- Author: Michelle Leinfelder-Miles
I recently visited a bean field in the southern part of the county with a PCA. From a distance, the beans in certain areas of the field appeared to be drying up and dying. A closer look showed that the leaf margins were drying up first before the whole plants declined. Pulling up plants by the roots, they appeared to show some reddish root lesions. Soil moisture was good – it seemed neither too wet nor too dry, but there was white crusting on the soil surface of the furrows.
As I was thinking about what could be happening with the beans, a couple things were running through my mind. The patchiness of the problem in the field and the reddish roots made me think that Fusarium root rot (Figure 1) may be a problem. The PCA believed that there had been tomatoes in the field the previous year but that there may have been beans in the field just two years ago. I wondered whether the white crusting on the soil was due to salt. The PCA said that he thought the field was irrigated with groundwater.
To put something behind my hunch, I sent plant samples up to the disease diagnostics lab at UC Davis. Tests confirmed that both Fusarium and Rhizoctonia inoculum were present on the plant roots and that the Fusarium inoculum was particularly high. Fusarium spores can survive in the soil for several years, and UC IPM guidelines suggest rotating out of beans for at least three years in Fusarium-affected fields. Unfortunately, Fusarium spores will live in the soil even when bean hosts are not present.
Stress conditions in the field can worsen Fusarium infection, particularly conditions of too much or too little water, compaction, and salinity. We tested the soil salinity at this site and found the electrical conductivity (EC) of the surface soil to be around 5.0 decisiemens per meter (dS/m). Beans are very sensitive to salinity, and yield declines are expected when rootzone soil salinity is as low as 1.0 decisiemens/meter. It would appear that salinity could be stressing the beans and causing them to be more susceptible to the Fusarium inoculum in the soil. Because this grower is irrigating with groundwater, I would recommend that he get his water tested for salinity. If the water salinity is acceptable, then he should consider how he will leach the field this winter, perhaps augmenting rainwater with irrigation water (assuming normal-to-low precipitation this winter). If his groundwater is high in salts, then he should consider using a different water source for irrigating and leaching (if available) and rotate to more salt-tolerant crops, like small grains, for at least three years.
- Author: Michelle Leinfelder-Miles
In late August, I visited a kidney bean field that was exhibiting stress symptoms, like necrotic leaves and stunting. Even though the field was near harvest, the consultant asked if I would take a look at it. Upon pulling up some plants, I noticed some brick red lesions on the roots, and when I pulled the roots apart, I saw that the brick red color ran the length of the conducting tissue. This is characteristic of Fusarium root rot (Fusarium solani).
Fusarium root rot can be a problem in mid- to late- season beans but generally only when the plants are experiencing some other stress. The other stresses may include lack of moisture, too much moisture (and low oxygen), poor nutrition, or salinity. I will speak to salinity below. Fusarium root rot chlamydospores can survive in soil for years, so the UC IPM recommendations are to rotate out of beans for at least three years. While there are no resistant varieties, some varieties are more tolerant than others, so check with your seed supplier for variety recommendations.
In the case of this field, I wondered whether salinity was the stress that encouraged the Fusarium root rot. Beans are considered sensitive to salinity, with yield reductions expected when the electrical conductivity of the soil saturated paste (ECe) exceeds 1.0 dS/m or when the electrical conductivity of the seasonal average applied water exceeds 0.7 dS/m. I sampled soil from about the top eight inches and found the ECe to be 1.025 dS/m. This barely exceeds the guideline salinity target, but it indicates that salinity could have been contributing to plant stress. The crop consultant was going to follow-up by testing the irrigation water salinity. Overall, I hope that winter rains will come and leach the salts below the root zone; nevertheless, the presence of Fusarium root rot would guide me away from planting beans in this field for a few years.