A ProMED-mail post
ProMED-mail is a program of the
International Society for Infectious Diseases <http://www.isid.org>
Date: Tue 14 Feb 2012
Source: Western Farm Press [edited]
Leaf spot in sugarbeets shows fungicide resistance
Cercospora leaf spot in sugarbeets is the most serious foliar disease in Michigan. Defoliation from this disease can affect both tonnage and sugar content. In the last couple of years, this disease has been more difficult to control in some areas. Typical fungicide rotations have included only the triazole and strobilurin classes. Recent testing results from both Michigan and North Dakota have confirmed that a high percentage of samples tested positive to strobilurin resistance.
Variables that can lead to resistance [build-up] include increased use of susceptible varieties, earlier planting dates, stretching spray intervals, not controlling the disease at the end of the season, spraying with inadequate coverage and failing to rotate fungicide classes. Use of strobilurin fungicides in other crops may also be problematic.
All fungicides should be tank-mixed with a fungicide of a different mode of action and different modes of action should be rotated.
Strobilurins should not be used more than once in a spray program and spray intervals should be tightened.
[byline: Steven Poindexter, Michigan State University Extension]
[_Cercospora beticola_ is considered the most destructive foliar pathogen of sugarbeet worldwide. The fungus affects most species of _Beta_ (cultivated and wild) and has also been reported on many weed species which can serve as pathogen reservoirs. Symptoms on sugarbeet (_Beta vulgaris_) include circular leaf spots with characteristic black centres that coalesce and, in severe cases, can lead to complete leaf collapse. During severe outbreaks, new leaf growth can be seen emerging from the centre of collapsed leaves. The disease reduces yield and quality of beet crops. Disease management generally relies on fungicides, but resistant _C. beticola_ strains are emerging.
Partial genetic resistance available in _Beta_ varieties is used in breeding programmes to develop new crop cultivars.
Strobilurins are strongly antifungal agents produced by fungi. Being derived from natural products, they are considered environmentally safe and are used against most major fungal crop diseases. Therefore pathogen resistance is of major concern to many primary industries worldwide. Rotating or mixing chemical classes of fungicides, as suggested above, is used to extend the useful life of the compounds.
Individual states via:
Cercospora symptoms on beet leaves:
Cercospora leaf spot, symptoms on whole beet plant:
<http://www.ag.ndsu.edu/pubs/plantsci/rowcrops/pp1244-04.jpg> and <http://www.forestryimages.org/images/3072x2048/5357579.jpg>
Affected beet field:
Cercospora leaf spot information:
_C. beticola_ taxonomy:
Information on sugarbeet cercospora in Michigan:
Information on strobilurins:
<http://www.answers.com/topic/strobilurin> and <http://grounds-mag.com/mag/grounds_maintenance_strobilurin_fungicides_natures/index.html>
Species and taxonomy of genus _Beta_:
<http://www.plantnames.unimelb.edu.au/Sorting/Beta.html> - Mod.DHA]
Cercospora leaf spot, sugarbeet - UK: alert 20100709.2297
Cercospora & powdery mildew, sugar beet - UK 20070801.2478]