Regional and varietal differences in prevalence and incidence levels of Bipolaris species in Brazilian rice seedlots

Abstract

A total of 722 rice seed lots were collected at six production regions of Rio Grande do Sul state during three consecutive seasons (2009/10 to 2011/12). For each seed lot, 200 seeds were assessed for the presence of Bipolaris spp. using a standard seed health blotter test. Results showed that B. oryzae and B. cynodontis were found in 62.5% and 10.4% of the seed lots, respectively. Overall mean incidence of B. oryzae and B. cynodontis were 0.5% and 0.06%, respectively. For the two most sampled varieties (75% of the seed lots), IRGA 424 and Puitá INTA CL, mean incidence levels were highest and lowest, respectively. Among regions, infection risk was highest in the southeastern and lowest in the western regions of the state, especially the Fronteira Oeste. The prevalence and the incidence levels of Bipolaris oryzae reported in this study were lower than previous reports in the same region -eighty percent of the seed lots showed incidence levels below the recommended 5% inoculum threshold. In conclusion, rice seeds produced by IRGA-certified growers showed an overall good health quality with regards to B. oryzae infection, which is the main Bipolaris species associated with rice seeds in southern Brazil.

Bipolaris oryzae seed borne inoculum and brown spot epidemics in the subtropical lowland rice-growing region of Brazil

“…The kernel infection levels were consistently higher at CA when compared to the BA location, and ranged from 10 to 30 %. This is above the mean incidence (9.5 %) across 197 seed lots of the cultivar used in the trials, IRGA 424, produced in the state during the three years of this study (Meneses et al 2014).…”

“…Yet, they may apply to the conditions of lowland irrigated rice systems similar to those of southern Brazil. A recent survey in the region showed that rice seeds produced in the state of Rio Grande do Sul with the current technology have led to improved health quality when compared to previous surveys; mean incidence levels of B. oryzae in seed lots varied according to region and cultivar, but were in general at low levels (79 % of 570 seed lots had incidence lower than 5 %) across the state over the three years (Meneses et al 2014). Collectively, these results indicate that the risk of yield loss associated with B. oryzae seed borne inoculum in the studied region is relatively low, especially considering that commercial fields are commonly sprayed with fungicides targeting the complex of foliar diseases, including rice blast.…”

“…Sporadic surveys of Bipolaris spp. associated with rice seeds produced in Brazil showed the dominance of B. oryzae, although three other species within the Bipolaris complex have been found, usually in very low incidence (Farias et al 2011;Meneses et al 2014). The role of seeds as a source of primary inoculum of B. oryzae in BS epidemics was recently highlighted as a knowledge gap and a research priority to this disease (Barnwal et al 2013).…”