I just raked up all the leaves under the avocado and it looks so nice. PUT THEM RIGHT BACK. The avocado is shallow-rooted and really depends on the natural leaf mulch to protect its roots. In fact the roots will actually colonize the rotted leaves as if it were soil. This mulch is also a first line of defense against root rot. The decomposing leaves create a hostile environment to the microorganism that causes the disease. The mulch also helps to reduce evaporative loss of water and therefore reduces water needs. Avocado growers will actually spread mulch in cases where trees are too young to produce adequate leaf drop for mulch or in windy areas where mulch has blown away. The key to remember is that the mulch should be kept at least six inches away from the trunk to avoid collar rot which can be causes by keeping a moist mulch against the trunk.
Mature avocados may be a big tree, but they have very shallow roots. The bulk of them are in the top 8 inches of soil. The tree therefore does not have access to a large volume of stored water. As opposed to a deep rooted walnut, they need frequent, small amounts of water. A young tree in the summer might need multiple applications per week, but because the root system is small, each application may only be 5-20 gallons. An older tree with its wider rooting pattern may go a week to a month between irrigations depending on the weather and rainfall. Proper irrigation is the best way to keep the avocado from getting root rot. Both over and under irrigation can induce the conditions for root rot, although over irrigation is more common. And remember, it is not just the amount applied at an irrigation, but the timing that is important, as well. Because you are managing such a shallow root system, just poking your finger into the root system will tell you if there is adequate moisture there before you irrigate again.
The canopy is thinning. The leaves are small and yellow. There is dieback in the canopy, with leafless tips on the branches. You dig around under the canopy in the wetted area of the sprinkler and you can't find roots within 6 inches of the soil surface or if you do find them they are black. There is little mulch under the tree. There are weeds growing under the tree. All these are signs of root rot disease. But it is also a sign of lack of water, because that is what is happening – there are no roots to take up water. And one of the things a gardener will often do, is start watering the diseased tree more, thinking it is lack of water, which if it is diseased only makes the condition worse. Adding more water to a tree that can not easily take it up only creates an asphyxiation that makes matters worse. Irrigation and mulch are the two most important factors for avoiding the disease.
So what do you do if you have disease? There are fungicides that are available from the nursery, but there are a number of things that you can do before applying something like that. First of all get a handle on the irrigation. Make sure you are irrigating to the tree's needs. Check soil moisture before irrigating. Make sure the tree is not getting supplemental water from another area such as a lawn sprinkler. Make sure there is a good thick, woody mulch under the canopy. Adding gypsum (15-20 pounds per tree) evenly spread under the canopy can also help, but reviewing and modifying the irrigation practice is the most important thing that you can do.
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- Author: Gary Bender
- Author: Dave Shaw
By now almost everyone growing tree crops in California is undergoing some amount of mandatory water cutbacks, especially if they are buying water from water districts. Some districts in the San Joaquin Valley have had really severe cuts, up to 80%. For a lot of growers who are not buying water, they may be facing severe water shortages due to water tables that are dropping.
San DiegoCounty is a good example. In 2008 all of the avocado and citrus growers in San Diego
County had a mandatory 30% water cutback, if they
were in the water discount program known as the Interruptible Ag Water Program. Fearing that the cuts would be even higher in 2009 if they stayed in the program, most growers opted out, but were then faced with paying full price for their water. Now, it looks like most of these growers will be faced with a mandatory cut of about 8% (this depends on the water district), and will have to pay full price for ag water. But wait! Because the districts aren't selling as much water, they need to raise prices to cover their fixed costs. For instance, Fallbrook
Public Utility District recently announced they were raising prices 13%. Many districts in San DiegoCounty have simply followed water wholesalers (MWD and SDCWA) in their price increases.
Please make sure that you are in touch with your local water district. They may have a different cutback rate than the 8% mentioned as well as conservation guidelines and regulations. You are responsible for knowing this information and you could be looking at some hefty fines for using more water than allowed.
So what can you as a grower do when faced with this scenario? Before we start with our recommendations, let's start by thinking about what you can't do. (This always confuses the issue, so let's get this off the table).
- You can't make it rain more. We are in a prolonged drought, this happens periodically in California, and that's just the way it is. Can you lobby for more storage for when it does rain in excess? Yes!
2. You can't solve the Delta smelt issue, not unless Congress cancels the Endangered Species Act. A solution for bypassing the Delta with a pipeline from the Sacramento River down to the State Water Project canal which supplies Southern California would certainly help. Researchers at UC Davis have determined that the canal would be the most economically feasible way to fix the delta issues. However, given California's budget woes, it probably won't happen soon.
- There are no magical solutions that work to “inactivate” the salts in your well water. There are a lot of devices sold that make lots of claims, but there is no University research evidence that shows that any of them work. The only thing that does work is reverse osmosis, but be careful because these systems produce brine which must be disposed of legally. The brine cannot go into the local creek.
