October 2, 2014

Wet Weather Increases Disease Pressure in Brazilian Wheat

Wheat producers in southern Brazil are expecting a record large wheat crop, but concerns are mounting about the quality of the crop due to the wet conditions over the last several weeks. Scientists in Rio Grande do Sul and Santa Catarina have been monitoring the impact of the gibberella fungus on the state's wheat crop and now they have detected a fungal disease that is rarely found in the state.

Rice blast is the common name of the fungal disease (Magnaporthe oryzea) that is usually found further north in the central regions of Brazil as well as the states of Mato Grosso do Sul, Sao Paulo, and northern Parana. Outbreaks of rice blast occur during periods of high humidity with temperatures in the range of 25 C to 28 C. The disease restricts the stem of the plant thus reducing the flow of nutrients above the infection point. If the disease is present before heading of the wheat, it can result in small head size and reduced seed growth.

In the cooler climates of Rio Grande do Sul and Santa Catarina the disease is usually confined to regions where the microclimate is more suited for the development of the disease, but this year the disease is much more widespread than normal.

The disease exhibits very similar symptoms to gibberella even though the causal organism is different. It makes the diagnosis of rice blast difficult without further laboratory tests. There are no wheat varieties in Brazil resistant to the disease and various fungicide treatments have met with limited success.

Rice blast is restricted to South America and it can infect various small grains and there are numerous host plants that can harbor the disease between growing seasons. This year the disease was first found in oats in Rio Grande do Sul during April and May which allowed the disease to get established early in the growing season. The extent of damage caused by this disease is yet to be determined.

The state of Rio Grande do Sul is the second leading wheat producing state in Brazil after Parana. Conab is estimating that Parana will produce 3.9 million tons of wheat in 2014 with Rio Grande do Sul producing 3.0 million tons. Together, these two states are expected to produce 92% of the 7.6 million tons estimated for the 2014 Brazilian wheat crop. The wheat in Parana is currently being harvested, but the wheat harvest in Rio Grande do Sul won't start until November.