Last of July/First August Update and Ernesto Advisory

Alan Henn, Extension Plant Pathologist, Mississippi State University
By Alan Henn, Extension Plant Pathologist, Mississippi State University August 5, 2012 23:53

Current update Last of July/First of August and Tropical Storm Ernesto Advisory

White mold
White mold began germinating and growing on dead organic matter in the south about 18 days ago in south central Mississippi, but really started to kill plants in this last week in the Delta, Hattiesburg and Lucedale areas. I have not yet seen any problems in the Waynesboro/Greene County or Amory/Aberdeen areas. All fields with white mold problems had good soil moisture and the canopy was lapping, so humidity near the soil was high. Ideal conditions for the disease. The white mold problems have mostly occurred in fields that were planted to soybeans in 2010 or 2011.

It seems to me that in my earlier note on white mold I did an inadequate job describing the difficulties that can be expected in controlling this disease. First the fungus invades plant parts near the the soil line, or if the soil texture is somewhat dry, light (sandy) and well aerated, the infection may be 0.5 in. or more deep in the soil, below the crown of the main stem of the plant.
– This means the infection is hidden beneath ca. 15 in. of dense plant canopy.

Remember that the white mold is growing on uneven soil, beneath and in dead organic matter.
– Difficult or impossible to reach with fungicides.

The white mold fungus (usually Sclerotium rolfsii) starts decaying the outside of the plant parts, and in the case of stems, will eventually enter them deeply enough that infection can travel inside of it some millimeters.
– Stem tissue is dense enough that even if penetrant type fungicides contacted the stem tissue, it would soon be exhausted and probably be inadequate.
– The fungus itself produces a lot of mycelia, and if the environment has been at all conducive, will have produced some sclerotia. The mycelia and sclerotia may be suppressed by fungicides, but rarely killed.

All this means that managing this fungus is difficult to do – AND – that once the white mold starts growing, multiple sprays will be needed at ca. 10 day intervals.

The best way to minimize some of the advantages white mold enjoys is to spray at night or early in the morning when the peanut leaves are folded up so that spray penetration into the canopy is improved, increase the water volume to at least 20 gallons/A or more (much better) and the increase the application pressure. Although I do not have the data to support these next suggestions, they work for late blight in Irish potato, another very difficult to kill pathogen in a dense canopy growing close to the ground: air assist sprayers seem to improve coverage, 50 gallons/A application rate, slow the speed of application and, adjust the boom height for best coverage.

Rhizoctonia blight
All growing regions. In every growing region I have found some rhizoctonia that was destroying the main stem about 0.5 in. to 1.5 in. above the soil line. I did find some slight blight moving inside the canopy near Wiggins, but it was not yet serious – however, I have a report of severe problems in two fields several counties to the west. It is possible that Rhizoctonia blight problems are waiting on “better” conditions.

Advisory warning
Tropical storm Ernesto may soon be bringing us rain. If so, please make sure that your fields are well and thoroughly protected BEFORE rainfall with BOTH a protectant fungicide (e.g. active ingredient chlorothalonil (sold as Bravo, Echo…)) AND a broad spectrum penetrant fungicide. Make another application with another protectant + penetrant fungicide combination as soon as labels call for after the storm.

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Alan Henn, Extension Plant Pathologist, Mississippi State University
By Alan Henn, Extension Plant Pathologist, Mississippi State University August 5, 2012 23:53
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