Franz Niederholzer, UCCE Farm Advisor, Colusa, Sutter & Yuba Counties
Luke Milliron, UCCE Farm Advisor, Butte, Tehama & Glenn Counties
Katherine Jarvis-Shean, UCCE Farm Advisor Sacramento, Solano & Yolo Counties
Emily J. Symmes, former UCCE Area IPM Advisor, Sacramento Valley

Updated by Clarissa Reyes, North Sacramento Valley Orchard System Staff Research Associate on February, 2023.

Please note that the following are general recommendations intended to help you keep track of regular practices in a busy time; the optimal timing for management practices may vary based on specific location and conditions.

• Got a crop? Measure crop load at reference date to determine if thinning is needed. Reference date is 7-10 days after pit hardening. Check for pit hardening starting in mid-April. If a sharp knife catches, even briefly, when cutting across the blossom end of the flower, the fruit has reached pit hardening. Once reference date is reached, check cropload in representative trees in the orchard and, if needed, thin as early as possible for best size results. Get ready to line up shakers if bloom weather is good. Thin early (once reference date is reached) for best size results.
• Irrigation: In dry springs, pay special attention to orchard water status and if irrigation is needed.
  • Monitor orchard moisture (soil moisture sensors and/or pressure chamber readings) to track orchard moisture status and determine when to apply first irrigation. Irrigating too early can saturate soils, leading to leaf yellowing from iron chlorosis. Despite heavy rains over the winter, we are still experiencing a drought. Find drought management strategies for prune here.
  • If a dry spring is expected, irrigation may be needed much earlier than “normal”. If the orchard is allowed to really dry out in the spring, rewetting can cause end cracking on fruit, especially in May and/or June. Don’t let your orchards go into those months with water stress. Keep an eye on 1) crop needs and soil water levels and 2) the forecast weather for the coming week and beyond. The most direct measure of water status is the pressure bomb, read more here.
  • Don’t apply irrigation before the crop has used more water than the first irrigation will apply. Irrigating too early can saturate soils, leading to leaf yellowing from iron chlorosis. Yellow trees due to wet soils in the spring should “green up” but may not feed the growing crop as well as if they never became yellow at all. For more on diagnosing yellow prune trees see this site.
• Fertilization program starts: Crop load is the major driver of nitrogen (N) and potassium (K) use in prune trees. The bigger the crop, the more of these nutrients that’re needed. Check crop load in mid-April and use this information to plan your fertilizer applications. To optimize uptake and avoid leaching, apply multiple N applications, avoiding a single heavy spring application. Consider an N application before the end of April if there is a good crop set. If considering foliar potassium nitrate applications as your K program or to supplement soil applied K, begin spraying in late April and make additional applications every 2-3 weeks.
  • More than 50% of annual N budget should be applied before June 1^st. A large prune crop (3-4 dry tons/acre) contains 40-50 lbs of N and that N will leave the orchard in the fruit bins at harvest. Assuming an additional 30 lbs N for the tree (shoot growth, spur growth, etc.) and 70% efficiency (0.7 lb N into the tree for every pound of fertilizer N applied to the soil) the annual N budget/acre for a mature prune orchard with a good to heavy crop should be 100-115 lbs N/acre. For best efficiency, make several smaller (for example, 25-40 lbs N/acre) N applications through the season and inject liquid fertilizer late in the irrigation set and then flush with 1-2 hours of clean water. More details on prune nutrition/fertility are found here.

• Aphid: Monitor for leaf curl plum aphid and mealy plum aphid since colonies can grow soon after bloom. Oil sprays anytime from petal fall to May 15 can reduce mealy plum aphid to acceptable levels with good to excellent coverage. Oil is not effective against leaf curl aphid during this period as the spray can’t reach inside the curled leaves.
• Obliquebanded leafroller (OBLR): Place pheromone traps (minimum 2 per block) at the beginning of April to establish a biofix (moths caught on two consecutive trap checks) and begin accumulating degree days to inform when to begin fruit inspections.
• Peach twig borer (PTB): Continue monitoring for PTB biofix. (Traps should be up in March.) PTB biofix in prune orchards is often later than in almond orchards. PTB damage can give brown rot disease entry into fruit. If you set a heavy crop, beware of PTB populations.
• San Jose Scale (SJS): If dormant treatments were not applied, the dormant spray didn’t do a good job, or spring SJS pressure appears high, consider treating at 600 to 700 degree days after pheromone trap biofix to target emerging crawlers. (Traps should be up in February.) Alternatively, SJS crawler activity can be monitored using double-sided sticky tape around limbs beginning in April to detect crawler emergence and time spring treatments if necessary. Caution: If you use neonic pesticides for aphid control (Actara^®, Assail^®, Leverage 360^®, etc.) scale populations may increase.