As growers have been irrigating their almond orchards in preparation for harvest, some may have been unnerved by the appearance of pale or yellow leaves in their canopies.
This is often the result of chlorosis, a condition in which leaves produce insufficient chlorophyll, which gives them their green color. A trio of University of California Cooperative Extension advisors suggests the cause may have something to do with irrigation decisions made in the orchard.
The first step in assessing the cause of canopy chlorosis and decline in an orchard is mapping the distribution of the symptoms, researchers Elizabeth Fichtner, Max Culumber, and Bruce Lampinen explain in the UC Nut, Olive, and Prune Programmatic News blog.
“If a pattern of chlorosis is similar across irrigation lines,” they note, “then the cause of the problem may be related to over-watering or under-watering.”
They say they see two scenarios regularly during summer farm calls: terminal tree chlorosis and within-row tree chlorosis.
TERMINAL TREE CHLOROSIS
In some orchards, the terminal tree along the irrigation line may become chlorotic in advance of mortality, the researchers say. If this is a trend throughout the orchard, the grower might want to assess the sprinkler distribution at the end of irrigation lines.
Sometimes that tree may be outfitted with a sprinkler that isn’t shared with a neighboring tree, meaning it gets about 1.5 times the water of other healthy trees. In fact, sometimes the sprinkler is so close to the tree that it results in direct wetting of the trunk, which can lead to Phytophthora infection, particularly when surface water is used.
To correct this, a grower can change the micro-sprinkler head to a lower flow rate and place it away from the base of the tree to prevent water from hitting the trunk, the advisors note. Also, remember when replacing dead or declining trees at the end of rows, that the replant will need considerably less water than older trees in the row.
If terminal trees appear healthy, but canopy chlorosis is consistent elsewhere throughout the orchard, a grower should assess the distribution of sprinklers around the terminal trees in comparison with the others, Fichtner, Culumber, and Lampinen advise.
It could be that the terminal trees are receiving less water than the chlorotic trees, meaning the orchard as a whole is being over-irrigated. To test this, a grower or orchard manager can use a pressure chamber to assess the midday stem water potential of the trees.
If the orchard is being over-irrigated, growers should change their irrigation strategy, using a pressure chamber to measure tree water stress, as well as weekly crop transpiration reports provided by the UCCE and California Department of Water Resources. The reports contain water-use information for a variety of crops, including almonds, walnuts, and pistachios.
The weekly water-use estimates account for the changing growth stage and weather conditions at stations at UC facilities at Madera, Parlier, Lindcove, Stratford, Panoche, and Five Points. Each report gives crop-specific evapotranspiration estimates for the previous and upcoming weeks.
For more information, including links to sites explaining how to use pressure chambers and the ET reports, read the blog post at https://bit.ly/2wE9SGW