- Author: Richard Smith
Leaf spots are a common issue that affects spinach and reduces its salability. Spots on spinach leaves are caused by both biotic and abiotic influences. Insects, such as leafminers, frequently cause spots on spinach leaves. Female leafminers stipple spinach leaves by puncturing the leaf surface with their ovipositors and then feeding on plant sap that exudes from the holes. The stippled areas often occur in clusters and have a characteristic look due to the broken epidermal cells in the center of the stipple (photo below).
Diseases that cause spots on lettuce include the following: Cladosporium causes round, tan lesions that have dark green spores and mycelia in the center of the spots (http://ipm.ucanr.edu/PMG/r732100311.html); Anthracnose lesions start as dark green water-soaked lesions that later turn tan with black fruiting bodies in the center (can be observed with a good hand lens, http://ipm.ucanr.edu/PMG/r732100211.html); Stemphylium causes circular lesions, but no fungal fruiting bodies or mycelia occur in the lesions making it difficult to distinguish this disease from abiotic causes (http://ipm.ucanr.edu/PMG/r732100411.html).
Abiotic leaf spots on spinach are caused by a variety of factors. The most common cause is from burn caused by herbicides, other pesticides or fertilizers; in addition, water and other stresses can cause spotting or lesions on spinach. Spinach leaves are quite sensitive to chemicals and will readily respond to them by developing chlorotic or tan colored necrotic areas. The size and distribution of the lesions can often provide clues as to the cause of the issue. For instance, spots caused by herbicides or other chemicals (Photos No. 1) often have a characteristic pattern which can reveal if the burn was caused by spray drift, herbicide on dust or even a direct spray. Spray drift or lift off of oxyfluorfen often causes small diffuse spotting on the leaves. The location of the affected plants in the field can indicate the direction that the chemical came from. Also, examining the pattern of the lesions on the plant can give an indication when the drift incident may have occurred, depending on which age of leaves are affected; for instance, younger leaves may be unaffected because they were protected down in the crown of the plant when the incident occurred. Also, if only the outer edge of leaves are affected, this may indicate that the base of the leaves were protected by shingling of other leaves; this symptom often helps to confirm that you are dealing with a drift issue vs some other cause. In many of these types of situations, having some background information about recent spray applications in the vicinity of the field helps piece together how and when the incident occurred. Spotting on the weeds also provides confirmation of the cause of the incident. Distortion of the leaves (Photos No. 2) occurs when the necrosis occurs early in the development cycle of the leaf; in this situation, the expanding young leaf continues to develop around the dead lesion and results in distorted growth. Chemical issues can also cause a sub-lethal response in spinach leaves which results in chlorotic lesions (Photos No. 3).
Other chemicals that commonly cause burn on spinach include salts. Salts in water can cause spotting and marginal burns on spinach (Photos No. 4), but it is rare for water to be salty enough to cause this issue. However, water-run injections of fertilizer are salty enough if the injected fertilizer is not adequately rinsed from the irrigation lines after the injection. Fertilizer burn from water runs typically look different than spray burn because the lesions often occur along the edge of the leaves and are typically larger and blotchier (Photos No. 5). Sometimes growers are surprised that their spinach was burned from fertilizer because they routinely apply fertilizer through the sprinkler with no issues; however, if the clean-out phase of the injection was not long enough, then fertilizer burn can readily occur. Topdress applications with dry fertilizer at the 1st – 2nd true leaf stage can also cause burn on spinach leaves if the fertilizer prills stick to moisture on the leaves (Photos No. 6).
Most of the spotting that we see on spinach is from the above-mentioned causes. However, other issues that can cause defects on the leaves include water stress (Photos No. 7) which is often tied to hot spells. Water stress characteristically will occur in large blotches in the interveinal region of the leaf. It mostly occurs on older to mid-aged leaves and on various locations on the leaf. Hail damage (Photo No. 8) occasionally occurs in the spring and causes light green spots due to damaged epidermal cells that lets the green from tissue lower in the leaf to show through. Tipburn of spinach occurs in the spring when air temperatures increase, but soil temperatures are still cool. Under such conditions a localized deficiency of calcium develops out on the tip of the young leaf; as the leaf continues to expand, the necrotic tissue on the tip inhibits expansion resulting in a “hooded” shape to the leaf (Photos No. 9). Occasionally, chimeras are seen in spinach fields which are the result of a mutation that occurs in the meristem of the leaf; the resulting leaves have a dramatic calico yellow and green appearance (Photos No. 9).
