Lima Bean Root and Crown Rot

I visited a lima bean field in the Delta at the beginning of June. The beans were showing what appeared to be dampening off symptoms (Figure 1), and dying plants were leaving large gaps down the rows. The problem was pretty widespread throughout the field. Two rows of beans were planted down 60-inch beds with a single drip line buried down the middle of the beds, at about 10-12 inch depth. The soil classification is a mucky clay. While the beans were planted in May, the grower admitted that the soil was still perhaps a bit cool for planting beans (48-52°F). (Ideally, we would wait for the soil temperature to reach 65-70°F before planting limas.) Additionally, the seed was planted quite deep (4-5 inches), when it probably should have only been planted 1-2 inches deep, provided good moisture in the seedbed. With these conditions, my tentative diagnosis was Rhizoctonia solani because of the reddish-brown lesions I was seeing on the roots. Usually, a seed treatment helps mitigate or control these types of seedling diseases. The grower and I checked a bag of extra seed in the barn, and there was a fungicide seed treatment on the seed.

I sent plant samples to the UC Davis Plant Pathology Diagnostics lab for confirmation on disease. Rather than Rhizoctonia, the lab identified Fusarium oxysporum on all of the plant tissue submitted. While Fusarium oxysporum is the causal agent of Fusarium wilt (a.k.a. Fusarium yellows), the symptoms on these plants indicated root and crown rot, and NOT yellows. Fusarium root rot is common on other beans; however, it has not been a common problem in California limas. For this particular field, its cropping history has been various vegetables for the last couple years, including tomatoes with a severe Fusarium problem in 2016. Rotating out of beans or vegetables to grains would be a good management strategy for future years because Fusarium can live in the soil for several years. Fungicide seed treatments may also help. In this particular case, we talked about managing water well to try to avoid moisture stress (too much or too little) of the current crop to optimize the crop that is there. The grower and PCA assured me that they can manage the moisture well with the drip irrigation on this soil type.  For future bean planting, waiting until soil temperature is warmer and not planting as deep would also be important strategies.