- Author: Brooke Jacobs
After an exceptional winter the UC pomology extension community has observed unusual bloom and leaf out patterns in a wide range of tree crops statewide. In general, bloom was delayed with less overlap among varieties within a crop. For example, Bob Beede (Farm Advisor emeritus, Kings County) observed poor overlap in bloom between female Kerman and male Peters pistachio varieties this spring. It is still unclear how much this reduced overlap in bloom timing limited pollination and fruit set.
In addition to reduced overlap in bloom among varieties, researchers in California have reported observations of uneven bloom and leaf out within individual trees. Rachel Elkins (Farm Advisor, Lake and Mendocino Counties), Dr. Bruce Lampinen (UCCE Walnut and Almond Specialist), and Alberto Ramos Luz (visiting doctoral student) have been tracking bloom and leaf out in pear and walnut to measure differences among varieties and understand potential effects on yield and harvest timing in the fall. Lampinen and Elkins observed almost normal bloom and leaf out in north facing buds in both walnut and pear. In contrast, south facing buds in both species were dramatically delayed (up to 6 weeks), dormant, or dead (Pictures 1 and 2). Preliminary measurements taken by Dr. Lampinen's research team at UC Davis indicate that north facing buds may have accumulated more winter chill because they experience substantially lower winter day time temperatures than south facing buds. South facing buds likely had chilling units cancelled by the fogless, sunny winter days in January when the south side of trees was substantially warmer than the north. A cold snap in early December 2013 could have also affected bud viability and provides an additional complicating factor.
Is it possible to say definitively that bloom and leaf out timing observed this spring are due to the low levels of winter chill as measured by the dynamic model?
No. Unfortunately, the biological basis for dormancy and chill accumulation is still poorly understood in tree crops. “The symptoms we've observed this year in many tree crops, including apple, walnut, pear pistachio and prune, are consistent with patterns we've seen in previous warm winters with low chill,” said Dr. Katherine Pope, a UC Davis pomology researcher. “However, without a properly controlled experiment we cannot say for sure whether the symptoms we have seen this spring are a result of low chill.” For now, growers and the UC extension community are continuing to watch and wait to see if this odd spring has any effect on yield at the end of the season.
Fruit and Nut Research and Information Center online chill accumulation models
Kern County newsletters by Bob Beede (Farm Advisor)