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Fond du Lac — Want to be able to predict when the mushrooms which infect soybeans with white mold will appear in a field? The fairly reliable answer comes in a 30-day lookback at the average mean of the daily temperatures at the location.

That was one of the messages that University of Wisconsin Extension Service field crops plant pathologist Damon Smith had for attendees at the 2017 round of area soybean conferences. He said the answer to the question proved to be right on at 17 of the 20 fields at which the combination of air temperature, relative humidity, and wind speeds have been tracked.

The magic number was 68 degrees — any number below it for a lookback period of 30 days was a strong indication that the mushrooms carrying the white mold pathogen would appear, Smith reported. He added, however, that this timing formula does not apply where irrigation is in place because of the effect of water on the temperatures.

Controlling white mold

White mold tends to be associated with the potential for high yields because of the canopy that has probably developed by the time at which the risk of infection is the highest, Smith remarked. He said low parts of a field and areas of poor air movement are other contributors.

Numerous studies continue to show that the timing of a fungicide application is also crucial to limiting a white mold infestation, Smith pointed out. He said the application is needed at the R1 to R3 growth or flowering stage and that it is definitely too late by the R5 stage.

The earlier timing also increases the chances of having the fungicide penetrate to the lower parts of the plant because that is where it needs to be to limit the spread of the infection, Smith explained. He pointed out that the risks are not only the potential yield losses for that crop but also the survival and distribution of the up to 25 to 30 sclerotia that can form inside the stem of an infected plant and then remain on the field in debris or be carried elsewhere through straw bedding.

A severity disease index (SDI), which calculates the amount of infection per plant (whether only on branches or on the main stem too), can be used to predict the amount of yield loss, Smith indicated. When the SDI scale tops 25, there's likely to be a yield loss — one that increases by 2 to 5 bushels per acre for every 10 percentage point uptick in the number of dead plants.

Precautionary practices

To cope with the potential for white mold, the list of precautionary practices includes an accurate history of the fields in which it has appeared, mapping of those areas, selection of varieties with resistance or tolerance, wider row spacings, lower plant populations, no consecutive years of soybeans in the same field and chemical or biological treatment, Smith advised.

If it comes down to a fungicide treatment, Smith reported that the Aproach, Endura and Proline with Stratego products most often place at the top for effectiveness in research plots for protecting yields. Another effective but not yet legally registered candidate is the herbicide Cobra but the growth setback it inflicts on leaves most often results in a yield loss, he warned.

Return on investment, which could run to $30 to $40 per acre, must be covered with a yield increase of 3 to 4.5 bushels per acre, Smith stressed. Based on studies in the plots at Hancock in 2013 and 2016, “hang on to your money” rather than deciding to treat until the R4 or R5 growth stage, he emphasized.

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