A white Christmas is a dream. White mold is a nightmare. And, it's a nightmare that many soybean growers in the Midwest were faced with due to ideal weather conditions.
“White mold is very strongly influenced by weather,” says Dean Malvick, plant pathology Extension specialist at the University of Minnesota. “High soil moisture during and after flowering along with cool conditions favor white mold. Most fields probably have the pathogen present to some extent.”
X.B. Yang, plant pathologist at Iowa State University, acknowledges that the 2009 growing season “had record cool weather in July that was ideal for soybean white mold occurrence,” despite the fact that white mold generally occurs in even-numbered years.
“The outbreaks in 2009 were the first in an odd-numbered year since 1997, and they covered almost the entire north-central region of the country,” Yang says. “The 2009 epidemic suggests that there are plenty of white mold inoculums in the soil and that it'll be necessary to take precautions to minimize the risk for 2010.”
Considering favorable factors — high soil moisture and rain during flowering, dense and tall plant stands, high populations, early planting/canopy closure, high soil fertility and weed populations — here are tips for managing white mold in 2010.
CHOOSE RESISTANT VARIETIES. “Avoid using susceptible varieties in 2010 and consider using white-mold-tolerant varieties for your next soybean rotation,” advises Yang. “High-yielding, white-mold-tolerant varieties are available.”
Malvick offers the same advice: “Choose the best varieties available with partial resistance or tolerance.”
MINIMIZE AND MAXIMIZE. “Minimizing plant populations while maintaining yield potential can help in managing white mold,” says Malvick. “Also, minimize fertility where possible. High soil fertility, especially manure, is favorable to white mold.”
Yang recommends increasing row spacing. “Avoid planting soybeans with narrow rows (less than 15 in.) in fields that had white mold in the past.”
Malvick offers the same advice, but says that “changing plant population seems to be a better solution.”
USE NO-TILL. “No-till seems to be beneficial,” says Malvick. “Using no-till may depress white mold more than rotation does. Iowa studies from the 1990s show that no-till fields had less white mold. ”
Yang agrees. “Tillage buries the sclerotia into the soil, increasing the survival rate of white mold fungus.
ROTATE CROPS. “Rotation can help in controlling the disease; it reduces the apothecia,” says Malvick.
“Fields that had white mold in 2009 should not be in soybeans in 2010,” Yang says. “It would be a mistake to plant seed corn or soybean after soybean in 2010.”
USE CHEMICAL CONTROL (if necessary). “Chemical control is an option if risk is high,” says Yang. “It's very important to use the right fungicides and read the labels when you make your plans this winter. Knowing the risk of white mold infection in July is key to an economical return in control of white mold.”
However, fungicides offer a challenge: “Delivering fungicide to lower soybean canopy at and below flower petals at the right time before much infection occurs is a challenge,” says Malvick. For fungicide results from a one-year/location study, see the chart.
- BIOLOGICAL CONTROL. Controlling white mold could be done with a biological control agent called Contans (coniothyrium minitans). Despite lack of data for beans, it “has proven effective in white mold control in many crops,” says Yang. “We tested other biological control agents in the 1990s and results were excellent.”