- Author: Michael D Cahn
Currently, we are experiencing a prolonged heatwave on the central coast. Heatwaves have become a recurring phenomenon in recent years, especially in late summer. With thousands of acres of cool season vegetables in the ground, irrigation will be critical for keeping crops cool and for supplying enough moisture to meet their water needs.
Crops can be kept cool by maximizing evapotranspiration (ET). As liquid water vaporizes heat is lost from the surfaces of leaves and soil and from the surrounding air, which cools the temperature of the crop. Under water stress leaf stomates close during the hottest period of the day (11 am to 4 pm) and the temperature of the plant tissue can rise above the temperature of the surrounding air. If the temperature becomes too great leaves and other plant parts may become scorched (Figure 1).
Figure 1. Heat damage in broccoli
Since most ranches have a limited number of wells and personnel to irrigate, it is challenging to assure that each field has adequate soil moisture to prevent plants from overheating. A good strategy is to irrigate just enough to refill the soil profile to the rooting depth of the crop.
To prioritize which fields to irrigate one should consider the water holding capacity and existing level of moisture of the soil, as well as rooting depth and developmental stage of the crop. For example, a lettuce crop near maturity with a high ET demand, growing on a sandy textured soil that feels dry, should probably be irrigated soon. A young lettuce crop with a low ET demand, growing on silt loam soil that still feels moist, likely can be irrigated later without suffering heat damage.
Another consideration for prioritizing which fields to irrigate are recent field operations. A recently transplanted vegetable field may need to be irrigated first but may not need a long irrigation to re-saturate the soil around the roots. A crop that was recently cultivated may have pruned roots, and therefore may need water soon to prevent wilting under these hot conditions.
Table 1 estimates how much moisture is available to a vegetable crop between saturation and moderately dry or dry conditions for different soil textures. This table can be a guide for how much water should be applied to re-saturate the soil. For example, applying 0.42 inches per foot of rooting depth will bring a moderately dry silty clay soil back to saturation. Applying more than this amount of water will likely over-saturate the root zone.
Table 1. Estimated plant-available moisture for different textured soils.
Also, estimating the cumulative crop ET since the last irrigation can guide how long to irrigate. Reference ET values between south Salinas and Soledad during this hot spell have been as high as 0.27 inches per day. If the crop has a full canopy, 0.25 to 0.3 inches for each day since the last irrigation would be a good rule of thumb for how much water to apply as long as the total does not exceed the water holding capacity of the soil.
Lastly, one needs to convert the amount of water to apply to an irrigation run-time. To make this calculation one needs to know the application rate of the irrigation system. For impact sprinklers, the application rate can be estimated using Tables 2-4. Note that pressure and nozzle size have a significant effect on application rate. For drip, the irrigation time will depend on the tape discharge rate and pressure, as well as the spacing of drip lines. Assuming that the drip system is operated at the pressure recommended by the manufacturer (usually 8 to 10 psi) one can use Table 5 to approximate the application rate. For example, for one drip line of medium flow tape (0.45 gpm/100 ft) on 40- inch wide beds the application rate of the drip system is 0.13 inches per hour. If there are several drip lines per bed then multiply the application rate in the table by the number of drip lines.
The appropriate run-time can be estimated by dividing the amount of water to apply by the application rate of the irrigation system. For example, to apply 0.6 inches of water to a field with drip using medium flow tape the water would need to run for 4.6 hours:
Hours to operate the irrigation system = 0.6 inches of water/0.13 inches per hour = 4.6 hours
Summary
Irrigating the right amount of time to bring the soil back to saturation will maximize crop ET during these hot days, and hopefully prevent any heat damage to crops. Also, consider visiting the CropManage website (cropmanage.ucanr.edu) for further guidance on scheduling irrigations. This online tool can assist growers in quickly estimating how much water to apply to meet crop water needs.
Table 2. Sprinkler application rate for varying pressures and nozzle diameters for a solid set spacing of 30 × 30 feet (Rainbird 20JH).
Table 3. Sprinkler application rate for varying pressures and nozzle diameters for a solid set spacing of 30 × 33.3 feet (Rainbird 20JH).
Table 4. Sprinkler application rate for varying pressures and nozzle diameters for a solid set spacing of 30 × 40 feet (Rainbird 20JH).
Table 5. Drip application rates for varying bed widths and tape flow rates estimated for 1 drip line per bed. Multiply the rate in the table by the number of drip lines per bed to determine the actual application rate. (For 3 drip lines on an 80-inch bed multiply by 3)
- Author: Alejandro Del Pozo-Valdivia
This will be a ZOOM Webinar
When? Thursday August 6th
At what time? From 10:00 –11:15 am
If interested in participating, please register at:
http://ucanr.edu/survey/survey.cfm?surveynumber=31781
We will send the ZOOM link 24 hours before the event
/span>Tim Hartz published a comprehensive guide to nutrient management for vegetable crops. The information provided in this book is of great relevance to growers, PCAs, fertilizer industry professionals and all who are interested in the principles of nutrient management and who need specific and relevant information on managing nutrients in vegetable crops.
