Assessment of Grain Yield Losses Caused by Rice Blast Disease in Major Rice Growing Areas in Tanzania

Pyricularia oryzae Cavara, [ (synonym P. grisea Sacc (teleomorph Magnaporthe grisea (Hebert) Barr)] causal agent of rice blast disease, is a major problem facing rice growers worldwide. In Tanzania, rice blast is considered as the most serious disease, resulting in severe yield losses especially, when susceptible rice varieties are grown. In order to assess yield losses caused by P. oryzae, studies were conducted in the screen-house using ten rice varieties viz, Jaribu 220, Supa, Kalamata, Shingo ya Mwali, Mwarabu, Mbawambili, Kihogo, IR 64, TXD 306 and TXD 85. Results showed that rice blast disease affected rice plants at all stages of growth and resulted in reduction in number of tillers per plant, grain weight, number of seeds per panicle and grain yield. Most of the rice varieties were susceptible to P. oryzae at seedling, early tillering and heading stages (reproductive stages). During the early growth stages symptoms were mainly found on leaves. Leaf blast disease severity reached maximum at tillering stage, then the disease symptoms disappeared gradually. Leaf blast development progressed significantly differently (Pless than0.05) between rice varieties. The varieties Mwarabu and Jaribu 220 were the most susceptible at 45 and 55 days after inoculation (DAI). The area under disease progress curve (AUDPC) increased with leaf age. The relationship between rice blast disease severity and grain yield loss indicated that each increase in the disease severity resulted in a simultaneous reduction in grain yield. Both leaf and panicle rice blast disease severities were positive and highly significantly correlated with grain yield losses (r = 0.96, P less than 0.001 and r = 0.91, P less than 0.001, respectively). The number of tillers and seeds per panicle were negatively correlated with disease severity and grain yield weight (r = -0.912 and -0.958 respectively). The varieties Jaribu 220, Mbawambili, Kalamata and Supa were susceptible to blast disease. Tiller loss/hill and seed loss/panicle due to rice blast disease ranged from 20 to 78.19 % and 7.97 to 64.48 %, respectively, and grain yield losses of between 11.9 to 37.8 % per hectare were recorded for such rice varieties.