Correct management of rice brown spot

caused by fungus Bipolaris oryzae, rice brown spot affects leaves, grains and, in addition to causing direct damage to productivity, it can survive in the seed and be transmitted to seedlings during germination. The use of quality genetic material, previously treated with fungicides, is one of the least costly management practices in preventing losses from this disease.

Rice brown spot, caused by the fungus Bipolaris oryzae, stands out for its endemic nature in Brazilian rice crops. The fungus is widespread throughout the world and its damage potential varies between regions and between years within the same region. In Asia, brown spot is as important or more important than the dreaded blast (Pyricularia oryzae), in terms of total damage to rice production on that continent. Historically, the disease almost completely decimated rice crops in the episode known as “the Bengal catastrophe”, a region close to India, in 1942. In Brazil, not only productivity is affected, due to the reduction in the photosynthetic area of ​​the plant , but also the yield due to damage to the grains, such as stains and plaster, which impair processing.

Damage to seeds

In addition to the direct damage to productivity and mill yield, B. oryzae it can survive in the seed for periods longer than six months and be transmitted to the seedling with germination. This fact highlights the fungus as an important seed pathogen with potential for damage during crop establishment. Furthermore, under conditions favorable to the disease, successful seed-to-seedling transmission can result in the early occurrence of brown spot epidemics in crops if conditions for the disease remain favorable.

Even with all the risks mentioned that involve B. oryzae and the production of rice seeds, there are few epidemiological studies that seek to elucidate the importance of B. oryzae in the seed and impacts on the crop. In a three-year research conducted by researchers from the Federal University of Rio Grande do Sul (UFRGS), Rio Grandense do Arroz Institute (IRGA), University of the Campanha Region (Urcamp) and Federal University of Pelotas (UFPel), monitoring of the seed health in the state of Rio Grande do Sul and studied the effect of using seeds infected with B. oryzae in the brown spot epidemic and irrigated rice cultivation.

Seed health monitored

Health analyzes of rice seeds produced in Rio Grande do Sul were carried out during the 2008/09, 2009/10 and 2010/11 harvests. The sampling totaled 722 lots from seed production fields certified by IRGA and distributed across the six main producing areas of the state of Rio Grande do Sul. In total, batches of 11 different cultivars were collected. Analysis of prevalence (frequency of infected batches) and incidence (percentage of fungus in a batch) of species of Bipolaris on seeds were carried out using the filter paper substrate test, identifying the presence of fungal structures in grains (Figure 1).

It was observed that 74% of the batches were infected with two species of Bipolaris. The most prevalent species was B. oryzae (62,6%) followed by B. cynodontis (10.2%), detected in the three crops analyzed in all regions and cultivars, with an average incidence in positive samples of 0,57%. Already B. oryzae it was found in all years and producing regions with an average incidence of 6,7%, but with a wide variation (0,5% to 81,5%). These variations can be explained due to the harvest sampled, being 6,1%, 3,7% and 2,9% for the years 2008/09, 2009/10 and 2010/11, respectively. There was variation in the average incidence of B. oryzae also in relation to the production region (Figure 2). The highest average incidence occurred in the internal coastal plain region with 15% in the 2008/09 harvest, varying from 0% to 81% in the 40 samples analyzed. The Western Border was the one with the highest number of samples (99 samples), but with a lower average incidence (0,6%), when compared to the others.

The variation between harvests and regions is attributed to environmental and crop management conditions, in addition to the climatic difference between years (mainly precipitation during the growing season). The six rice growing regions of Rio Grande do Sul have distinct soil and climate characteristics that imply variations in the risk of disease occurrence. In the region of the Inner and Outer Coastal Plain, characterized by sandier soils and high relative humidity during the rice growing period, the risks of foliar diseases are greater and consequently grain contamination. On the other hand, conditions contrary to those observed in those regions are attributed to the Western Border region, which explains the lower overall average incidence of B. oryzae observed.

The study also showed that the cultivar used can influence the incidence of B. oryzae in seeds produced. For the two cultivars most sampled in the study (75% of the lots collected), Puitá INTA CL and IRGA 424, the incidence levels were 1,6% and 9,5% (average of the three harvests), respectively.

