A Case Study: Are Volunteer Soybean Plants, During the Fallow Period, Source of Phakopsora pachyrhizi Inoculum for the Following Crop in Mato Grosso State, Brazil?

The soybean sanitary-free period (SFP) was implemented in Mato Grosso state, Brazil, based on the fact that soybean and rust (Phakopsora pachyrhizi—Pp) remain alive/active year-round, favored by irrigated seed production fields, especially RR cultivars, from January/February to October in a 270 days period. This study aimed to generate data to confirm the presence of soybean plants during the SFP. In 43 observation areas located at random in the seven edaphoclimatic regions of the state, plant density and soybean cycle length were quantified. The volunteer first-generation plants completed their cycle in a 127 days average (115-153). No second-generation of volunteer plants were found in the studied areas, leading to the conclusion that, in the absence of these green bridges due to water deficit, Pp does not survive in the state. This is the first report showing the existence of only one generation of volunteer plants in Mato Grosso. It is inferred that, in the absence of the second generation, there are no green bridges in the state and that, the legislation regarding soybean free period should be reviewed to meet the necessary requirements for the creation of a phytosanitary measure.