OK! Let's Strategize. There are four steps for everybody to consider, it doesn't matter if you have a backyard lawn and landscape or if you have 700 acres of avocados.
1. Maintenance: Irrigation System and Cultural Practices
2. Improve Irrigation Scheduling
3. Deficit Irrigation
4. Reduce Irrigated Area
a. Irrigation System.
- Fix leaks. Unfortunately, there are almost always leaks for all kinds of reasons. Pickers step on sprinklers, squirrels eat through polytube, branches drop on valves, coyote puppies like to chew….the system should be checked during every irrigation
- Drain the lines. At the beginning of each year every lateral line should be opened in order to drain the fine silt that builds up.
- Maintain or increase the uniformity of irrigation so that each tree or each area gets about the same amount of water. Common problems include different sized sprinklers on the same line or pressure differences in the lines. Where there are elevation changes, every line should have a pressure regulator, they come pre-set to 30 psi. Having all of your lines set up with pressure regulators is the only way you can get an even distribution of water to all of the trees, and it solves the problem of too much pressure at the bottom of the grove and not enough at the top.
- Clean the filters often. You don't have a filter because you think that the district water has already been filtered? Hah! What happens if there is a break in the line in the street and the line fills with dirt during the repairs? All of your sprinklers will soon be filled with dirt.
- Is water flow being reduced at the end of the lateral line? It could be because scaffold roots are growing old enough to pinch off the buried line. The only cure is to replace the line.
b. Cultural Management.
- Control the weeds because weeds can use a lot of water.
- Mulch? Mulching is good for increasing biological activity in the soil and reducing stress on the trees, but the mulch will not save a lot of water if you are irrigating often….the large evaporative surface in mulches causes a lot of water to evaporate if the mulch surface is kept wet through frequent irrigation. Mulches are more helpful in reducing water use if the trees are young and a lot of soil is exposed to direct sunlight.
2. Improve the Irrigation Scheduling.
- CIMIS will calculate the amount of water to apply in your grove based on last week's water evapotranspiration (ET). You can get to CIMIS by using several methods; for avocado growers the best method is to use the irrigation calculator on the www.avocado.org website. If you need further instruction on this, you can call our office and ask for the Avocado Irrigation Calculator Step by Step paper. You need to know the application rater of your mini-sprinklers and the distribution uniformity of your grove's irrigation system.
- CIMIS tells you how much water to apply, but you need tensiometers, soil probes or shovels to tell you when to water.
- “Smart Controllers” have been used successfully in landscape and we have used one very successfully in an avocado irrigation trial The one we used allowed us to enter the crop coefficient for avocado into the device, and daily ET information would come in via a cell phone connection. When the required ET (multiplied automatically by the crop coefficient) reached the critical level, the irrigation system would come on, and then shut down when the required amount had been applied. Increased precision can be obtained by fine tuning these devices with the irrigation system precipitation (application) rate.
3. Deficit Irrigation.
- Deficit irrigation is the practice of applying less water than the ET of the crop or plant materials. Deficit irrigation is useful for conserving water in woody landscape ornamentals and drought tolerant plants where crop yield is not an issue. Water conserved in these areas may be re-allocated to other areas on the farm or landscape.
- There hasn't been enough research on deficit irrigation of avocado for us to comment. We suspect, however, that deficit irrigation will simply lead to dropped fruit and reduced yield.
- Stumping the avocado tree could be considered a form of deficit irrigation. In this case, the tree should be stumped in the spring, painted with white water-based paint to reflect heat, and the sprinkler can be capped for at least 2 months. As the tree starts to re-grow, some water should be added back, probably about 10-20% of the normal water use of a mature tree.
- Regulated Deficit Irrigation for Citrus is an important method for saving water, and in some cases will reduce puff and crease of the peel. In one orange trial done by Dr. David Goldhammer in the San JoaquinValley, an application of 25% of ETc from mid-May to Mid July saved about 25% of applied water for the year and reduced crease by 67%, without appreciably reducing yield.
- 4. Reduce Irrigated Area.
- Taking trees out of production. Trees that are chronically diseased and do not produce fruit (or the fruit is poor quality) should be taken out of production during this period. Also consider: trees in frosty areas, trees in wind-blown areas, trees near eucalyptus and other large trees that steal the water from the fruit trees.
- Changing crops. You may want to take out those Valencias during this period and replant to something that brings in more money, like seedless, easy-peeling mandarins. The young trees will be using a lot less water.
- Fallow Opportunities. You may decide to do some soil preparation, tillage or cultivation, or even soil solarization of non-irrigated areas.
We have found that this four step process is a logical way to achieve water cutbacks with least impact. It is possible to achieve a ten percent reduction in water by only improving irrigation system uniformity and scheduling procedures. Often, these two measures also result in better crop performance and reduced runoff. Reducing irrigated area or taking areas out of production should be a last resort and a well thought out decision. Plan for the future, hopefully water will be more available in future years.