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A log of observations from the fields of San Joaquin County and the Delta.
Alfalfa field
Lima Bean Heat Stress
Michelle M Leinfelder-Miles
Published on: September 3, 2020

Last week, I visited a baby lima field in the southwest part of San Joaquin County that had overall poor pod set. Pods were filling lower in the canopy, but flowers had not set higher on the plants. The field, which was planted in late-June/early-July had an excellent stand, and ostensibly, good fertility and moisture status (Figure 1). There were two possible reasons for the poor pod set that immediately came to mind: 1) lygus damage and 2) heat stress. 

Figure 2. Adult Lygus. (Courtesy of UC IPM.)
Figure 2. Adult Lygus. (Courtesy of UC IPM.)
Lygus bugs (Figure 2) have piercing and sucking mouthparts that can damage flowers and buds, resulting in poor pod set, or pierce developing pods, resulting in blemished beans. Alfalfa and safflower are preferred hosts for lygus bugs, but when these crops are cut, the lygus may migrate to nearby bean fields. While this particular bean field was near alfalfa, there were no lygus flying in the field as we walked through it, nor was there injury to the pods that had developed. The grower indicated that he had already treated for lygus twice to knock down populations early-on. Lygus damage tends to be in patches; whereas, this damage was uniform and widespread.  In other words, the culprit did not appear to be lygus, but heat stress seemed very likely (Figure 3).

I checked data from the CIMIS stations nearest to this field, which are the Brentwood and Manteca stations. Between May 1st and August 31st, the Brentwood station recorded 16 days with a temperature over 100°F, and the Manteca station recorded 9 days over 100°F. Most notably, the heatwave in mid-August struck at perfectly wrong timing. This field was about 50 days after planting, which is generally the prime time for bean flowering. The heatwave brought daytime temperatures over 105°F and nighttime temperatures that barely, if at all, dropped below 70°F. In fact, it is the high nighttime temperatures that will impair pod development by hindering pollen movement and rendering it sterile. This is not just the case for limas; it can happen with other dry beans, as shown by Rachael Long in this blog post from a couple weeks ago. Had the heatwave occurred earlier in the summer, it could have caused a split set, which is not desirable. The mid-August timing, however, means that day length is now too short for further pod development, and yield will likely be lower than expected.

Naturally, one should ask what would be considered a ‘high' nighttime temperature? Rachael remembers having a conversation roughly 20 years ago with UC dry bean breeder of that time, Steve Temple, who said that nighttime temperatures above 68°F will cause poor pod set. This is corroborated by recent work out of the University of Delaware that indicates nighttime temperatures of roughly 70°F impairing pod set. CIMIS recorded nighttime temperatures in that range. The grower indicated that some nights stayed closer to 80°F, which is the temperature at which breeders screen varieties for heat tolerance.

So, what can a grower do during a heatwave? Obviously, we can't control the weather, but it's important to ensure that beans are not moisture-stressed at bloom, and especially not when bloom occurs during a heatwave. Check the top 12 to 24 inches of the soil profile, and irrigate if the soil is dry. If in doubt about how much water is needed, check the reference evapotranspiration (ETo) and irrigate to replace at least 120 percent of your daily ETo. Daily August ETo in the San Joaquin Valley ranges from 0.2 to 0.3 inches per day, so growers would want to apply 120 percent of that amount. In the field that I visited, the crop looked very healthy and non-stressed, so there is no clear suggestion for a management practice that could have saved the set during our recent heatwave.

For more information on lima bean production in California, please see the UC production manual. 

Tags: heat stress (1), lima beans (3)
Comments: 0
Lima Bean Root and Crown Rot
Michelle M Leinfelder-Miles
Published on: July 11, 2018

Figure 1. Lima bean root and crown rot.
Figure 1. 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 (48-52⁰F). (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.

Tags: disease management (2), diseases (2), lima beans (3)
Comments: 0
White Mold in Lima Beans
Michelle M Leinfelder-Miles
Published on: September 28, 2016

Hard black structures on bean roots.
Figure 1. Sclerotia of white mold disease.
I was recently contacted by a Pest Control Advisor (PCA) regarding a lima bean field in the Tracy area. The plants were showing some unusual symptoms, and while the field was nearing harvest, the grower was hoping to get another couple weeks out of it for pod fill. The PCA had already collected plant samples and sent them to a disease lab; the lab diagnosed the plants as having Sclerotinia sclerotiorum, or white mold. Inside the roots of those plants were hard, black sclerotia (Figure 1), which are compact masses of mycelium and characteristic of the disease.

Figure 2. White mycelium of white mold.
Figure 2. White mycelium of white mold.
I later visited the field with the PCA in order to assess the situation for myself. The disease appeared to be most apparent in one corner of the field where the water tends to sit a little longer. (This is a furrow irrigated field that has had beans for several consecutive years.) Moist or humid conditions favor white mold, as do heavy canopies. The leaves were showing some unusual brown lesions, and some plants had white mycelium growing on the pods (Figure 2). The most detrimental symptom that we observed, however, was complete infection of the stem at the soil line, causing the tissue to dry up (Figure 3). In this case, the upper canopy looked ok, but I am uncertain, perhaps doubtful that the pods will fill on these plants.

Figure 3. Stem die-back from white mold.
Figure 3. Stem die-back from white mold.
What we can hope is that the disease won't appear on more plants in the next two weeks because there are really no management strategies at this stage in the game. The grower needs to think about how to manage this disease in the future, and rotating out of beans to a non-host crop (like small grains or corn) is probably a good strategy. Weeds around the field can still harbor the disease, and the hard, black sclerotia can survive in the field for several years. If the grower plants beans again, then it is important not to over-apply nitrogen, which can result in an excessive canopy, limiting air movement. If wider row spacing is an option, this can also improve air movement in the canopy, which makes conditions less favorable for white mold. Monitoring for the disease should begin at flowering, which is also the best time to apply fungicides (like thiophanate methyl or boscalid) because the plants are small enough for the treatment to penetrate into the canopy.

Tags: disease management (2), lima beans (3), mycelium (1), sclerotia (1), white mold (2)
Comments: 0
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