Asian Soybean Rust - Effect on soybean yield and grain quality.

Since Asian soybean rust (Phakopsora pachyrhizi) was first confirmed in North American on November 10, 2004, a lot of questions have been raised on the impact of Asian soybean rust (SBR) on soybean yield and grain quality. Because the disease is so new, potential yield loss due to SBR can't yet be estimated for the United States. However, in countries where rust is common, yield loss as high as 90 percent has been observed in the worst cases. We have summarized how we might expect SBR to affect soybean yield and grain quality based on observations from other countries where rust occurs.

Soybean yield components

Soybean yield can be divided into three major components: the number of pods per plant, the number of seeds per pod, and seed size. Stressful conditions such as high temperature, moisture deficiency, or poor management practices decrease yield by reducing one or more of these yield components. Large yield increases most often result from more seeds per pod and greater seed size.

The soybean plant is unique in the way that it can compensate for a reduction in one component with an increase in another component, so that yields are not significantly changed. For instance, more seeds may be produced in fewer pods. Which yield component is reduced or will compensate depends on the growth stage of the plant when the stress occurs.

Stress that occurs in early reproductive stages can be compensated for by a greater number of pods, or later seed set once the stress is relieved. As the soybean plant ages from flowering to the seed-filling stage, however, it is less able to compensate for a stressful condition, and the potential for yield loss increases. Late-season stress reduces seed size and yield, although it does not have as severe effect as pod and seed abortion. In general, increased seed size never fully compensates for reduced seed number.

The effect of SBR on yield depends on the soybean growth stage at which infection occurs

In most cases, SBR appears first on the lower leaves of the plants. Under normal conditions, actively-functioning leaves are a major source of nutrients for seed growth, and proper leaf functioning is therefore important to maximize seed yield, especially seed weight. Translocation of materials from other plant parts to the seeds (mobilization), can be reduced by SBR infection, especially if the infection causes premature defoliation. Premature defoliation can lower the number of filled pods, the number of filled seeds, and lower the seed size.

Data from China shows that under moderate disease pressure, soybean rust reduces yield mostly through reduction in seed number and seed weight. When rust was severe from the early reproductive stage, the number of pods was reduced. Fortunately, if a SBR epidemic occurred in Iowa, it would likely occur in later, rather than earlier, reproductive stages.

As the soybean plant matures past the R6 growth stage (full seed), the potential yield loss due to SBR declines

Potential yield loss from stress occurring in the late R6 growth stage is much smaller because the seeds have already accumulated a sizable portion of their mature dry weight. Stress occurring at R7 (physiological maturity) or later has essentially no effect on yield.

The effect of SBR on grain quality is not clear
The effect of SBR on seed quality is not as clear because it is influenced by the timing, severity, and duration of the infection. There are no reports from Brazil that indicate that SBR affects grain quality.

More rust topics

Asian soybean rust - overview
Spray equipment considerations for foliar fungicide application on soybean
Impact of cultural practices on Asian soybean rust