Optimisation of protocol for large-scale screening of rice populations against bakanae disease under controlled conditions

The aim of this work was to develop a simple, rapid and reproducible method to screen large numbers of rice cultivars for resistance to bakanae disease, and included four objectives: i) to develop optimum conditions for spore production by two strains of F. fujikuroi and two of F. verticillioides ; ii) to compare seed and growth substrate inoculation in screening rice cultivars for responses to bakanae disease; iii) to differentiate between the virulence of two Fusarium spp. in causing bakanae on Bangladeshi rice landraces, and iv) to screen a panel of aus rice lines for resistance to bakanae. Culture of two strains of F. fujikuroi (M1149 and VAE5) and two stains of F. verticillioides (CBS299.96 and 19-115) in constant darkness at 30°C induced sporulation, whereas, no spores were produced under near-UV light or at 40°C. All four strains were able to grow and sporulate across a wide pH range, with different growth patterns. Using spore suspensions, seed inoculation and growth in a soil+sand substrate gave consistent and reproducible infections, compared with inoculation in autoclaved sand alone. All four bakanae-causing strains were tested to identify the most virulent against four Bangladeshi rice cultivars (Kali boro, Jagli boro, Sada boro, Lahi boro). Kali boro was consistently resistant to bakanae disease while Jagli boro was susceptible. The two F. fujikuroi strains were more virulent than either F. verticillioides strain , with F. fujikuroi VAE5 being the most virulent strain tested in terms of symptoms induced. Subsequently, thirty-eight rice cultivars were screened for resistance to bakanae disease using F. fujikori VAE5 as inoculum: nine cultivars showed high resistance to F. fujikuroi VAE5. These results improve our understanding of the biology of these important pathogens and will be useful in pathogenicity and disease management experiments where mass production of inoculum is required. The seed inoculation method and most virulent strain F. fujikuroi VAE5 may be used for screening of rice when examining variation in bakanae resistance and to test novel disease management approaches. Bakanae resistant rice cultivars identified using this approach may be used in breeding programmes to improve genetic resistance to this pathogen. • Optimum environmental conditions (light, temperature and pH) for mycelial growth and sporulation of bakanae pathogens are identified. • A reliable artificial inoculation method and a virulent strain of bakanae pathogen are identified. • Bakanae resistant rice cultivars are identified – could be used in breeding programmes to improve genetic resistance to this pathogen.