Agronomic ALERT - White Mold
Jul 03, 2018
White mold is a major soybean disease that impacted many fields in 2017. White mold infection occurs during the flowering or reproductive soybean stages which are quickly approaching or already upon us. Soybean growers should begin scouting now as the majority of soybeans should be flowering now or in the near future. Weather events taking place before flowering may encourage early spore production, increasing the likelihood of infection occurring as soon as flowering begins (R1). Each 10% increase in diseased plants at the R7 growth stage has been estimated to result in a 2 to 5 bushel/acre yield loss.
Estimated Precipitation for the next 14 days from Geosys:
Favorable Conditions for White Mold
To understand if white mold could be a threat this year, it’s important for you to understand how it develops and which conditions favor its development. Disease development is dependent upon the three components of the Disease Triangle (susceptible host, pathogen, and favorable environment) being present at the same time. Favorable conditions for white mold development are as follows:
White mold overwinters as sclerotia, which are hard black particles that resemble rat droppings.
Sclerotia on or within the top 2 inches of the soil produce apothecia (mushroom-like structures). Extended periods of cool soil temperatures (40-60° F), moderate air temperatures (less than 85° F), moist soil conditions, and partial canopy closure are ideal for apothecia development.
Apothecia produce millions as ascospores, which can infect soybean plants through dying or dead soybean flowers.
After infection, lesions girdle and damage stems. As a result, moisture and nutrient uptake and movement can be inhibited. The pathogen produces a white, cottony mycelium (white mold) that develops on lesions and sclerotia that can serve as a future source of inoculum. If sclerotia are abundant in harvested grain, it may cause a discount at the elevator.
Limiting Conditions for White Mold
Warmer air temperatures (90° F or higher) for a period of time can stop lesion development. Also, development may be slowed or stopped when canopy temperatures of 82° F or greater are reached in dry conditions. Canopy closure reduces air flow, which leads to damp conditions. Soybean fields that have not developed canopies are less prone to white mold infection.
Apothecia development prefer many light rains (think less than a tenth each time it rains) compared to one heavy rain (.5” +) each week. So far, this season we haven’t experienced many light rains, most of them have been heavier rains (2).
Apothecia development is quickest in cooler soils. Our soils in South Dakota have been much warmer this season compared to normal, which may hinder white mold development.
It requires 40 hours of continued soil wetness to get white mold to infect.
There are several fungicides the are listed as ‘good’ for white mold management including picoxystrobin, flutriafol, prothioconazole, tetraconazole, thiophanate-methyl, and boscalid. In-season applications of fungicides are recommended at R1(flowering). This is the stage when disease inoculum can infect the plants through dying blossoms. Applications made at R3 (early pod) may reduce the severity of the infection. Fungicide coverage (spray penetration) is the most limiting factor for how well fungicides work on white mold. Once white mold is present, fungicides may no longer be effective. Fields that have high levels of white mold should be harvested last to reduce the spread of sclerotia.
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