Preliminary evaluation of root disease of winter oilseed rape in the UK

Summary

Sector:
Cereals & Oilseeds
Project code:
PROS62
Date:
01 January 2001 - 31 August 2002
Funders:
AHDB Cereals & Oilseeds.
AHDB sector cost:
£7,826 from HGCA (project no. 2642).
Project leader:
N EVANS, J STEED & B FITT, Rothamsted Research, West Common, Harpenden, Hertfordshire AL5 2JQ. K NORMAN, R BRYSON, J ALFORD, Velcourt Ltd., e-space South, 26, St Thomas Place, Ely, Cambs. CB74EX. P CAVELL, Bayer CropScience, Hauxton, Cambridge, CB2 5HU.

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pros62

About this project

Abstract

During the 2000/1 season, significant variability in the senescence of oilseed rape (OSR) stems was observed in Velcourt-managed fields. These symptoms tend to occur more in the southern part of the country, i.e. Kent and Wiltshire and in fields that have a history of OSR in the rotation.  Roots of affected plants appeared blackened and stunted, very similar in appearance to take-all infected roots in cereals. Significant losses were observed at harvest from fields in an OSR rotation compared with adjacent virgin oilseed rape fields. The aims of this pilot project were to: a) determine the causal agent of any associated root disease, b) to determine the scale of the problem and c) to begin to identify solutions to address the problem.

Root samples were taken from matched pairs of virgin and rotational oilseed rape fields and sent for analysis at a number of sampling times; stem extension (March) and pod fill (May).  Samples were visually inspected and incubated in moist chambers.  After a period of 7 days, root samples were visually assessed and cultures were taken from any lesions present.  A nematologist also assessed stem samples. Results from the March and May sampling times were inconclusive as nothing 'exotic' or novel was found, either fungal or nematological.

As the season progressed the "blackened roots" observed in previous years did develop at a number of sample sites. In additional work outside that detailed in the project proposal, samples were taken at harvest at four sites.  Visually infected roots appeared to be infected with the stem canker pathogenLeptosphaeria maculans.  To verify this conclusion, root samples were placed in mesh covered plastic trays, placed outside and left under natural conditions.  At weekly intervals, sections of stem from each site were observed under a binocular microscope and the development of any fungal structures was monitored.  The only structures observed were pseudothecia (the sexual fruiting body) of the L. maculanspathogen. Progress was monitored using a development key (A - pseudothecia immature, asci and ascospores not differentiated; D - asci fully developed, ascospores fully differentiated and ready for release).  Due to warm, dry weather, pseudothecial maturation was slow and did not occur until mid-late November. Isolations were done which confirmed that the fungus was L. maculans.

The results suggest that the saprophytic phase of growth of L. maculans may be more important than previously thought.  In this study it would appear that the pathogen was able to colonise the root system of oilseed rape plants very rapidly at and around harvest time as the host plant underwent maturation and senescence.  It is unlikely that such late season invasion would be responsible for any yield loss. Current knowledge of the stem canker pathogen life-cycle would suggest that the associated severe basal stem cankers would primarily cause yield loss earlier in the season. However, infected roots may act as a significant source of inoculum aiding carry-over of phoma/stem canker to the newly sown crop.

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