Evaluating a Natural Fumigant for Apple Orchards

Apple growers seeking to use naturalsubstances produced by decomposing Brassica plants to"biofumigate" their orchards may want to first consider new findingsby Agricultural Research Service (ARS)scientists in Wenatchee, Wash.

Rapeseed, mustard and other Brassica species are gaining popularityin Washington and California as a natural means of controlling soilborne pestsbefore planting time. That's because they release a variety of chemicalbyproducts upon decomposing--particularly isothiocynates. But according toMarkMazzola, mechanisms other than biofumigation are at work againstRhizoctonia solani, a fungal culprit behind apple replant disease.Mazzola is a plant pathologist with ARS'TreeFruit Research Laboratory in Wenatchee.

Mazzola and collaborators want to improve the use of Brassicas inintegrated approaches to managing replant disease, which is traditionallyfought with chemical fumigants. In the Pacific Northwest, this growth-sappingaffliction of young apple trees can cause diminished crop returns up to $40,000per acre over 10 years.

In trials using ground-up rapeseed as a soil amendment, Mazzola observedthat release of isothiocynates had nothing to do with Rhizoctoniacontrol. Rather, the control stemmed from changes the rapeseed caused to thesoil environment and microbes living there. Some flourished, while othersperished.

For example, Pythium fungi -- another replant disease culprit -- andStreptomyces bacteria strains that produce nitric oxide both thrived. Inplants, nitric oxide is an important signaling compound that musters apest-fighting response called systemic acquired resistance. Mazzola theorizesthat Streptomyces increases resulting from rapeseed amendmentsstimulated this resistance response in apple tree roots, suppressingRhizoctonia survival long after the isothiocynates had disappeared fromthe soil.

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.

Readmore about the research in the April 2006 issue of AgriculturalResearch magazine.

ARS is the U.S. Department ofAgriculture's chief scientific research agency.