Two New “Verticillium” Threats to Sunflower in North America

A new strain of Verticillium dahliae, the fungus causing Verticillium leaf mottle and wilt, was identified from northwestern Minnesota 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. All V. dahliae isolates collected to date from North Dakota and South Dakota have been identified as the “original” biotype, still fully controlled by the V-1 gene, suggesting that the distribution of the second biotype may be limited to northwestern MN. Another fungus, Phialophora asteris f. sp. helianthi, was isolated from oilseed sunflower plants showing leaf mottle symptoms in Minot, North Dakota, and presumed to be due to Verticillium dahliae. 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.