.

Date: September 3, 2007
Source: Farming UK [edited]
<http://www.farminguk.com/index.asp?show=newsArticle&id=4273&country=>

The difficult growing season so far encountered by brassica growers is set to become even worse with high levels of leaf disease prevalent this year [2007], reports Andy Richardson of the Allium & Brassica Centre (ABC). Localised pockets of ringspot, light leaf spot, and _Phoma_ are all beginning to cause unsightly leaf spots, but more worryingly _Xanthomonas_ has already been identified, with the major potential to cause real problems in the months ahead, he warns. "Wet soil conditions have put plantings well behind normal, and continuity of supply is going to be a huge issue for the brassica supply chain." he advises.

Mr Richardson highlights that _Xanthomonas_ bacterial infection can cause significant losses in marketable yield, with cabbage and late autumn cauliflower particularly badly affected. He aims to combat the impending problem through improved control of ringspot, _Alternaria_, and _Phoma_, which seem to make plants more susceptible to _Xanthomonas_ infection. "Although fungicides have no direct effect on the bacteria, our experience in trials is that the enhanced control of leaf fungal diseases has a significant effect on reducing the damage from _Xanthomonas_," he says.

Ringspot remains the most important leaf disease in Lincolnshire.
Light leaf spot, although hitherto only a real problem in Scotland, has been found as far south as Lincolnshire this year [2007] due to the wet conditions and increase in oilseed rape cultivation, reports Mr Richardson. "We are also predicting a risk of early and intensive attacks of _Phoma_ this season," he adds. Wet soils and morning dew could be sufficient to trigger _Phoma_ spore release from the high level of stem cankers on recently harvested oilseed rape stubbles.

Monitoring of brassica diseases by the ABC for the Brassica SMS disease forecasting service has identified the combination of weather conditions and infective spore release to create an exceptionally accurate assessment of disease risk for _Alternaria_ and ringspot.
"Difenoconazole will improve control of both ringspot and _Alternaria_ in brassicas, and gave good protectant control of white blister in ABC trials last year [2006]," according to Mr Richardson.

He believes the synergistic combination of pre-mixed azoxystrobin and difenoconazole brings significant added benefits over the 2 individual components -- particularly with enhanced effects seen on powdery mildew. "We recommend that a rigorous fungicide programme is maintained by all growers on susceptible crops, in conjunction with use of the Brassica SMS disease forecasting service to maximise returns."

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

[The bacterium _Xanthomonas campestris_ pv. _campestris_ is the causal agent of black rot, which affects crucifers such as _Brassica_ and _Arabidopsis_. It is an important seed-borne pathogen and can also be spread by mechanical means. Symptoms include marginal leaf chlorosis and darkening of vascular tissue, accompanied by extensive wilting and necrosis. Full leaf yellowing, wilting, and necrosis occur as the disease advances. Pre-existing fungal lesions may increase the susceptibility of crops to black rot by providing entrance points for the bacteria.

_Leptosphaeria maculans_ (previously _Phoma lingam_) causes black leg, also called _Phoma_ stem canker, on oilseed rape (_Brassica napus_). Black leg is the most serious disease of oilseed rape in most of the world's production areas. The fungus can also affect other _Brassica_ species but rarely does very serious damage in these. It is spread by infected seed and crop litter.

The fungus _Alternaria brassicicola_ causes black spot disease (also called dark leaf spot) on virtually every important cultivated _Brassica_ species including broccoli, cabbage, canola, and mustard.
It is of worldwide economic importance resulting in up to 50 percent yield reductions in affected crops. Symptoms are necrotic lesions on leaves. However, the fungus can also infect all other parts of the plant including pods, seeds, and stems, and is of particular importance as a post-harvest disease. The related species _Alternaria brassicae_ causes dark leaf spots and dark stem lesions immediately after germination and can result in "damping-off," or stunted seedlings. The fungus causes major disease in oilseed rape and often spreads to and causes damage in other brassicas.

