By Jarrod Hardke, Rice Extension Agronomist
Several low yielding rice fields recently have been identified as having stem rot. In the past few years I have not observed much stem rot, but some of the cases this year have been particularly severe. If you have low-yielding fields and are uncertain of the cause, it may be useful to evaluate the stubble residue left in the field to see if stem rot was present in the field. To be clear, this may not be a widespread problem, but it’s worth looking into.
All of the cultivars that we currently grow, varieties and hybrids, are rated as susceptible or very susceptible. We do not have adequate ratings for all current cultivars – remember we haven’t observed much stem rot in recent years so ratings are limited on some newer cultivars.
Essentially the stem rot fungus persists for long periods of time in the soil. It floats in the water and once it contacts the sheath it infects and grows inward into the stem. If the infection reaches the stem before grain fill is complete it can cause anywhere from partial to complete blanking of the panicle. It can also cause lodging as the stem breaks down. Reduced plant health may also increase plant infection from other diseases such as blast and narrow brown leaf spot. In some of these cases, the stem rot infection is likely the primary cause of yield loss and the other diseases secondary.
What are the causes of stem rot? First, the fungus must be present in the field at sufficient levels to cause issues. Low soil potassium (K) levels increase the risk of infection. In several fields observed recently, the K levels wouldn’t necessarily be considered low, but the nitrogen (N) fertilization rates were excessive, or at least above optimum. In the situation of high N levels and low K levels, stem rot issues can be exaggerated. A tissue N:K ratio of 3:1 or great after midseason strongly favors severe stem rot.
What now? Burn the residue if at all possible – difficult with the rainy weather pattern we’re in. If burning, follow the Voluntary Smoke Management Guidelines for Row Crop Burning available online. In addition to burning, work the residue into the soil and attempt to winter flood. It is also preferable to rotate to an alternate crop such as soybean which is not a host for stem rot. For future seasons, evaluate soil test K and fertilize according to soil test recommendations. Also, use the recommended rate of N fertilizer or attempt to avoid excessive N rates.
Fig. 1. Rice stem infected with stem rot and a late tiller not showing infection.
Fig. 2. All older tillers of rice in this field infected with stem rot. The only green tillers are late emerging ones.
Fig. 3. Additional rice stems infected with stem rot.