Risk associated with the use of infected seeds

Field experiments were conducted in the same three harvests, in the municipalities of Cachoeirinha and Bagé in Rio Grande do Sul, at different sowing times, totaling 11 experiments. The treatments consisted of the use of six lots of seeds artificially inoculated with B. oryzae following an exponential increase (0%, 3%, 6%, 12%, 24% and 48%) of the incidence of the fungus in the seeds. The assessments involved the characterization of factors linked to rice cultivation and the brown spot epidemic. In relation to the epidemic, the incidence of brown spot in the vegetative stage (proportion of plants with symptoms), severity of brown spot in the reproductive stage (proportion of area with lesions in relation to the total area of ​​the flag leaf) and incidence of harvested grains were evaluated. infected with B. oryzae. For the crop, the initial plant stand (at the four-leaf stage) and the final grain productivity were evaluated (Figure 3).

The results show that the use of seeds infected with B. oryzae It has an effect on the epidemic in climate conditions that are more favorable to the disease. Among the 11 experiments, a significant effect of seed infection levels on the area below the progress curve of disease incidence and severity (classic variable for epidemiological studies) was found in only two trials, both conducted in Bagé during the 2009 harvest. /10. The results are related to the environmental conditions that occurred during that harvest, notably under the influence of the phenomenon El Niño. The accumulated monthly precipitation for the months of November to March were the highest among the three harvests observed, with accumulated precipitation for the month of November five times greater than expected for the municipality of Bagé. Conditions that, together with the higher average temperature for the months of December to February and the lower luminosity in the period due to the greater amount of rain, led to the early and more intense manifestation of the epidemic for that harvest.

Regarding variables related to rice cultivation, the effect of seed infestation was observed with a reduction in plant stand in seven of the eight experiments where this variable was evaluated (Figure 4). Stand reductions reached 48% for a trial in Bagé, implying that no infected seeds would germinate in that case. However, even with the effects observed in the brown spot epidemic and initial stand of rice plants, no effect of treatments on final rice productivity was observed in any of the experiments. It is possible that the compensatory capacity conferred by tillering of the rice plant was sufficient to recover the stand losses caused by the use of infected seeds.

The observations made in this research help to clarify the general picture regarding the health of certified seed production crops in the state of Rio Grande do Sul. Furthermore, they point to the risks associated with the use of rice seeds infected by B. oryzae. The results of both studies constitute good news for the irrigated rice production chain. Firstly, they demonstrate good seed quality, with levels of B. oryzae well below previous reports from the 1990s and early 2000s. This fact may be associated with the higher quality of management adopted by certified seed producers or the production of a greater volume of seeds in regions with lower risk, such as the region Western Border of Rio Grande do Sul. Second, it shows that the impact of using infected seeds tends to be minimized by the plant's ability to compensate for the reduction in plant density. However, it should be noted that the use of infected seeds is inadvisable under any circumstances, as the seed can carry several other pathogens capable of causing damage to the crop. In the present study, seeds of high sanitary quality were used and which were artificially infected with a single pathogen. Management practices that are relatively low-cost to producers, such as seed treatment, result in greater safety and less risk of losses for producers who already have rice seeds with good health quality.

Productivity and challenges 

Irrigated rice productivity in southern Brazil has seen significant increases in the last 15 years, especially due to the wide adoption of best management practices by producers, such as those recommended by Project 10 of the Rio-grandense Rice Institute (IRGA). The increase is around 70% between 2002 and 2014, according to data from the institute. However, some biotic factors still threaten crops, especially attacks by insect pests and diseases, especially those caused by fungi.

André A. Schwanck, Federal University of Rio Grande do Sul; Emerson M. Del Ponte, Federal University of Viçosa; Priscila R. Meneses, Federal University of Pelotas; Candida R. J. de Farias, Federal University of Pelotas; Gustavo R. D. Funck, Instituto Rio Grandense do Arroz

Article published in issue 201 of Cultivar Grandes Culturas.