Dormant-Applied Nitrogen Fertilizers Advance ‘French' Prune Bloom

Manipulating deciduous fruit tree bloom timing in temperate climates can spread the risk of crop loss from extreme temperatures.  Heavy rates of horticultural oil (4-5% v/v in 100 gallon/acre = 935 l/ha) applied in the dormant season can advance bloom in prune orchards in California.  Risk of phytotoxicity limits this practice.  Calcium ammonium nitrate (CAN17) fertilizer (25% v/v) plus non-ionic surfactant (NIS) at 2% v/v as a canopy spray in mid-winter (30-60 chill portions) can advance prune bloom in the Sacramento Valley of California.  Use of a nitrogen (N) fertilizer spray to break rest and advance bloom may be a viable alternative to horticultural oil for California prune growers. A field trial was established to compare the established CAN17 rate to two other commercially available N fertilizer materials – dry potassium nitrate (KN) and a 28-0-0 urea-triazone solution (UTS).  The KN and UTS treatments were applied at 5% w/w and a 1% v/v rates, respectively.  The three N fertilizer treatments, each applied with 2% NIS, were established in a randomized complete block design (n=4) in January, 2008 and January 2009 in commercial orchards near Marysville, California.  Additional treatments included horticultural oil (4% v/v) and an untreated control.  All sprays were applied between 50 and 54 accumulated Chill Portions.  Treatments were applied to individual trees by backpack mistblower at a spray volume per tree equivalent to 100 gpa (935 l/ha).  Separate orchards under similar management were used in 2008 and 2009.  Three small branches (< 2 cm diameter) equally spaced around each tree and containing 40-100 flowers each were flagged prior to bloom.  Open flowers were counted every 2-3 days through bloom.  In 2008, all rest breaking treatments advanced time of 50% full bloom by 5-6 days over the untreated control.  2009 data will be presented.