Verticillium on sunflower

Verticillium is caused by a soil-borne fungus Verticillium dahliae.  The fungus has a wide range of host plants.  Vert, as it is most commonly called, initially infects the lower leaves which show easily defined symptoms (see picture).  Necrosis occurs between the main veins of the leaf.  The outside of the leaf curls up as it dries.  The infection occurs on the bottom leaves of the plant and moves upward.  The infection usually begins at flowering or later.  However, the disease can begin to show symptoms as early as the six leaf stage.
Sunflower leaf infected with

Life Cycle: The disease forms microsclerotia in the pith of the stalk.   The microsclerotia over winter in plant debris and infect the plant root when contact is made.  Toxins are produced by the fungus and translocated from the roots to the leaves. The microsclerotia cannot be observed without the aid of a microscope. 

Damage: The disease can reduce yield, test weight, oil content and seed size.  The extent of damage depends on when in the plant’s life cycle that infection occurs.  Early infected plants may die before reproduction takes place.  The microsclerotia are known to persist in the soil for eight years.  The disease appears to be more common on lighter sandy soils as opposed to heavy clay soils. Infection also weakens the stalk making the plant prone to late season lodging.

Scouting:  The symptoms of Vert infection are readily recognizable with the unique necrosis of the infected leaf.  Most often infection is recognizable at flowering or shortly thereafter. 

Management:  Planting resistant hybrids is the only effective tool. Verticillium is controlled by a single dominant gene.  Most U.S. oilseed hybrids contain the V-1 gene which has provided consistent long term resistance.  However, a new strain or race of Vert has been identified in the U.S. and Canada that is able to overcome the resistance gene. There is no fungicide labeled for Vert in the U.S.

Research:  Verticillium infection in commercial fields has been identified in all states and Manitoba in the annual NSA crop survey in 2008.  No significant economic damage has been determined.  Because the resistance gene (V-1) appears to be compromised by a new strain/race, the National Sunflower Association has funded a two year research project which will run concurrent to the 2009-2011 seasons.  The cost of the project is $112,000 and will employ a post-doctoral scientist with a strong background in potato Verticillium. The objectives are:
  1. To determine the vegetative compatibility groups (VCG) or races/stains of Vert that infect sunflower in the U.S. and overseas. A large collection of isolates will be made throughout the U.S. in the fall of 2009.  Overseas collections are available from other sources.
  2. Determine the aggressiveness of a representative sample of sunflower isolates from each VCG/race/strain identified.  The VCG/race/strain will be determined using amplified fragment length polymorphism (AFLP) primers.  Samples will be analyzed using a state-of-art fluorescent DNA sequencer. Aggressiveness will be determined by a root dip method, needle inoculation or other techniques that may be developed.
  3. Develop (if time permits) a sequence characterized amplified region (SCAR) marker for Verticillium Dahliae.  From this a PCR marker will be developed and used in real-time format to evaluate pathogen accumulation in sunflower and thus evaluate genetic resistance.
Outcome:  With this knowledge a breeding program will have confidence in their ability to develop and incorporate resistance to the most appropriate VCG/strain/race.  It will save time and money in the breeding process.  The marker technology will be an efficient tool to screen resistant lines.

Other:  Private seed companies may be inserting resistant genes already identified in Argentina or other producing countries with pathology research programs.  The research outlined above will assist those private entities to efficiently determine the gene’s resistance to the most virulent VCG(s) in the U.S. 
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