CROP MONITOR

2004 was a tricky year for judging disease risk

With the roller coaster changes in weather this season crop managers have faced a tougher task than usual in keeping track of disease risk. As a consequence, risk alerts were issued from across industry sources for most of the major wheat diseases. Provisional end-of-season results are now available from the Defra-funded CropMonitor project, providing a basis to judge which of those threats actually materialised (Table 1).

Greatest concern centred on Septoria tritici, which has been the most damaging foliar disease of wheat over the past two decades. Weekly monitoring of crops at 15 reference sites across England showed that S. tritici infections were apparent on leaf 2 in untreated crops at least a week earlier than in 2003. For example, at the Norfolk site disease was visible on leaf 2 on 16th June 2003 whereas in 2004 the disease was apparent on 24th May. This was typical of many commercial crops and led to widespread expectations and warnings of high disease risk. However, subsequent weather conditions did not favour further disease development, and disease levels on leaf 2 at GS73-75 reached an average of 10% in untreated crops on the reference sites. Nationally however, septoria levels in commercial crops were generally lower than feared. Levels on leaf 2 at this growth stage were only slightly higher than in 2003 (a low risk season) and also similar to the 10-year mean. This level of septoria in treated crops (4.6% leaf area affected) is relatively low compared to some recent years: 6.7% leaf area was recorded in 1999, 7.2% in 2000 and 9% in 2002. The surveys of commercial crops do indicate a larger proportion of fields than usual with disease levels well above the national mean. These are likely to be crops where T1 sprays were delayed due to the prolonged spell of wet weather during April.

In the past, crop managers have been advised to judge septoria risk mainly from the occurrence of rainfall. However, this season has highlighted the importance of temperature in driving crop growth and the expression of disease symptoms. Recent Defra-funded work has also shown that temperature plays a major part in determining the risk from septoria, so it is important that work is taken-up by industry to develop better seasonal disease warnings.

Four cultivars were grown at each reference site (Consort, Malacca, Claire and Robigus) and disease levels throughout the season clearly illustrated the benefits of cultivar resistance in controlling disease. For example, in untreated plots, S. tritici levels at GS 73-75 on the susceptible cultivar Consort were more than double those on the more resistant Robigus. These differences could be exploited by crop managers to provide savings in fungicide cost.

In commercial crops, Septoria nodorum, brown rust, yellow rust and mildew were found at only low levels, similar to those in 2002 and 2003. Mildew pressure at some sites was somewhat higher than recent years. For example, on some of the reference sites mildew epidemics did establish on untreated susceptible cultivars. But the measurements in commercial crops show that most growers controlled mildew well below damaging levels. Yellow rust was found at low levels on Robigus at monitoring sites in the Cambridge area. Growers should consider seed treatments for yellow rust on very susceptible varieties in high-risk areas e.g. eastern England.

The incidence of moderate or severe symptoms of eyespot was slightly below the level recorded in 2003 and only half of the 10-year mean. Higher levels were reported in crops which emerged early, but the widespread delay in emergence nationally reduced the extent of the problem.

Table 1: National foliar stem base disease levels (GS 73-75)


Disease	10-year mean (1994-2003)	2002	2003	2004 *

Mildew	0.2	0.1	0.2	0.2
Septoria tritici	4.2	9.0	3.8	4.6
Septoria nodorum	0.1	trace	trace	trace
Brown rust	0.1	trace	trace	trace
Yellow rust	trace	trace	0	trace
Eyespot	11.2	6.7	7.3	6.5

Foliar disease - percentage area of leaf 2 affected
Stem base diseases - percentage stems with moderate or severe lesions
Trace - <0.05% area affected
* - provisional data

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