Mechanisms of resistance to Fusarium ear blight in winter wheat (<i>Triticum aestivum</i> L.).

The addition of anthers from five cultivars of winter wheat to 10μl of a conidial suspension of F. culmorum significantly increased conidial germination, germ-tube branching and growth compared with a water control. Although anthers from the cultivars Beaver and Mercia caused significantly more branching of germ tubes of F. culmorum than Hussar and Riband, there were no differences between cultivars in % germination and total germ-tube length. The role of anthers in the resistance of wheat cultivars to initial infection by F. culmorum is discussed. Resistance to spread of infection was examined by measuring the % of necrotic spikelets after a single spikelet in the middle of the ear was inoculated with either F. culmorum or F. graminearum. Significant differences between cultivars were observed in % necrotic spikelets below the point of inoculation 4 weeks after inoculation with F. culmorum. Premature bleaching (scalding) occurred in the top half of ears above the point of inoculation and was associated with cultivars showing high levels of necrosis. The levels of necrosis and scalding in these cultivars could not be related to severity of Fusarium Ear Blight (FEB) observed in the field and reasons for this have been discussed. Field studies revealed that cultivar morphology including total straw height and compactness of ear could also significantly affect the severity of FEB. In 1995/96 field trials artificially inoculated with a mixture of Fusarium spp and Microdochium nivale, (concentration 2.5 X 105 ml-1 of water) showed that short strawed cultivars with lax ears had more symptoms of FEB than taller strawed cultivars with dense ears. Among random F3 populations derived from cultivars of varying height there was a clear tendency for tall strawed lines to show less disease symptoms than shorter lines following inoculation suggesting that the relationship between straw height and disease severity had a genetic basis. Monitoring relative humidity at ear height in a short and tall isogenic line of Maris Huntsman revealed no significant differences between these genotypes from GS 65 to GS 85, suggesting that microclimate cannot explain differences in severity between tall and short lines. The implications of these results are discussed in terms of breeding cultivars resistant to FEB of any height and with suitable ear characteristics. Symptoms of FEB seen on different cultivars of wheat in the field are, therefore, not only due to differences to initial infection in the anthers and spread of necrosis within the ear but also differences in scalding. It has also been shown that a number of morphological characters, including cultivar height, significantly affect severity of FEB