- Author: Elizabeth J Fichtner
- Author: Mae Culumber
- Author: Bruce Lampinen
The first step in assessing the cause of canopy chlorosis and decline in an orchard is mapping the distribution of the symptoms. If a pattern of chlorosis is similar across irrigation lines, then the cause of the problem may be related to over- or under-watering. Two scenarios present themselves regularly during summer farm calls: a) terminal tree chlorosis, and b) within row tree chlorosis (Figures 1 and2).
Terminal Tree Chlorosis. In some orchards, the terminal tree along the irrigation line may become chlorotic and decline in advance of mortality. If terminal tree chlorosis is a trend throughout the orchard, it is worth assessing the sprinkler distribution at the end of the irrigation lines. In some orchards, the terminal tree is outfitted with a sprinkler that is not shared with a neighboring tree (Figure 2A). This terminal tree receives 1.5 x the amount of water as the other ‘healthy' trees down the irrigation line. In an otherwise adequately-irrigated orchard, these terminal trees are over-irrigated and develop chlorosis and decline. Sometimes the terminal sprinkler is positioned adjacent to the trunk (Figure 3), resulting in direct wetting of the trunk, a condition that predisposes the tree to Phytophthora infection, particularly when surface water is utilized.
Correcting terminal tree chlorosis: To correct the over-irrigation of the terminal tree, the microsprinkler head can be changed to a lower flow rate. Sprinklers should be placed away from the base of trees to prevent direct contact of the trunk with the stream of water. Additionally, when replanting dead or declining trees at the end of rows, consider that the irrigation needs of the replant are considerably lower than that of the neighboring older tree in the row.
Within-row chlorosis. If canopy chlorosis is consistent throughout the orchard, but terminal trees appear healthy, assess the distribution of sprinklers around the terminal tree in comparison to the trees along the irrigation line. If the terminal tree receives less water (Figures 1B and 2B) than adjacent chlorotic trees, consider the potential that the orchard, as a whole, is over-irrigated. To test this hypothesis, growers and orchard managers can use a pressure chamber to assess the midday stem water potential of the trees. Almond trees maintained from -6--10 bar are under low water stress, but may be more susceptible to disease. Maintenance of almonds at -10--14 bar (mild stress) from mid-June through hull split, minimizes risk of disease (ie. hull rot) and supports shoot growth. For information on use of a pressure chamber for enhanced irrigation management of almond, walnut and prune, download UC ANR Publication #8503 (http://ucanr.edu/datastoreFiles/391-761.pdf).
Correcting within-row chlorosis: If the orchard at large is over-irrigated, a change in the overall irrigation strategy is warranted. A combination of pressure chamber use to measure tree water stress, and consideration of weekly crop transpiration may enhance irrigation scheduling. The California Department of Water Resources and UCCE have teamed up to provide Weekly ET Reports to agricultural water users to assist with irrigation scheduling. The reports include water use information for a variety of crops including almonds, pistachios, walnuts, grapevines, citrus, and stone-fruit of mature bearing age. Adjusted on a weekly basis, water use estimates account for the changing growth stage and weather conditions at the Madera, Parlier, Lindcove, Stratford, Panoche, and Five-Points CIMIS weather stations. Each report gives crop-specific evapotranspiration (ETc, total crop water use including soil evaporation) estimates for the previous and coming week. To learn how to use these reports, please refer to the following article: http://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=26858. Crop ET reports can be found online (ie: http://cetulare.ucanr.edu/Agriculture782/Custom_Program911/).
- Author: Mae Culumber
- Contact: Elizabeth J Fichtner
The California Department of Water Resources and the University of California Cooperative Extension have teamed up to provide Weekly ET Reports to agricultural water users to assist with irrigation scheduling. The reports include water use information for a variety of crops including almonds, pistachios, walnuts, grapevines, citrus, and stone-fruit of mature bearing age. Adjusted on a weekly basis, water use estimates account for the changing growth stage and weather conditions at the Madera, Parlier, Lindcove, Stratford, Panoche, and Five-Points CIMIS weather stations. Each report gives crop-specific evapotranspiration (ETc, total crop water use including soil evaporation) estimates for the previous and coming week.
Beginning the Irrigation Season
One of the objectives of the weekly ET report is to help managers decide when to initiate the first irrigation.Irrigating too much and too early can lead to reduced growth and yield due to loss of fine feeder roots as well as root disease. Stored soil moisture from winter and early spring precipitation will reduce the need to irrigate early in the season. As ET rates increase and the seasonal crop water use exceeds accumulated rainfall, water managers should consider if there is adequate soil moisture to supply the difference or to begin to irrigation. The weekly ET report provides “Accumulated Precipitation” since January 1st and the “Accumulated Seasonal Water Use” since leaf-out. When water use exceeds precipitation, it may be time to irrigate. Irrigation decisions should be confirmed with field monitoring of soil moisture and plant water status.
Irrigation Frequency and Duration
Managers need to know the application rate of their irrigation system in either inches/hr or gallons/hr and the effective wetted volume (surface wetting and sub-surface lateral subbing) of the root zone as a % of the orchard floor (Figure 1).Different soils will have larger or smaller wetted zones as soils with different texture hold different levels of moisture. This will influence when seasonal irrigation should begin, the appropriate set duration, and frequency of irrigation events throughout the season.
Example: If the effective wetted volume for a sandy loam soil is 25% of the whole orchard floor and crop ET for the week is given as 1.5” (0.21”/day), then the moisture extraction from the wetted area for the week = 1.5”/25% = 6”. A sandy loam soil at field capacity that has approximately 0.8 inches plant available water per foot of soil would have 5” of available moisture in a 5-foot rooting zone. The weekly water use (6” every 7 days) will exceed soil plant available water in about 5 days. In this example, irrigation sets need to be more frequent than once a week, roughly every 3-4 days. The application time for a system with one gallon per hour (gph) emitters that deliver about 1”/day would be calculated as:
Application time = (0.21”/day ET * 3 days)/ 1”/day irrigation * 24 hr/day = 15 hours every 3 days
Irrigation recommendations for common crop spacings are shown in the report in units of inches or gallons of water needed per week. To convert inches per tree to gallons:
Gallons = (Inches * 27,154 gal/acre inch water) / # plantings per acre
Adjusting for System Efficiency
Different irrigation systems can vary greatly in efficiency. Those with high uniformity such as drip micro-irrigation, are roughly 80-95% efficient and require less output to meet crop needs than other systems like flood-furrow, with efficiencies as low as 50%. The recommended amount of water to apply is corrected for a range of irrigation system efficiencies. If the mature almond orchard in the example was 90% efficient, you'd find the 90% column in the second table of the Weekly ET Report, and put in the almond value at 90% (1.67 inches or 0.24 in/day) into the equation above, instead of 1.5 acre-inches.
If you would like to receive weekly reports, have questions or need more assistance contact:
Mae Culumber UCCE Fresno County Nut Crop Advisor, cmculumber@ucanr.edu, 559-241-7526
Elizabeth Fichtner UCCE Tulare County Orchard Systems Advisor, ejfichtner@ucanr.edu, 559-684-3310
Phoebe Gordon UCCE Madera County Orchard Systems Advisor, pegordon@ucanr.edu, 559-675-7879 George Zhuang UCCE Fresno County Viticulture Advisor, gzhuang@ucanr.edu, 559-241-7506
Steve Ewert, California Department of Water Resources, sewert@water.ca.gov, 559-230-3334
