Deficit Irrigation and Hull Rot – A Practical Approach

By Ben Smith
Hortau Grower Support Specialist

Suture. Dehisce. Abscise.

Not only are they cool words to throw out when you want to look intelligent, they all can be used to describe an important part of almond development prior to harvest.

Almond growers care much about almond sutures, their progress in abscising, and their eventual dehiscing as they mark a very vulnerable period for the nut right before it is ready for harvest. Late June, or early July each year, the ridge running lengthwise along the developing almond nut begins to form a deep crevice, or suture.  As the days go by, the crevice deepens and widens as the tissues abscise (disconnect) and the two halves of the hull pull apart, eventually dehiscing (separating) and opening to reveal the almond and shell on the inside.

It has always been an exciting thing for me to watch. I don’t know of any other stone fruit that does it and it is one of the final processes to occur before the nut is ready to harvest.  However, while the process occurs, the shell is soft and vulnerable to insects, and the newly separated tissues provide really good conditions for mold growth which lowers the quality of the fruit and tends to kill the fruiting spurs on the tree.

One of the mostly commonly recommend practices to reduce occurrence of these problems and to limit the amount of time the nut is vulnerable, is to perform “regulated deficit irrigation”. 

… without good real-time monitoring, the stress becomes too great and the soil is never rehydrated before harvest
In theory, causing a limited amount of stress in the tree to help encourage the drying of the hull and shell, making them much less susceptible to pests. The general recommendation goes something like, “when the suture begins to appear begin applying 50% ET for 3 to 4 weeks.” In my experience, it is very difficult to perform on a farming scale.  I don’t know if I can say that I have ever performed or seen it performed successfully. Usually, trees are already too stressed to provide any benefit, or without good real-time monitoring, the stress becomes too great and the soil is never rehydrated before harvest.  That being said, I receive many questions each year asking how to properly perform regulated deficit irrigation and I am determined to find a way to make it work.

After a lot of failures, and a lot of pondering, here are some of my thoughts on the matter:

  • Hormones within the tree signals and carries out the formation of the suture and abscission. Stress, especially drought stress too early on, will limit the production and movement of the hormones, thus increasing the time it takes for the process to take place and increasing the vulnerable period.
    • In July, a tree can pass from a good water state to stressed within just 2 or 3 days. Don’t start stressing the trees until you can see that the suture has formed all the way to the shell.
    • At this point provide stress. Hormone production and delivery is not as critical and stress will promote the drying and flaring open of the hulls (this will also aid in reducing stick-tights).
  • Don’t over-stress. Most programs recommend stressing the tree until Stem Water Potential (Tension at leaf petiole) reaches somewhere around 14 bars.  Using a pressure bomb to measure stem water potential is very time consuming and is difficult to perform accurately without research-level detail orientation, calculating R-values, and standard deviations.  Furthermore, you can only get a good reading maybe once per day.  In other words, by the time you measure the appropriate tension and react by applying water, the trees will be stressed beyond the beneficial level.
    • Soil tension measures water stress at the root level, in very precise increments, and in real-time. This makes it ideal for allowing and controlling stress.
    • More experience can help nail the exact number down, but from what I have seen letting the soil tension reach 60 kPa (cbars) provides plenty of drought stress to encourage hull dry down without over-stressing and making it too difficult to recover good water status.
    • It isn’t possible to reach 60 kPa through the entire root zone and then to hold it there for days or weeks. Tension will continue to rise.  But the idea is to let the tension move across a greater zone.  Through the rest of the season, we recommend keeping the soil tension between 10 and 40 kPa.  During deficit irrigation, I would let the tension move between 10 and 60 kPa.  Irrigation will still be needed during this period, but you can cut back a little on the hours and the frequency to hold the tension a little higher.
  • Most recommendations suggest stressing the tree for 3 to 4 weeks. However, if we wait to apply stress until the suture is fully formed, 3 to 4 weeks isn’t needed.  I wouldn’t allow stress any longer than that. Two weeks is enough to facilitate drying without compromising tree health.  Furthermore, waiting too long won’t leave enough time to rehydrate the soil before harvest.
  • After the controlled deficit irrigation is past, start dumping on the water! This is where most deficit irrigation programs fail.  The stress gets out-of-hand and only limited amounts of water is applied afterwards, so the trees stay in a stressed state through harvest and often into dormancy.
    • This is where you will need to be thinking about the ideas discussed in the article, “Rehydrating Soils After a loss in Deep Moisture”. In a nutshell, put as much water on as the soil will take until soil tension decreases.  Realistically, with a double-line drip system, this may be 120 hours of irrigation per week for 3 weeks.  In a fan-jet system, it can easily be 60 to 70 hours a week for two weeks.  It is going to take a lot of water to get everything in a good condition before harvest.  I know things need to be dried down again right before harvest, but the losses from drought stress are not worth it.  Do everything to get the stress under control before harvest.

A Couple More Considerations

How much damage does hull rot cause you?  Depending on your answer, it may not be worth the effort and potential losses of regulated deficit irrigation.  August and September is when bud differentiation occurs in almonds.  That is when the tree is forming buds for next year and is deciding if those buds are going to push blossoms or leaves.  Extended periods of stress will have a negative impact on that ratio. Some areas really do not suffer from hull rot.  If you are lucky enough to be in one of those areas, save yourself and the trees the stress and forget deficit irrigation.

Similarly, if your fields are heavy in clay (30 percent or more), or have limited infiltration, I would think carefully and consider the value of deficit irrigation.  In these cases, it is going to be immensely difficult to rehydrate the soil between hull split and harvest.  The loss from mold and spur death really may not be worth the months and months of stress that would occur.

Finally, let’s be realistic.  How far can your energies and attentions extend?  Performing regulated deficit irrigation takes a lot of attention and time.  Do you have the time to control it without causing more damage than good?  If you don’t, it may be worth skipping it.

No doubt deficit irrigation can provide real benefits in limiting hull rot and limiting the crop’s vulnerability to insect pests.  But it is very tricky to perform.  If you have the time and energy to do it correctly, it should prove very beneficial.  But, this is the real world, and if the best you can do is keep everything hydrated and stress-free until harvest, I would opt for skipping deficit irrigation.

ABOUT THE AUTHOR

Ben Smith provides grower support in California’s Central Valley for Hortau. He grew up on a small dairy farm in northern Utah, where he spent most of his summers in the alfalfa and corn fields shoveling dirt, desperately trying to get the water to flood the fields evenly. He knew that there had to be a better way.

Ben went to Utah State University and received a B.S. degree in Crop Science with a minor in Soil Science. He has an extensive background working for irrigation equipment manufacturers and in large-scale production agriculture  installing irrigation systems, maintaining existing systems, and scheduling irrigations over large acreages.

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