Abstract
Sclerotinia disease caused by the plant pathogenic fungus Sclerotinia sclerotiorum continues to be a major problem for UK agriculture and horticulture. S. sclerotiorum has a host range of more than 400 plant species and hence many crops can be affected including oilseed rape, lettuce, carrot, vegetable brassicas, peas, beans and potatoes. Recent years have seen the highest Sclerotinia levels ever recorded in oilseed rape and the trend for increasing Sclerotinia incidence is likely to continue because of the large area of oilseed rape grown, shorter crop rotations and inoculum build up in the soil as other important susceptible crops such as lettuce and carrot also become infected more frequently. Control of Sclerotinia for all crops relies on fungicide treatments which prevent spore germination. However, there are problems in timing these applications effectively and because severe Sclerotinia infection has a high impact on yield, the use of prophylactic sprays is becoming increasingly widespread. However, this current reliance on fungicides is under threat because of the proposed changes to EU Directive 91/414 and also has negative environmental and social impacts. An alternative approach is to investigate the potential of host resistance to reduce the impact of Sclerotinia in susceptible crops. Although there is no evidence for complete resistance to S. sclerotiorum in any of the current cultivars of the major crops affected, there is some indication that there are differences in their relative susceptibility. At the same time, there are also reports that different S. sclerotiorum isolates can also vary in their aggressiveness. Warwick HRI is developing important complementary genetic resources to understand interactions between S. sclerotiorum and its hosts. Firstly, we have developed unique diversity sets for susceptible crop plants such as brassicas and lettuce and development of a similar set for carrot is proposed under the Defra funded Crop Improvement Project. These diversity sets include wild plant relatives which may be sources of novel resistance factors compared to other resources based on the domesticated crop. Secondly, we have now assembled a large collection of characterised S. sclerotiorum isolates from oilseed rape, lettuce and carrot crops from across the UK. Results from Defra project HH3230SFV demonstrated extensive genetic diversity in these S. sclerotiorum populations but a few genotypes were consistently found in different crops and locations. In the future, the aim will be to use these resources to screen different plant genotypes for resistance to S. sclerotiorum and to examine the relative aggressiveness of different UK S. sclerotiorum genotypes on a range of host accessions. However, before this work can proceed it is necessary to develop reproducible and appropriate plant screening tests using S. sclerotiorum, which is the primary aim of this preliminary project. The first objective of the project will be to collect and characterise S. sclerotiorum isolates from a wild host in order to expand the diversity range of the current collection. This is because in the anticipated future work, a range of both wild and agricultural plant genotypes will be evaluated for Sclerotinia resistance. Hence it will be important to have representative wild and agricultural pathogen genotypes with which to fully study interactions. Meadow buttercup (Ranunculus acris) has already been identified through preliminary work as being a widespread wild host of S. sclerotiorum and will be used to source new isolates which will be characterised using established DNA-based methods. The second (main) objective of the project will be to develop an appropriate and consistent screening test which can detect resistance to S. sclerotiorum and this will be done by adapting methodologies reported in the literature based on cotyledon and petiole inoculations. Different commercially available F1 brassica hybrids will be used as test plants for a range of different S. sclerotiorum genotypes. A successful outcome will be is the development of a test that i) results in high infection rates and ii) demonstrates consistent phenotypic reactions for the different brassica / S. sclerotiorum genotype combinations.
United Kingdom