Regional survey and identification of Bipolaris spp. associated with rice seeds in Rio Grande do Sul State, Brazil

Abstract

In the harvest year of 2004, a random sample of seed lots (n=165) from six rice-growing regions of Rio Grande do Sul (RS) State, Brazil, were examined under seed health blotter test. Fungi growth resembling Bipolaris or Drechslera spp. was isolated and identified at the species level. Observations on the anamorphic structures (conidiophores and conidia) allowed separation of isolates into three groups. Pseudothecia, asci and ascospores from sexual crosses between isolates from a same anamorphic group but originated from different regions showed the typical characteristics of three recognized species (anamorph/teleomorph): Bipolaris cynodontis (Cochliobolus cynodontis); B. curvispora (C. melinidis) and B. oryzae (C. miyabeanus). All species were widely distributed across the rice-growing regions. Overall, the most prevalent species were B. oryzae (44.6%) and B. curvispora (42,9%) followed by B. cynodontis (24,1%). This is the first report of B. cynodontis associated with rice seeds in Brazil and the preliminary identification of B. curvispora in rice by the observation of the teleomorph phase was confirmed. The regional occurrence of a complex of pathogenic Bipolaris species related to brown spot epidemics in other countries, may have epidemiological consequences in the region which deserve further investigation.

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

“…This prevalence was lower compared to a previous report of B. cynodontis (one quarter of 165 seed lots) in seed samples originated from the same production regions of the country (Farias et al, 2011). Commonly associated with the wild grass Cynodon dactylon (L.) and previously reported in Oryza sp.…”

“…In previous surveys conducted in southern Brazilian rice seedlots, two Bipolaris species possessing morphological features that were distinct from B. oryzae were found associated with rice seeds from all rice-growing regions of Rio Grande do Sul State in the 2004 season: B. cynodontis and B. curvispora with mean prevalence of 24.1% and 42.9%, respectively (Farias et al, 2011). Although critical, the information provided on that study was only the presence or absence of each species in a seed lot, and not the extension of the infection within a seed lot.…”

“…During the visual assessments, morphologically distinct groups of Bipolaris based on conidiophore and conidia features were further isolated to petri dishes containing potato-dextrose-agar media for vigorous growth. Slides were prepared to visualize the pattern of conidia insertion in the conidiophore, which is a key marker that allows to segregate the most commonly species of this genus found in rice in the region (Farias et al, 2011). Results were expressed as percentage of seed lots (pooled or by each level of a factor) exhibiting at least one infected seed (prevalence), and percentage of infected seeds in the lot by each species (incidence).…”

“…Sporadic surveys have been conducted to identify Bipolaris associated with rice seeds produced in Brazil (Franco et al, 2001;Farias et al, 2007;Farias et al, 2011) but few attempted to identify Bipolaris at the species level (Malavolta et al, 2007).…”

“…While B. oryzae is the most commonly species infecting rice seeds, likely due to its association with foliar epidemics of brown spot, other Bipolaris species have been found during routine analysis of rice seedlots around the world, including B. bicolor, B. curvispora, B. cynodontis, B. halodes, B. hawaiiensis, B. indica, B. spicifera and B. victoria (Sivanesan, 1987;Mathur & Neergard, 1973;Motlagh & Kaviani, 2008;Farias et al, 2011;Archana & Prakash, 2013).…”