1. Herbicide or other Chemical Burn
2. Necrosis and Distortion
3. Yellow Lesions
4. Salts in Water
5. Water-run Fertilizer Burn
6. Dry, Topdress Fertilizer Burn
7. Water Stress
8. Hail Damage
9. Other Leaf Issues
Testing the soil for nitrate is critical for managing fertilizer in crop production. Soil nitrate levels are continually in flux due to inputs of nitrate from fertilizer, mineralization of soil organic matter and crop residues, and irrigation water, as well as from losses of soil nitrate from leaching, crop uptake and denitrification. As a result, we advise to measure soil nitrate as close to a fertilization event as possible to make your decision based on current soil nitrate levels. Soil samples can also be sent to a laboratory for nitrate analysis, but there can be a lag in getting the results back which can reduce the usefulness of the analysis for making fertilizer decisions.
The soil nitrate quick test has the advantage over laboratory tests of analyzing soil nitrate in a timely fashion so that accurate fertilizer application decisions can be made. The soil nitrate quick test is often used just before a fertilizer application. If soil nitrate values are high enough, it is possible to reduce the fertilizer rate or even skip the fertilizer application without jeopardizing crop yield because the existing nitrate in the soil supplies the plant with nitrogen in the same way as applied fertilizer.
Soil cores are taken to a 12-inch depth for lettuce and spinach; however, for deeper rooted crops such as broccoli and cauliflower, soil cores of the second foot of soil during the last half of the crop cycle provide additional information on residual soil nitrate available for crop growth. Scrape away the soil from the top 2 inches of soil as it may be high in nitrate (due to upward movement of salts), but too dry for the plants to access. We have found that on some soil types (e.g. clays, silty clays) it is important to angle the soil probe in the direction of the fertilizer bead or drip tape (in fertigated situations) (See Figures 1 & 2). The reason for this is that in these soils, the fertilizer sometimes does not move far with the irrigation water and by angling the probe, you collect a more representative sample. As a matter of habit, we angle the probe on all soil types to keep our sampling method uniform.
Sample the field in a pattern that goes from one end of the field to the other, both sides of the field and through the middle – generally an “X” or “N” shaped pattern is fine. For a representative sample, it is important to collect enough samples, generally, 15 to 20 soil cores.
After collection, the sample needs to be thoroughly homogenized. Sandy soils are easily homogenized, but sticky clays or even wet loams are too gummy to mix. In these situations, we do the “pinch” method by laying out the soil cores and pinching off small, uniform amounts of soil from up and down each core. We then mix the pinches and use them for placing in the calcium chloride solution (see below). The strips are read with colorimetric test strips (see photo). The cheapest are the MQuant nitrate test strips described below. They can also be read with the Reflectoquant reader which provides a more accurate reading of the color development using Reflectoquant test strips.
Figures 1 & 2. Insert the probe in the seedline, but angle it to go beneath the bead of fertilizer or beneath the drip tape.
Procedure for conducting the nitrate quick test:
- MQuant 1.10020.0001 nitrate and nitrite test strips (0 to 500 ppm nitrate). They are available from MilliporeSigma or Amazon and contain 100 strips. They should be stored in a refrigerator when not being used for field testing. Also because color development may change as the strips age, it is advisable to store solutions of known nitrate concentration in a refrigerator to test if the strips are still accurate.
- 50 ml polyethylene centrifuge tubes (Figure 3) and a rack to hold sample tubes. These can be ordered from scientific supply companies, but they want to sell large batches that cost more. Amazon will sell smaller batches that are cheaper.
- Calcium chloride dihydrate. Can be ordered from scientific supply companies, but aquarium or food grade (e.g. canning supply companies or bulkfoods.com) calcium chloride is also fine for conducting the test.
- 1 gallon of distilled water
- Add 5.6 grams of calcium chloride to 1 gallon of distilled water to make up the 0.01 M calcium chloride solution
Note: Nitrate test strips should be stored in a refrigerator when not being used for field testing. Also because color development may change as the strips age, it is advisable to store solutions of known nitrate concentration in a refrigerator to test if the strips are still accurate. Using water samples from several wells with different concentrations of nitrate could be used to test if the strips continue to provide consistent readings.
