In the past few years, the number of calls I have received about disease management has increased considerably. Most of them were about stem rot, a disease that seems prevalent in many areas of the Sacramento Valley.
Stem rot is a fungal disease. In the fall, the fungus forms resting structures called sclerotia inside infected tillers. These sclerotia survive in crop residue during the winter and are incorporated into the soil in the spring with ground work.
When the field is flooded, the sclerotia float to the surface and infect rice tillers when conditions are appropriate. Infected tillers develop black lesions on the outer sheaths. These lesions can grow and penetrate the tiller and rot it through, interfering with the movement of water and nutrients from the roots to the panicles.
Stem rot is not a new problem; it’s been present in California since the beginning of rice cultivation. In the past, the main control method was residue burning. Burning kills the sclerotia that are inside the tissue, therefore keeping the sclerotia levels in the soil at a minimum.
Since burning is now difficult, the second best strategy is to chop and disk the straw in the fall to get good straw decomposition during the winter. The sclerotia don’t survive long if they are not inside rice tissue.
Another good alternative is to bale, but try to cut the straw as close to the ground as possible to remove the sclerotia, which are mostly located in the tiller at the water level. Leaving the straw on the soil surface is not a good idea since sclerotia will survive in the tissue and accumulate in the soil over the years.
At this point, varietal resistance is not a feasible strategy. All of our varieties are equally susceptible to stem rot. If there are differences, they are due to nitrogen fertilization.
Past research has shown that higher N rates result in more severe stem rot. Therefore, varieties that require lower rates of N tend to have fewer stem rot problems. Another factor to consider is plant density. Thick stands tend to be more susceptible to stem rot than thinner stands.
Fungicide trials I conducted last year showed that Quadris application at the maximum label rate during the heading stage can reduce the incidence and severity of stem rot. Stem rot severity reduction ranged from 43 to 79 percent and resulted in yield increases that ranged from 250 to 700 pounds per acre.
One common question I get is whether it would be better to control stem rot earlier, around propanil application time. I tried using this timing last year, but I did not get good control. I will repeat the early application timing again this year to confirm the result.
The best strategy to manage stem rot is to use several of the tactics I discussed, but the key is going to be residue management.
Good straw decomposition will help reduce the number of viable sclerotia in the soil, minimizing the severity of the disease during the season. In cases where the disease is already well established, a fungicide application will help.