New strain of Verticillium dahliae in North America

Abstract

A new strain of Verticillium dahliae, the fungus causing Verticillium leaf mottle and wilt, was identified from northwestern Minesota in 2002. The new strain or biotype is characterized by its ability to overcome the single, dominant V-1 resistance gene employed in oilseed and confection hybrids. Samples collected in 2003 from diseased plants confirmed the new biotype also exists in Manitoba, Canada. Limited survey, conducted in September 2004, identified the new strain in roughly one-quarter of fields showing Verticillium wilt in North Dakota. In greenhouse trials in 2004/5 we tested 221 cultivars for reaction to the new Verticillium strain. One entry from the USDA Plant Introduction collection, developed in Russia as VNIIMK 8883, was immune and would be an ideal source of resistance. Other commercial oilseed hybrids from the U.S., Argentina and Europe, were also noted with ratings of 0.5 or lower on a 0 to 5 scale. On a related note, another fungus, Phialophora asteris F. sp. helianthi, was isolated from oilseed sunflower plants showing leaf mottle symptoms in Minot, North Dakota in 2002, and presumed to be due to Verticilliumdahliae. In greenhouse inoculations, Phialophora produced leaf mottle symptoms on sunflower lines with and without the V-1 gene, which were very similar to those incited by V. dahliae. Proper identification of this pathogen from plants displaying leaf mottle symptoms will be necessary to avoid confusion between the two pathogens.