Fusarium Head Blight Infection following Point Inoculation in the Greenhouse Compared with Movement of Fusarium graminearum in Seed and Floral Components

blight symptoms under natural or spray inoculated con- ditions. Fusarium head blight (FHB) caused by Fusarium graminearum Spread of scab within a spikehas been recognized as a Schwabe can cause extensive losses in wheat (Triticum aestivum L.) relatively reliable index of Type II resistance in cultivars seed quality and yield. Greenhouse studies were conducted in 2000 (Bai et al., 1999). Histological studies of pathogen spread and 2001 to investigate the movement of F. graminearum into floral components of the wheat spike following point inoculation (PI) of in the spikelet provide evidence that the path of infec- a middle floret. The visual greenhouse rating of spikelet infection tion follows the vascular tissue, with most rapid growth (severity) following PI for all genotypes in the Uniform Northern and in the longitudinal direction with slower transverse Southern FHB Screening Nurseries with various levels of Type II growth (Schroeder and Christensen, 1963; Ribichich resistance was also related to seed infection by F. graminearum in the et al., 2000). Susceptible wheat cultivars infected with samespikesandtofieldinfectionof allgenotypesin2001.Greenhouse F. graminearum have greater indices of spread in the ratingsofFHBspikeletinfection werepoorlyassociatedwithF.gram- rachiswhencomparedwithresistantcultivars.Ribichich inearuminfection in seeds and other floral components from the same et al. (2000) reported thickening of parenchyma walls spikelets. Pathogen movement was primarily localized around the and occluded but intact phloem in an infected resistant PI (resistant cultivars) or moved basally from the PI (susceptible cultivar, and no defensive reaction with occluded and cultivars).GreenhouseFHBspikeletinfectionrangedfrom5(resistant destroyed phloem in a susceptible cultivar. Mesterhazy genotypes) to 100% (susceptible genotypes) with a wide but unrelated (1995) noted variation in fungal movement is more sta- range in seed infection in the same spikes. Thus, Type II resistance to FHB (measured by spikelet severity) in the greenhouse was not a ble in resistant varieties. In describing the relationship good indicator of F. graminearum seed infection. Seed infection with between resistance and movement, Savard et al. (2000) F. graminearum exceeded 70% in all genotypes when grown under found DON to be localized primarily downward from near epidemic conditions in the field nursery and was poorly related the point of inoculation in a susceptible cultivar, but to visual measurements (spikelet severity and scabby seeds) of Type failed to note the presence or absence of F. graminea- II resistance. It is proposed that a greenhouse rating of those spikes rum or the relationship to the visual rating of spikelet having 30% spikelet FHB infection can be combined with a labora- infection. tory evaluation of the same spikes with 30% F. graminearum seed Pandeya and Sinha (1998) reported a poor relation- infection to provide a more definitive method of selecting for Type ship between visual FHB disease rating of spikelets in II resistance. the greenhouse and presence of DON in wheat seeds. Schroeder and Christensen (1963) proposed that the clogging of vascular tissue in the rachis above the point