Integrated Crop Management

Soybean replanting decisions

This planting season, soil temperature generally has been colder than normal and most regions received plenty of precipitation. Many fields have high moisture. Cool and wet soil temperature slows soybean seedling emergence and increases the risk of seedling diseases, especially Pythium damping-off. Some growers may have stand reduction severe enough to consider replanting. This article discusses a few questions related to replanting.

How can you determine the cause of a poor stand?

Because of the short planting season, it is difficult to conduct laboratory tests to determine the cause of stand problems. Instead, spot identification is being used in making replanting decisions. Some generalizations can help determine the cause of poor stand establishment. For example, if poor emergence is due to low germination rate (poor seed quality), poor emergence should occur uniformly across a field. If poor stand establishment is due to diseases, the problematic areas are patchy, often in low spots in a field, or on the slope of a hill when it is caused by Phytophthora. In addition, fields that had seedling disease problems in the past are more likely to have disease problems again, especially when soil moisture is high. If soybean seedlings die after emergence, stand reduction is caused by diseases rather than seed quality. If you observe seedlings damping-off on the soil surface, it suggests disease as the cause of poor stand development.

Stand reduction by damping-off.

When should you replant?

When stand counts are lower than a threshold, replanting should be considered, which is an agronomic issue. However, when a disease is involved in stand reduction, it also becomes a pathology issue. If the stand reduction is caused by Phytophthora, plants may continue to die after no replanting decision is made although the counts may be at or above threshold. Continue to monitor the disease as needed. If stand reduction is caused by Pythium, plants become tolerant/resistant as they grow, and infection will not be a concern, especially as soil temperatures increase.

Up until May 19, soil conditions have been generally cool and thus favorable to Pythium damping-off. However, Phytophthora damping-off cannot be ruled out. This latter disease has similar symptoms of Pythium damping-off. In the past several years, Phytophthora has been isolated from diseased seedlings sampled in May.

What about seed treatment?

If poor stand is due to diseases, seed treatments with fungicides should be used in replanting to avoid future infections. In this growing season, use seed treatments that are targeted at Phytophthora and Pythium in replanting. When poor stand is due to low germination rate, use of seed treatment cannot improve seed vigor or seed germination rate.

Should you till?

One question related to replanting is whether to work the land before replanting or simply plant over the plants from earlier plantings. Tillage such as disking does not affect fungal populations significantly. However, seed emerges quicker in a well prepared seed bed, which reduces risk of preemergence damping-off.

This article originally appeared on page 85 of the IC-488(10) -- May 27, 2002 issue.

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