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 (48-52⁰F). (Ideally, we would wait for the soil temperature to reach 65-70⁰F 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.

A field in Sutter County was confirmed to have charcoal rot, also known as dry root rot or ashy stem blight, which is caused by the fungus Macrophomina phaseolina.  The disease generally occurs under dry soil conditions paired with high temperatures and can be especially problematic when irrigation is delayed during periods of drought stress. This pathogen infects the crown and stem of garbanzo plants near the soil line and produces black cankers, which are sunken with distinct margins and often contain concentric rings. The disease is usually scattered in the field and often occurs during the flowering and pod stages (although infection can occur at all growth stages). The pathogen infects the stems of seedlings at the base of the developing cotyledon near the soil line. In older plants, symptoms include stunting, leaf chlorosis, early defoliation, and ultimately plant death. A sudden drying of whole plants scattered in the field is observed. Additionally, a “charcoal dust” can appear near the soil line on the surface of roots and stems of older plants. Canker development may kill the plant's growing tip and weaken the stem, causing stems to break, separating roots from the rest of the plant when plants are removed from the field. Infection can move into the hypocotyl and root region, as well as primary leaf petioles. The plant taproot often becomes dark, necrotic, and devoid of lateral and fine roots. 

 

 

 

Several garbanzo fields in the Sacramento Valley were infected with alfalfa mosaic virus, a disease that's vectored by aphids. Plant symptoms of viral infections include yellowing wilting, stunting, and dieback. The degree of plant loss and yield decline will depend on the timing of the infection (later infections may not be as damaging). A common theme for viral infections in garbanzos is that there is no pattern to the disease incidence in the field.  That is, individual plants scattered throughout the field will show dieback. This is because the infection depends on where the aphids land and feed. Aphids do not colonize (reproduce) on garbanzo plants due to the acids secreted by the plants. Instead, they usually die, so there is minimal lateral spread of the disease from the point of infection.

To identify alfalfa mosaic virus and other viral infections in garbanzos, cut a stem longitudinally and observe any discoloration in the vascular tissue (xylem and phloem). If the discoloration (brownish) runs along the edges of the cut stem, then it is likely the phloem (sugar conducting tissue) and caused by an aphid-transmitted virus, which moves down into the plant from the leaves. To manage alfalfa mosaic virus, avoid planting garbanzo fields next to alfalfa fields where they can pick up and transmit the virus (lima beans are also very susceptible). It is not economical to spray for aphids because by the time you see the plant damage, the aphids will be gone. In addition, plant garbanzos during cooler months (generally December to January), after or before aphid flights occur. This year, aphid flights might have occurred during the particularly warm spell in early February when temperatures were in the high 70's.

If the discoloration in a stem is in the center of the stem (xylem or water conducting tissues), it is likely Fusarium wilt which moves up into the plant from the roots and stains the xylem tissues dark-brown to almost black. Fusarium wilt is different from Fusarium root rot and it has primarily been found in the Central Coast of California in some of the older garbanzo growing areas. In fields with a history of Fusarium wilt, plant resistant cultivars such as UC-27, which is adapted to the Central Valley.

This spring, we found a garbanzo bean field in the Sacramento Valley with fusarium root rot, identified by our new UCCE Plant Pathologist, Cassandra Swett at UC Davis. Fusarium root rot, caused by the fungus, Fusarium solani f. sp. ciceris, attacks the underground stems and roots of plants. Early infection is characterized by elongated reddish streaks on the roots. As the disease progresses, these eventually form reddish-brown lesions that will surround the entire root, causing decay.  The above ground plant symptoms of affected plants include yellowing, wilting, stunting, and dieback. This Fusarium root rot is very specific to garbanzos and field peas (it will not go to other field crops). Plants are more susceptible to this disease when they are stressed, such as under drought or water logging conditions. We suspect that our recent relatively warm, dry winter stressed the garbanzo plants in the affected field, favoring disease development that healthy plants would normally grow out of.  Use of seed treatments will help manage this disease along with long-term crop rotation. More information on diseases in dry beans can be found on the newly revised UC IPM guidelines for dry beans.