Cereal seed health and seed treatment strategies

Abstract

Experimental results with seedling blight (Microdochium nivale) and bunt (Tilletia tritici) have confirmed the importance of both diseases in wheat production.  Measured rates of seed-borne bunt multiplication were lower than those described in the literature.  However, there was evidence that soil-borne infection may be more important than previously thought.  This may explain the low level of seed contamination commonly found in surveys of seed bulks.  Results from bunt spore dispersal experiments suggest a significant threat to wheat crops sown in soil which has recently been contaminated by harvesting of bunt-infected crops nearby.  The importance of this source of disease depends on the rate of loss of viable spores in soil.  The rate of loss should be quantified, in order to properly assess the risk.  Until this has been done, the advisory threshold for bunt should be maintained at one spore per seed, and the use of seed treatments effective against the soil-borne phase should be actively encouraged. 

Work within the project has allowed the development of rapid diagnostic tests for seed-borneMicrodochium nivale and Tilletia tritici to be developed. These tests have been developed for commercial use and will allow growers to make more timely decisions on whether to treat winter wheat seed against seed-borne diseases.  For these seed tests to be meaningful, the results must relate to the actual disease levels in the seed bulk.  Sampling experiments were carried out which showed that seed-borne disease distribution is uniform in most wheat seed bulks.  However, pockets of bunt infection existed in a small number of contaminated bulk samples. The observed variation indicates a 95% confidence interval of 0.7 to 1.5 spores/seed around a threshold of 1 spore/seed, provided 40 primary samples are taken. 

Epidemiological studies on seedling blight have confirmed the worst-case scenario for the relationship between M. nivale seed infection and field emergence.  Under such conditions, for every 1% of seed infection a 1% reduction in plant establishment can occur.  On the basis of a cost-benefit analysis, an increase in the current advisory threshold of 5% seed infection with M. nivale to 10 % is recommended.  Above this level, seed should be treated.