Nutrient management is critical to successful vegetable production, given the high value and exacting market standards for size, appearance, and postharvest quality for vegetable crops. Growers are now facing increasingly stringent regulations designed to minimize nutrient losses to the environment and this guide provides the detailed information needed to address these issues by providing detailed background information on nutrients and current information on best practices management practices for the industry.
Drawing on over 25 years of industry experience, Tim Hartz outlines the principles of nutrient management that are specifically of interest to vegetable producers on the Central Coast and other areas of the state.
Ordering Information:
112 pages
ANR Publication #3555
ISBN 978-1-62711-070-9
$35.00
Direct product link: https://anrcatalog.ucanr.edu/Details.aspx?itemNo=3555
- Author: Alejandro Del Pozo-Valdivia
Upcoming online meeting on July 30th 2020 from 8:30am to 12pm.
Register for this ZOOM event at:
https://ucanr.edu/survey/survey.cfm?surveynumber=30593
You will receive the ZOOM link for the meeting by email after the registration.
Agenda for this meeting is below.
Richard Smith, Vegetable Crops and Weed Science Farm Advisor
Steve Koike, TriCal Diagnostics
Clubroot disease can be a serious production issue for broccoli, cauliflower, and other brassicas in the Salinas Valley. The disease is caused by a unique organism (Plasmodiophora brassicae) that is closely related to ciliate protozoans but is classified in its own taxonomic group. It survives over 20 years as resting spores in the soil that are released as the clubbed root tissue decays. At temperatures above 65 °F, the resting spores release zoospores that swim to host plant roots and infect through root hairs. Once inside the plant, the organism grows into a large multinucleate plasmodium (a multinucleate mass of protoplasm) which stimulates changes in the plant hormones, resulting in enlarged root cells and the characteristic clubbing of the roots (Photo 1). Root infections by the clubroot pathogen can occur in both acid and alkaline soils; however, acidic soil conditions favor the development of the root symptoms. In addition to the main brassica crops, Plasmodiophora can infect arugula, radish, mustard cover crops, and weeds such as shepherd's purse and even some grasses. Plants that develop severe root swellings will exhibit above ground symptoms (Photo 2) indicative of non-functioning root systems, which includes yellowing, wilting, poor growth and stunting, drying and death of lower leaves, and eventual plant death.
Clubroot in the Salinas Valley is mostly controlled by maintaining soil pH above 7.2 to 7.3 by liming. The high pH does not kill the pathogen but inhibits the formation of the root clubs. Soils where control of clubroot by liming is achieved are called “responsive” soils. However, soils where liming is less effective are called “unresponsive” soils.
In 2020 we had calls regarding the incidence of clubroot on brassicas. In each situation the grower/PCA had soil lab results that indicated that the soil pH was greater than 7.2. To investigate this situation, a small study was conducted. At three fields soil was collected from symptomatic and asymptomatic areas of the crop and soil pH was determined using a pH meter at the UCCE or UC Davis Analytical Lab. The results shown in Table 1 indicate that clubroot was more severe in soils with lower soil pH levels. These findings are consistent with what we know about clubroot, that higher pH soils should have less concern with this disease.
So why did clubroot occur in soils that had test pH values greater than 7.2? It is important to keep in mind that soils have a great deal of inherent variability. The goal is to determine if the soil pH for a 5 or 10 acre field is ≥7.2. This is typically done by collecting 15 – 20 soil cores from various parts of the field and mixing them together as a composite sample. However, if sample collection by chance missed areas of lower soil pH, the lab results may be skewed to represent areas of the field that had relatively higher pH values. If this is the case, such a sample could have an artificially high pH (greater than 7.2) while some parts of the field may have a lower pH value. One way to have greater confidence in the soil pH is to collect more soil cores in fields where clubroot disease has been noted in the past.
At present, we have not seen evidence in Monterey County that soils are unresponsive to liming or that the liming treatment is failing to control clubroot, given variability in soil pH and pH testing. In our intensive vegetable production system, soil pH tends to decrease over time through the use of ammonium fertilizers. The loss of calcium, magnesium and potassium from crop removal and leaching can also contribute to lower soil pH on lighter soils. Given the longevity of clubroot resting spores in the soil, it is important to maintain a liming program to assure that soil pHs are above 7.2 to 7.3 to thoroughly suppress clubroot throughout the field.
Table 1. Three evaluations of soil pH in clubroot affected fields
Site |
Soil pH |
Crop |
Location |
Soil Type |
|
Symptomatic |
Asymptomatic |
||||
1 |
6.01 |
6.25* |
Broccoli |
Blanco |
Pacheco clay loam |
2 |
6.60 |
7.58 |
B. sprouts |
Eastside |
Chualar loam |
3 |
7.13 |
7.67 |
Cauliflower |
Near river |
Metz complex |
* These plants were also affected but to a lesser degree.
Photo 1. Typical clubbing of broccoli roots
Photo 2. A weak localized area in a cauliflower field infected with clubroot (note stunting, wilting and yellowing of affected plants)