By Katherine Jarvis-Shean, UC Orchard Systems Advisor, Sacramento, Solano & Yolo Counties

We all know that trees need water to grow but figuring out how much water and when to apply it is a trickier matter. Generally, this decision largely depends on the growth stage of the tree and the fruit, along with the weather leading up to that point in the season. Irrigation management in June is particularly important because it impacts some of the most critical components of fruit quality: sizing, cracking and sweetness.

What makes June a critical month for irrigation management? June to late July is the window in which prune fruit are increasing in size. Each fruit is just a collection of cells, and earlier in the spring, the fruit cells were multiplying, and in June and July those cells expand in size. The pressure of fluids in the cells force cell walls out, sort of like the water from a hose into a water balloon. When trees are water stressed, there’s less pressure moving those cell walls out. All those smaller cells add up to smaller fruit. Cells with enough turgor pressure to expand to meet their size potential result in larger fruit.

With that in mind, a word of caution! Fruit are susceptible to end cracking in June and July if they are exposed to sudden or dramatic shifts in water pressure – long periods of water stress followed by irrigation. This can then result in potential sites for disease infection, fruit loss and sometimes even substandard grades for an entire load. So, it’s not just a matter of meeting the trees water needs on average, when you look at the big picture of a season or a month or a week. It’s particularly important to avoid severe stress. 

Water stress can limit fruit cell expansion. It can also limit sugar production. Leaves are the sugar-making factories of the tree, through the process of photosynthesis. But they need carbon dioxide, which enters the leaves through tiny gateways called stomata, to make those sugars. When stomata are open, carbon dioxide can get in, but water can escape. Thus, when trees are water stressed, stomata will close in response, which significantly slows down sugar production. UC research found photosynthesis decreased by more than 25% during the heat of the day when prune trees were severely water stressed. The take away: If you want high Brix (sugar levels), don’t water stress your trees. This becomes even more important in July and August, when the majority of sugar accumulation in the fruit occurs.

So, hopefully you’re convinced that it’s important to avoid water stress in your prune trees in June. How do you go about doing that?

The most basic approach to irrigation scheduling – deciding how much to irrigate and when – is rooted in refilling water that the trees use from the moisture bank that is the soil. That water use is measured as evapotranspiration or ET (evaporation from the soil and transpiration (water use) by the tree and ground cover in the rows). Looking at historic weather records, we know prune trees use about 2 inches of water per week in June, but this can vary year to year and location to location. UC Cooperative Extension advisors in the Sacramento Valley have teamed up with the Department of Water Resources to give more exact estimates of water use based on CIMIS weather station data. You can find weekly reports of water usage here: http://www.sacvalleyorchards.com/et-reports/. You can also sign up there to receive reports by email. There you’ll also find details on how to estimate how many inches of water your system applies. 

It’s also worth remembering that all systems require maintenance and can decrease in the uniformity of water application over time. A low distribution uniformity will mean some parts of a block will be over-irrigated and some under-irrigated if they are, on average, having their water needs met. You can read more about irrigation system maintenance here: http://www.sacvalleyorchards.com/almonds/irrigation/irrigation-system-maintenance/. 

Using ET is a great starting tool for irrigation scheduling. However, it’s important to keep in mind that the numbers given in those reports are developed from average orchards in experiments and weather station data that could be miles from the orchard you care about. On-farm measurements are a much more precise way to know whether trees are experiencing water stress, and how irrigation can be managed to avoid that. 

In the May issue of this newsletter, my colleagues Allan Fulton, Luke Milliron and Franz Niederholzer discussed using the pressure chamber to measure stem water potential, and the different threshold to target for different times of the year. A few excerpts from that which are pertinent for June: For early June, -8 to -12 bars will favor fruit sizing. Anything consistently wetter than that (higher than -8 bars) is likely to encourage root rot and foliar disease. Mild stress between -12 to -16 bars in late June slowed shoot growth without affecting fruit size. However, anything more stressful (more negative) than -16 bars can decrease fruit size and increase susceptibility to fruit cracking once irrigation is applied, so avoid those moderate to high levels of stress. More information on pressure bomb use is available at http://anrcatalog.ucanr.edu/pdf/8503.pdf.

Soil moisture sensors are another tool to inform irrigation scheduling. These sensors, if properly installed and calibrated to the soils of a specific orchard, can indicate when soil is overly dry and irrigation is warranted. Thresholds for action depend on sensor and soil type. There are a number of different types of soil moisture sensors, with different degrees of cost, upkeep, ease of use and reliability. For some insight into the merits of different sensors for different conditions, check out http://www.sacvalleyorchards.com/blog/soil-moisture-sensor-selection-is-confusing/. 

There are many tools available to monitor for water stress in prunes. Keep an eye out for that stress this month, in particular, because size and cracking can all be seriously impacted by water stress this month.

Source: California Dried Plum Board