar fungicide may be warranted in fields with a history of white mold. Be sure to use a product that has been rated as providing good control. Fungicide applications must be made between R1 and R3 to protect flowers from infection. A common sign of the white mold fungus is the presence of apothecia (white mold mushrooms; Figure 1), however there are many other mushrooms that can be confused with the white mold fungus occur including beneficial species.
The worst yield devastation from white mold in soybean comes from managing irrigated soybeans in a similar manner as irrigated corn. This is common in small seed corn setback areas that require a non-corn crop as a border from adjacent corn for pollination purity reasons. Areas managed for corn receive too much water too soon, and sometimes nitrogen (N) fertigation. The additional water and N creates large, full succulent soybean plants that tend to stay wet with a lack of air flow resulting in a lodged white mold-ridden mess by late August.
Between the R3 and the R7 (beginning of maturity) stages, soybeans have the greatest yield benefit to irrigation and are the most susceptible to drought-related yield losses. for more information on irrigated soybean management contact, Lyndon Kelley (firstname.lastname@example.org), Mike Staton at (email@example.com) or Dr. Shawn Casteel (firstname.lastname@example.org).