Franz Niederholzer, UCCE Farm Advisor, Colusa and Sutter/Yuba Counties

Hot weather (82-85^oF, maximum average hourly temperature) around full bloom coincided with local or statewide prune crop failures in 2004, 2005, 2007, 2013, and 2020. Orchards in the Sutter/Yuba area were hard hit in four of those five years.

For the last fifteen years, UC research supported by the CA Prune Board funds and Sunsweet Growers, Inc. staff has focused on 1) determining what bloom weather conditions produce crop failure and 2) how to save a crop when those conditions appear. So far, we have learned a lot about the weather causing crop failure, but no consistent, effective “fix” has been identified.

Researchers at UC Davis (2006-2008) showed that when air temperatures were artificially elevated from prebloom through full bloom with maximum temperatures in the 85^oF range, pollen tubes died before reaching the ovule. They concluded that this effect on pollen tube growth could stop flower fertilization and fruit set. Note: In this research, it took 4 – 6 days for pollen tubes to reach the ovule after pollination, with cooler temperatures slowing the growth of pollen tubes to the base of the flower.

So, sustained high temperatures (around 85^oF) can reduce prune fruit set. However, sustained heat is rare in the Sacramento Valley in March and daily maximum temperatures can vary by 15^oF through the 5-10 day period when flowers are opening. This raises the question; does the timing of extreme heat during bloom make a difference in crop set? To better understand crop risk under natural temperature patterns in prune orchards, UC Farm Advisors, staff and Sunsweet Growers, Inc. employees tracked flower stage and temperature through bloom, annually since 2009, on a number of prune branches in multiple orchards from around the state. Fruit set on those branches was determined late spring. and then determined % fruit set on those branches.

The conditions that lead to crop failure in prunes included the following:

1. Warm February temperatures (max temps = 65-80^oF) leading to…
2. Full bloom in the first two weeks of March
3. Short bloom period (averaging 5 days between first flower and full bloom)
4a. High temperatures (82^oF or more) for 1-2 days within 2 days of full bloom
OR
4b. Low temperatures averaging around 55^oF for 4-5 days after full bloom.

The data indicate  that prune flowers are most vulnerable to high temperatures when bloom is early, it progresses rapidly (5 days or so) and full bloom (roughly 80% open flowers) is followed by as little as a day or two of extremely warm temperatures (82^oF or more). Interestingly, high temperatures prior to full bloom, followed by cooler temperatures around full bloom, has not impacted crop set since the project started (2009).

What can growers do to improve set when hot weather is forecast at bloom? The only proven practice is running micro-sprinklers when temperatures are above 75^oF to provide some orchard cooling — usually 1-2 degree temperature drop. That degree of cooling could be the difference between a decent crop set and crop failure.

Dormant season oil application (3-4% 440 hort oil applied in late December through mid-January) may move bloom ahead of heat. In the Yuba City area, application of heavy rates of horticultural oil (for example, 3-4 gallons of 440 oil/acre) as part of a scale control program advances bloom time by a few days. In years where extreme heat passed quickly (2005, 2020), dormant oil moved bloom ahead of the heat spike and so helped improve set. Growers harvested a good crop.  In years with extended heat (2004, 2007) or cool weather (2016), oil didn’t move bloom out of the bad weather and set was not improved.