Light leaf spot in _Brassica_ spp. is caused by the fungus _Pyrenopeziza brassicae_ (previously _Cylindrosporium concentricum_).
It is a major disease of oilseed rape but rarely very damaging on other brassicas. Leaves become infected soon after sowing, but remain symptomless until the appearance of pale green or water soaked lesions (which are brittle and crack easily when bent) on older leaves. These symptoms develop into large (1 cm diameter) spots, fawn in colour with a crusty appearance.

Ringspot disease on _Brassica_ crops is caused by the fungus _Mycosphaerella brassicicola_. Symptoms are small dark spots on leaves, which become larger lesions with concentric rings of growth and a definite margin. In the UK, it has become more damaging in recent years especially in Cornwall and Lincolnshire.

Brassica powdery mildew is caused by the fungus _Erysiphe cruciferarum_. It is widespread with possibly severe attacks on brassica crops. Powdery mildews are a group of closely related species of fungi with a narrow host range. Low levels of fungal infection may show no symptoms but these may develop when conditions are favourable. Spores are spread by wind.

White blister, or white rust, is a disease caused by _Albugo candida_. This fungus is quite common on species in the _Brassicaceae_ and affects most crucifers causing conspicuous leaf patches. It is rarely a problem on oilseed rape but has become increasingly damaging on other brassicas.

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

Pictures
Black leg leaf symptoms on oilseed rape:
<http://www.gmoinfo.ie/images/Blackleg.jpg>
White blister on brassica:
<http://www.shootgardening.co.uk/uploaded/images/plant_disease_20051204160354.jpg>
Powdery mildew on brassica:
<http://www.bbc.co.uk/gardening/advice/pests_and_diseases/images/medium/powdery_mildew_on_brassica.jpg>

Links
_Xanthomonas_ taxonomy:
<http://www.bacterio.cict.fr/xz/xanthomonas.html>
Information on foliar fungal pathogens of brassicas, with pictures:
<http://www.hri.ac.uk/site2/research/path/pathogen.htm>
_Leptosphaeria maculans_ taxonomy:
<http://www.indexfungorum.org/Names/SynSpecies.asp?RecordID=230154>
Information on _Alternaria_ on brassicas:
<http://genome.wustl.edu/genome.cgi?GENOME=Alternaria%20brassicicola>
_Alternaria brassicicola_ taxonomy:
<http://www.indexfungorum.org/Names/SynSpecies.asp?RecordID=292407>
_Alternaria brassicae_ taxonomy:
<http://www.indexfungorum.org/Names/SynSpecies.asp?RecordID=214057>
_Pyrenopeziza brassicae_ taxonomy:
<http://www.indexfungorum.org/Names/SynSpecies.asp?RecordID=322140>
_Mycosphaerella brassicicola_ taxonomy:
<http://www.indexfungorum.org/Names/SynSpecies.asp?RecordID=100064>
_Erysiphe cruciferarum_ taxonomy:
<http://www.indexfungorum.org/Names/SynSpecies.asp?RecordID=330697>
Information on white blister of brassicas:
<http://www.growcom.com.au/_uploads/22331white_blister_handout.pdf>
_Albugo candida_ taxonomy:
<http://www.indexfungorum.org/Names/SynSpecies.asp?RecordID=122113>
Information on oilseed rape and pathogens:
<http://www.gmoinfo.ie/osr.php>
Allium & Brassica Centre:
<http://www.abcentre.co.uk>
Press release on SMS brassica monitoring system:
<http://www.syngenta-crop.co.uk/NR/exeres/6FCD9973-C7E0-4DB2-BD1E-99A3512423CF,frameless.htm>. 
- Mod.DHA]

[see also in the archive:
2006
----
Blackleg, canola, path.gr.3 - Hungary: 1st report 20060419.1156
2005
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Cabbage leaf spot - Ukraine (Odessa) 20051008.2942 Blackleg, canola - USA (ND): 1st report 20050626.1807 Blackleg, canola - Argentina: 1st report 20050319.0807
2003
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Blackleg, canola - Canada (Manitoba) 20030918.2367 Blackleg, canola - Brazil (Rio Grande do Sul) 20030225.0478
2001
----
Blackleg, root infection, canola - Australia (SA, NSW) 20010819.1959]

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