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

Abstract

The effect of increasing seed borne incidence levels (0, 3, 6, 12, 24 and 48 %) of Bipolaris oryzae on brown spot epidemics and crop performance was studied in eleven field trials. These trials were conducted at two sites (Bagé-BA and Cachoeirinha-CA) in the major rice-growing region of Brazil over three seasons (2008 to 2010). Disease variables assessed over time were disease incidence (INC, %) on leaves prior to flowering, and disease severity (SEV, %) on flag leaves after flowering. Kernel infection (KI, %) by B. oryzae was assessed after harvest. Crop-related variables such as plant population density (PD) and yield (YLD) were also assessed. In only three trials, all in the 2009/10 season, which had well above-normal rainfall in the early season, was the disease found at vegetative stages. In those same trials, a significant effect of seed borne inoculum was found for the area under the disease progress curve of INC and SEV. Overall mean SEV at CA (1.67 %) was higher than at BA (0.22 %). Seed borne inoculum levels did not affect final SEV and KI, which was not correlated between each other. PD was significantly reduced with the increase of seed borne inoculum levels in seven out of eight trials and at levels as high as 48 % (2009/10 season). The seed borne inoculum levels did not affect YLD, although significantly reducing PD, which may be due to the rice having a low population compensated through tillering. The risk of yield loss by sowing B. oryzae-infected seeds seems to be low and the early onset of the disease caused by increased levels of seed borne inoculum was dependent on seasonal weather conditions.

Combination of biocontrol bacteria for rice seeds microbiolization: incidence and transmission of Bipolaris oryzae, and plant growth

“…However, their inoculum level on the seeds depends on the collecting time, location and cultivar. On the other hand, the position and structure of the seed and the environmental conditions during the seed development also contribute for the establishment of the pathogen population (Ghosh et al, 2018), and its transmission is strongly related to the inoculum level on the seed (Schwanck et al, 2015). Therefore, differences among cultivars and seed lots are expected, which could explain the difference on the efficiency of bacterial treatments.…”

“…The fungus (B. oryzae) may cause damages during germination, such as death of the seeds and seedlings, weakening of adult plants, increased sterility and reduction of the grain quality (Ludwig et al, 2009). The pathogen is also one of the main fungi to cause seed discoloration, reducing their commercial value (Schwanck et al, 2015).…”

Measuring lesion attributes and analysing their spatial patterns at the leaf scale using digital image analysis

“…The disease is typically polycyclic with the primary infections originating from seed‐, soil‐ or weed‐borne inoculum. Disease onset occurs during the reproductive or vegetative stages of the crop cycle, depending on favourable weather early in the season (Ou, ; Pannu & Chahal, ; Schwanck et al ., ). The disease spreads during the season via dispersal of airborne conidia from sources that are not well understood (Barnwal et al ., ).…”

“…The importance of lesions as sources of inoculum can be demonstrated directly (Lannou et al ., ) or can be inferred from spatial analysis (Pires et al ., ). However, quantitative information on the spatial aspects at any hierarchical level and information on the temporal progress of the disease is lacking (Pannu & Chahal, ; Schwanck et al ., ).…”

“…Moreover, it is not known whether the attributes and the spatial pattern of lesions change at a certain severity threshold during the course of the epidemic, especially in the presence of coalesced lesions and halo expansion. To the authors’ knowledge, these two aspects have never been quantified together for a leaf‐spotting disease such as rice brown spot that rarely reaches >35% in the field (Savary et al ., ; Pannu & Chahal, ; Schwanck et al ., ). Knowledge concerning the variability of lesion size and aggregation may be useful for (i) generating hypotheses about epidemiological processes occurring at the leaf level for rice brown spot, such as autoinfection (Lannou et al ., ); (ii) providing information to improve simulation models (Savary et al ., ); and (iii) developing disease assessment tools, such as standard area diagrams, for use in the field (Bock et al ., ).…”