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Micrograph of Rhizoctonia mycelium showing
the classic hyphal branching. Click the image for more information about
Evaluating a Natural Fumigant for Apple Orchards
By Jan Suszkiw
April 12, 2006
Apple growers seeking to use natural
substances produced by decomposing Brassica plants to
"biofumigate" their orchards may want to first consider new findings
by Agricultural Research Service (ARS)
scientists in Wenatchee, Wash.
Rapeseed, mustard and other Brassica species are gaining popularity
in Washington and California as a natural means of controlling soilborne pests
before planting time. That's because they release a variety of chemical
byproducts upon decomposing--particularly isothiocynates. But according to
Mazzola, mechanisms other than biofumigation are at work against
Rhizoctonia solani, a fungal culprit behind apple replant disease.
Mazzola is a plant pathologist with ARS'
Fruit Research Laboratory in Wenatchee.
Mazzola and collaborators want to improve the use of Brassicas in
integrated approaches to managing replant disease, which is traditionally
fought with chemical fumigants. In the Pacific Northwest, this growth-sapping
affliction of young apple trees can cause diminished crop returns up to $40,000
per acre over 10 years.
In trials using ground-up rapeseed as a soil amendment, Mazzola observed
that release of isothiocynates had nothing to do with Rhizoctonia
control. Rather, the control stemmed from changes the rapeseed caused to the
soil environment and microbes living there. Some flourished, while others
For example, Pythium fungi -- another replant disease culprit -- and
Streptomyces bacteria strains that produce nitric oxide both thrived. In
plants, nitric oxide is an important signaling compound that musters a
pest-fighting response called systemic acquired resistance. Mazzola theorizes
that Streptomyces increases resulting from rapeseed amendments
stimulated this resistance response in apple tree roots, suppressing
Rhizoctonia survival long after the isothiocynates had disappeared from
However, Pythium increases required chemical control with mefenoxam.
Thus, Brassica's pest control effectiveness isn't so clear-cut,
according to Mazzola, whose studies appear in the journal Plant Disease.
more about the research in the April 2006 issue of Agricultural
ARS is the U.S. Department of
Agriculture's chief scientific research agency.