Managing Heat Stress At Bloom In Prunes

Irrigation In Prune OrchardAbrupt changes in cash flow make short- and long-term farm management very challenging. So, consistent production of large crops is vital to commercial success in farming. While most tree crop growers might think of cold/frost damage when it comes to weather-related crop loss, heat at the wrong time can also be a major problem.

In the last decade, prune growers in California have faced three years (2004, 2005, and 2007) of unpredicted, severe crop reductions. The damage appears due to reduced fruit set following bloom temperatures above 80°F. Small fruits form, but yellow and drop within three to four weeks of bloom.
In 2013, bloom temperatures were almost as high as in previous disaster years, and the crop was very light for many growers. Exactly what conditions at bloom harm the crop and the specific mechanism(s) affected by the heat is an ongoing research subject at the University of California.

Complex Problem

Until recently, little was known about the relationship between heat at bloom, flower health, and fruit set in Improved French prune. In a two-year study at UC-Davis that began after the 2004 crop year with prune industry support, Dr. Vito Polito and Matt DeCeault found that the optimum temperature for Improved French prune pollen germination and pollen tube growth was between 72°F-76°F. When temperatures are higher than 76°F, the pollen viability and the pollen tube growth decline rapidly.

A field study was established in 2008 to track weather conditions and bloom timing in the Sacramento Valley to better understand what and how weather conditions impact prune fruit. The work is being conducted by UC farm advisors including Rich Buchner, UC-Tehama County. So far, the results suggest that it is not just very warm temperatures at bloom that harm the crop, but when that heat occurs during bloom and the length of time those temperatures remain high.

Dry north winds appear to contribute to reduced fruit set, although not in all years. The absolute threshold temperatures and duration are still unknown, but our working threshold for significant crop loss is exposure to 10 hours or more of temperatures greater than 80°F.

Light Crop Factors

Weather in extremely light crop years in Sutter and Yuba Counties has been similar: Early to normal bloom timing (March 10-15) with no rain from first flower to full bloom, often with dry north winds.
Warm temperatures (70°F-plus daily maximum) begin early, even before any flowers open, and continue as bloom begins. These conditions help make a short, compact bloom period that can be more vulnerable to a spike in temperatures at the wrong time.

Extreme heat (83°F-plus) occurs around full bloom and is sustained for at least two days. Extreme heat (85°F-87°F) right around full bloom seems to have a big role in damaging the crop set. Appearance of extreme heat early in bloom followed by cooler temperatures doesn’t appear to harm the crop as much as heat at full bloom or closely after.

How To Limit Crop Loss

While no effective solution to the problem has emerged from the research to date, some management practices that may help growers reduce heat-at-bloom damage in prunes have been identified from conversations with growers. First, cool the orchard with irrigation water. Evaporative cooling may reduce temperatures enough to help set a crop. Impact sprinklers or micro-jet irrigation systems have an advantage over flood irrigation systems for orchard cooling. There were reports of good crops in 2005 after running water, while other growers ran water with no benefit.

Here are some key points to consider when using irrigation water to try to reduce temperatures in an orchard:
• The top 1 foot of soil should be moist (not saturated) when warm weather hits.
• If you can only irrigate part of the orchard per set, run water long enough to wet the soil and then shift flow to another part of the orchard. “Flash” irrigation water across irrigation checks and move on to others when using flood irrigation. If the soil surface dries and isn’t re-wet, the potential for evaporative cooling decreases significantly.
• Concentrate irrigation/cooling efforts on the upwind side of the orchard. Let the wind move the cooled air through the orchard.
• Consider running water (impact sprinklers or microjet sprinklers) if the temperature gets above 70°F during bloom. If you only have flood irrigation to work with, try to wet the soil surface in advance of predicted warm (more than 70°F-75°F) weather. If the warm weather stayed and the soil surface dried, irrigate again, but just to re-wet the top foot of soil.
• Get bees in the orchard. Rent bees, as native bee populations have weakened due to bee mites and poor food availability. Experience suggests better fruit set in 2005 and 2007 on trees close to hives, and poor fruit set away from the hives. It may be beneficial to spread hives throughout the orchard. In larger almond orchards, beehives are distributed at 1/10 to 1/4 mile intervals through the orchard for optimum pollination. If the orchard is smaller than 40 acres, hives can be distributed around the perimeter.
• Leave grass long in the orchard if heat at bloom is predicted. Tall, well irrigated vegetation should be 1°F-2°F cooler compared to short mowed vegetation on the orchard floor. If frost is a threat at bloom, keep the orchard ground cover as short as possible. Delay the orchard floor management decision as long as possible so that a better forecast of bloom weather is available and can be included in the final decision.

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