Diagnosing problems in the field is never an easy task, but it is one of the most interesting aspects of my job because I usually never encounter the same set of circumstances twice. This summer, I have been called out on a few interesting diagnostics calls, and I wanted to share some observations.
I visited a blackeye bean field that was planted at the end of June. Since June was unusually cool this year, the grower's planting was delayed. Blackeyes shouldn't be planted until the soil temperature reaches at least 65 degrees F, and the cool spring conditions kept the soil cool. About six days after planting, the beans were only sporadically emerged. The plants that had emerged looked healthy, but the overall stand was poor. The grower said that soil moisture was good at the time of planting. I scratched down and found the seed about three inches deep, which is perhaps a little bit deep for blackeyes. Seed had germinated, and the germinated seed looked healthy with no apparent seedling diseases. Seedling diseases include Pythium and Rhizoctonia. Pythium symptoms appear as water-soaked lesions, and the hypocotyl eventually ‘dampens off'. Rhizoctonia symptoms appear as reddish-brown lesions that can girdle the stem. We felt better that it didn't appear to be a disease problem. I reached out to my farm advisor colleague, Rachael Long, to get her take on the situation, and she agreed that this was likely delayed emergence due to cool soil and a deeper planting depth. In the end, we decided to test our patience, wait a few more days, and see what would happen. After another five days, the grower let me know that the plants had emerged, and the stand looked good!
In mid-July, I was walking through a rice field and observed reddish-brown spotting on leaves that were above the water (Figure 1), and older leaves had turned brown and were in the water. I thought about stem rot, which is a disease we have observed on some Delta farms, but I wasn't observing black lesions at the water line. I contacted my colleague, Luis Espino, who has done disease management work in the Sacramento Valley. He didn't think the symptoms resembled a disease, but rather, a nutrient deficiency, like possibly potassium. I think a potassium deficiency is a reasonable hypothesis. We know that soil potassium is low in some Delta soils. Also, potassium is removed from the system in large quantities after harvest, especially in fields where the straw is baled. UC Rice Specialist, Bruce Linquist, has researched this and summarized it in this fact sheet. For a 90-cwt yielding crop, approximately 26 lb K/ac is removed in the grain, but about 28 lb K/ac is removed for every ton of straw. At this field, we advised leaf tissue testing. Between tillering and panicle initiation, the Y-leaf should have a K concentration of at least 1.5%. At heading, the flag leaf should have a K concentration of at least 1.2%. Bruce has also created a fact sheet on rice tissue nutrient concentrations. If growers are baling straw, I would advise soil sampling to determine whether soil K concentrations are declining over time.
Figure 1. Leaf spotting on rice from suspected K deficiency.
In late July, I visited a corn field with my new Agronomy Advisor ‘neighbor', Giuliano Galdi. Throughout much of the field, there were plants that were drying up and dying. We didn't have a soil analysis, and what information we had on field history was limited. We noticed that manure had been applied to the field. One of our initial thoughts was salinity injury, but the symptoms didn't seem quite right. With salt damage, we would have expected to see necrosis along the leaf margins. Instead, what we were observing were leaves that were fused and unfurled (Figure 2). We called our colleague, Nick Clark, and Nick noted that he had observed symptoms like this in the southern San Joaquin Valley. He noted that when extreme weather heats up the soil, it can cause these symptoms when the plants emerge. Symptoms will vary based on the amount of plant emergence when the hot weather (i.e. hot soil) occurs. If the growing point of the plant is above the soil at the time of the heat, the plants may only have minimal damage, like a band across the older leaves. However, we suspect that the burned-up plants met their demise because the growing point was at the soil line at the time of the heat. Of course, it wouldn't hurt to get the soil analyzed just to make sure there aren't other conditions impacting the crop.
Figure 2. Suspected heat damage on corn due to hot soil temperatures at emergence.
The UCCE network has a breadth of experience to help identify problems and provide potential management solutions. Please don't hesitate to contact me if you'd like help with diagnosing problems in the field.