Comparative slow-blasting in rice grown under upland and flooded blast nursery culture

Evaluation of rice for resistance to rice blast disease caused by Pyricular ia grisea usually is conducted in upland (nonflooded) disease nurseries, al though all commercial U.S. rice is produced under flood irrigation. Upland rice is more susceptible to leaf blast than is flooded rice, and the magnit ude of this differential susceptibility can vary among cultivars. This 2-ye ar study was undertaken to determine (i) the relationship between rates of disease development (slow-blasting) in upland and flooded rice and (ii) the value to a rice breeding program of establishing a flooded blast nursery, a facility far more difficult to manage than an upland nursery. Among 200 r ice lines compared for leaf blast susceptibility under upland and flooded c ultures, only 7 were rated as slightly more resistant under upland culture and 136 rated more resistant under flooded culture. Disease ratings under u pland and flooded cultures were highly correlated (R = 0.819). Among 14 pre selected cultivars over 2 years, disease development curves under upland an d flooded cultures were highly correlated (R = 0.990). The cultivars with i ntermediate susceptibility under upland culture appeared to benefit most fr om flooded culture. Upland culture provided more opportunities to assess sl ow-blasting than did flooded culture, since many lines that produced suscep tible-type lesions in upland culture failed to do so under flooded culture. It was concluded that adequate information on comparative leaf blast resis tance among rice lines was attainable from upland blast nurseries and that routine evaluation of rice breeding lines for blast resistance in flooded n urseries was not necessary.