Bronze Soybeans a Response to Variety, Weather

Bronze Soybeans a Response to Variety, Weather

September 19, 2008

This year you may have noticed an unusual soybean leaf color — bronze or dark gold rather than the more typical bright yellow — as soybeans begin to mature. This is not due to a fungus and does not indicate a problem.

This bronze coloration is quite evident in certain varieties (notably Pioneer P93M11) that contain the gene for producing significant amounts of anthocyanin pigmentation when the fall days are sunny bright, but not hot, and the nights are cloudless and cool. This pigmentation does not become evident until the leaf's green coloration, which masks the color, begins to fade as the plant's chlorophyll degrades and the plant matures and dries.

Varieties producing this kind of pigmentation have

  • purple (not white) flowers,
  • tawny (not grey) colored pubescence (i.e, "hairs" on the plant stems, leaves, and pods, although the term "hair" is not a scientifically correct term),
  • brown (not tan) colored pods, and
  • black (not other) colored hilum on the seed coat (i.e., the hilum is the point where the seed is attached to the pod).

Plants must undergo a gradual maturation for the coloration to be visible before leaf shed. With these varieties, you will see a slight but deep purple coloration in the

  • pods near the top of the canopy,
  • petioles connecting the upper leaflets to the stems, and
  • the leaflets themselves.

As the plants dry down and start losing their green pigmentation, the pigmented leaves will take on a bronze color (deep purple when observed up close) before the leaflets and petioles are shed from the plant and the pods dry down.

Consider this coloration to be a beautiful but very transient expression of color in your soybean field, much like the fall color in trees but not as long-lasting.

Thanks to Gregg Fujan (Weston) for sharing this question from producers.

Jim Specht
Charles E Bessey Professor of Agronomy and Horticulture