- Collect a composite soil sample as described above.
- Fill a volumetrically marked tube or cylinder to the 30 ml level with 0.01 M Calcium Chloride (CaC12) solution.
- Add soil to the tube until the liquid level rises to 40 ml; cap tightly and shake vigorously until soil is thoroughly dispersed. Let sit until soil particles settle out.
- When solution is reasonable clear, dip the nitrate test strip into the solution, shake off excess solution, and wait 60 seconds. Compare color with the color chart provided (Figure 4)
- To minimize variability inherent in soil sampling, run duplicate samples for each field soil evaluated.
Figures 3. An empty polyethylene centrifuge tube. Figure 4. Dip the test strip in the clear supernatant and allow it to develop color for 1 minute
Figure 5. Use the scale indicated by red arrow for calculating soil nitrate concentration for mQuant test strips.
The MQuant test strips are calibrated both in parts per million ppm NO3 and ppm NO3-N. Reading the ppm NO3 value (Figure 5), use the table and equation below to convert the reading to ppm NO3-N in dry soil:
Strip reading (ppm NO3) ÷ correction factor = ppm NO3-N in dry soil
For instance, a reading from the test strips of 25 ppm NO3 from a moist loam would have 12.5 ppm NO3-N in the soil (25 ÷ 2 = 12.5). The ppm NO3-N values can be converted to pounds of nitrogen per acre in the top foot of soil by multiplying by 3.7, and in this example, that would equal 46 pounds of nitrogen per acre.
In general, soils with less than 10 ppm NO3-N are considered low for fast growing vegetable crops and soils with levels above 20 ppm NO3-N may have enough available N to supply crop needs for a limited period. Intermediate concentrations between 12 and 15 ppm NO3-N may warrant a half rate of fertilizer. However, it is important to get familiar with the nitrate quick test by doing small trials on your farm. As you gain more confidence in using the test to adjust fertilizer applications, you can do larger trials. Keep in mind that nitrate is very mobile, and in light textured soils, heavy irrigation/rainfall events can reduce the amount of available nitrogen in the soil. That is why it is always good to be cautious in reducing fertilizer applications based on the soil nitrate test. Feel free to contact either of us if you have any questions.
Additional information resources on soil nitrate testing and nitrate in irrigation water:
- Author: Alejandro Del Pozo-Valdivia
There are sex pheromone traps for Diamondback moth set up across the Salinas Valley. This pheromone only attracts males of this pest. These traps were first put out on February 20th, 2019. Traps are located in Castroville, Marina, Buena Vista, Chualar, Gonzales, by the Prison (near Soledad), and Soledad. Thanks to the PCAs who are helping me with this project.
Basically, numbers of moths per day per trap have been zeros, with the exception of the trap located in Castroville. Interestingly, moths were captured in all traps last week. The actual values of those captures are presented in the below figure as yellow dots. The bigger the dot represents a larger moth capture.
It seems like a new flight for the diamondback moth is about to begin across the Salinas Valley. Additionally, there has not been a break on the life cycle of this pest in the Castroville area. Population of this moth are persistent throughout the year in that area. The trap in Castroville has always captured moths since it was set up. Populations of this moth are residents of brassica weeds, as noted in previous scouting trips.
But, what does it mean to have less than one moth per trap per day, compared to 5 moths per trap per day? Is 5 moths a high value? How is that translated to caterpillars in the field? The next step will be to pair moth trap captures with actual scout data for caterpillars found in the surrounding areas of the traps. In the meantime, the information from these traps could help us to potentially predict when caterpillars might be present in the system in larger numbers. It is more likely that we will be able to see an increase of diamondback moth caterpillars in the next two weeks. It may be good to pay attention to cole crop fields, with the goal to early detect potential damaging populations of this pest in scouted fields.
I will be updating this map with moth captures at least every other week. Stay tune!
If you would like to learn more about this project, do not hesitate in contacting Alejandro Del-Pozo at email@example.com or 831-759-7359.
- Author: Lennis Arriaga
- Contributor: Richard Smith
For more information: https://svagtechsummit.com/