The evaluation of varietal disease resistance is a critical element of VCU testing, reflecting the importance attached to reducing fungicide inputs by a gradual improvement in the genetic disease resistance of varieties available to UK farmers. Consideration of a variety's disease resistance is a major factor in the decision to accdept or reject the variety for National Listing. To minimise the risk of wrong decisions being taken, it is essential that the method of evaluation is capable of giving an accurate indication of a variety's resistance within the 2-year trial period.
Resistance ratings on a 1-9 scale are currently estimated from visual assessments of disease symptoms in untreated NL trials and disease observation tussocks and, for some diseases, in artificailly inoculated trials. Visual assessment suffers from a number of inherent technical problems.
1) Symptoms of some diseases can be difficult to digtinguish from those of other diseases, natural senescence, or physiological damage by pests or agrochemicals.
2) Visual assessment lacks sensitivity and it is therefore usually difficult to discriminate between varieties in tirals where disease levels are low.
3) Visible symptoms do not always give a good indication of the extent of colonisation of the host variety and may not be reliable indicators of the damage done by the disease.
4) Visual assessment is subjective and differences between observers are unavoidable. highly trained and experienced personnel are required to identify and assess diseases in the field.
Quantitative diagnostics, such as PCR, should provide the means of overcoming most of these problems. Using PCR it should be possible to estimate disease resistance with greatly improved accuracy. This in turn should present opportunities to cut costs. For example, saving should arise from a reduction in the number of trail assessments required to produce a reliable rating. At the same time, savings in staff costs would be acheived by replacing time-consuming visual assessments by experienced staff with simple plot sampling by relatively untrained personnel. It is envisaged that these saving should more than offset the costs of processing the samples in a central laboratory.
Currently the most widespread and damaging foliar disease of wheat is Septoria tritici. Septoria nodorum has declined in importance since the early 1980s but is still significant and there are indications that it may be increasing in some regions. The symptoms of the Septoria diseases are very difficult to distinguish visually in the field, both from each other and from the symptoms of various secondary pathogens and natural senescence. The two species often occur together, making separate assessment almost impossible. Hence in many trials disease is simply recored as "Septoria spp". There is an obvious requirement for an effective and species-specific test to differentiate between, and quantify, the two species.