Disease ratings are a very important component of deciding which cultivar to grow. Cultivar disease resistance ratings are based on a combination of scores from disease nurseries, CPT2 trials, and previous season’s scores. Both autumn and spring disease nurseries are planted at Lincoln while there is a spring only nursery at Plant & Food Research, Palmerston North. A cultivar or mix of cultivars which are very susceptible to the disease in question are sown evenly throughout each nursery for stripe rust, leaf rust and powdery mildew. In the case of Septoria tritici blotch, infected stubble is also spread throughout the nursery. Assessments are carried out in some of the CPT2 trials, especially in trials where fungicides have not been applied on one replicate.
Each cultivar is then given a rating for resistance to each disease: R (resistant); MRR (mostly resistant); MR (moderately resistant); MS (moderately susceptible); MSS (mostly susceptible); S (susceptible). Where resistance varies according to the presence of particular races of the pathogen, an average is given and this is indicated with an asterisk on the rating.
Some diseases are more difficult to rate. For example Fusarium head blight (FHB) scores are highly variable from site to site and season to season, as the primary time for infection is flowering, and the symptoms are most apparent 15 to 20 days later. Therefore, both the maturity of a cultivar and the rainfall patterns around flowering need to be taken into consideration when assessing susceptibility. Typically, FHB nurseries favour later maturing lines which appear more resistant because their flowering coincides with drier weather. This season a specific trial was run at Palmerston North. The trial was infected with maize straw to encourage infection by FHB. Regular rainfall over flowering gave perfect conditions for infection resulting in a successful trial. This trial will be repeated this season to strengthen the FHB ratings.
FAR welcomes any queries or suggestions to further improve these booklets. Alternatively, if you require any additional information that we have not included please contact us.
Rob Craigie
Research Manager