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Date: 17 Jul 2007
Source: Farmers Weekly Interactive [edited] (LINK)

Signs of cercospora and powdery mildew in beet

The 1st signs of cercospora and powdery mildew have been found in sugar beet crops, British Sugar has confirmed. The appearance of cercospora -- in crops in North West Norfolk -- may come as a surprise to some, as the disease normally prefers hot conditions, which have been relatively few and far between so far this summer [2007]. But, recent wet, humid weather has encouraged its development. Cercospora will develop when there are 5 or 6 days when the temperature is above 17 degrees C [62.6 degrees F] and humidity above 98 percent.

British Sugar's Colin Walters advised growers to be vigilant, but was not unduly concerned by the appearance of either disease. "Now is the time to be going in with a decent triazole and get everything treated within the next 2 to 3 weeks." Treatments for controlling powdery mildew usually last 4-8 weeks (depending on product and disease severity), so growers considering spraying before symptoms appear should take into account disease history and variety susceptibility, the British Beet Research Organisation added.

Applications now would lose 2 weeks of that effect if disease did not appear until late July, it said. Where crops were backward (i.e., not covering or only just covered across the row) growers were advised not to apply a triazole or strobilurin fungicide yet. If powdery mildew was to come into these early, consider a treatment of quinoxyfen or sulphur, followed by a triazole in mid-August, it suggested.

[Byline: Paul Spackman]

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Communicated by:
J. Allan Dodds
Former ProMED-mail plant disease moderator <dodds@ucr.edu>

[_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 sugar beet (_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. It reduces yield and quality of sugarbeet. In addition, control by fungicides adds costs to producers and has selected for fungicide-tolerant _C. beticola_ strains. Partial genetic resistance in _Beta_ varieties is also used in disease management.

Powdery mildew of sugar beet is caused by the fungus _Erysiphe betae_. It is a serious foliar disease resulting in sugar yield losses of up to 30 percent. The fungus occurs worldwide in sugar beet and also infects other edible beet crops, but its host range is confined to the genus _Beta_ only. The precise mechanisms by which it infects its host are still unclear. Sources of genetic resistance have been identified in cultivated and wild _Beta_ germplasm and resistant varieties of sugar beet are being developed. The pathogen can also be controlled by fungicides.

Map of the UK: <http://www.lib.utexas.edu/maps/europe/united_kingdom.gif>

Pictures:
_Cercospora_ leaf spot symptoms, whole plant:
<http://pdc.unl.edu/sugarbeet/CercosporaLeafSpot/images/Cercospora%20leaf%20spot%20main.jpg>
_Cercospora_ lesions on sugar beet leaf:
<http://pdc.unl.edu/sugarbeet/CercosporaLeafSpot/spotted.htm>
Powdery mildew symptoms:
<http://www.bspp.org.uk/publications/pathprofiles/pathprofile14a.jpg>
_E. betae_ electron micrograph:
<http://www.bspp.org.uk/publications/pathprofiles/pathprofile14b.jpg>

Links:
_Cercospora_ leaf spot information:
<http://www.bspp.org.uk/publications/pathprofiles/pathprofile38.htm>  and <http://pdc.unl.edu/sugarbeet/CercosporaLeafSpot/text.htm>
_C. beticola_ taxonomy:
<http://www.indexfungorum.org/Names/NamesRecord.asp?RecordID=162050>
Sugar beet powdery mildew information:
<http://www.bspp.org.uk/publications/pathprofiles/pathprofile14.htm>  and <http://www.ag.ndsu.edu/pubs/plantsci/pests/pp967w.htm>
_Erysiphe betae_ taxonomy:
<http://www.indexfungorum.org/Names/SynSpecies.asp?RecordID=330695>
British Sugar website:
<http://www.britishsugar.co.uk/RVE2f631f8c15f044089c0a2739ba5c814f,,.aspx>
British Beet Research Organisation:
<http://www.bbro.co.uk/>
- Mod.DHA]

[see also in the archive:
Fungal diseases, sugar beet and barley: alert 20070628.2078
2004
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Powdery mildew, sugar beets - USA (NE) 20040825.2375]

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