Use of dormant oil in pistachio is the subject for this month. You have until mid-February to decide on whether or not to oil for rest breaking and slightly earlier harvest. I question the practice of oiling mature trees in a good chilling year, and would wait until young trees are at least six years old before treating them, unless your five-year-olds are monsters with a huge flower bud count.
Over cropping baby trees can really be hard on their structure, since the branches are usually not of sufficient diameter to support the large crop without possibly circle tying the main scaffolds.
Horticultural mineral oil (HMO), is referred to by many as “dormant oil”, since it is most often applied to certain deciduous trees in the winter for the control of scale insects. I originally got HMO registered on pistachios for the control of immature soft scale and Phytocoris, both which overwinter in the egg stage on the tree.
While performing this insect research, I discovered that oil assists in overcoming delayed leafing and erratic bloom caused by inadequate chilling. Additional research over 12 years showed that it can also significantly increase nut production in young and mature trees, providing they have an abundance of fruit buds. Harvest is also advanced by about four days, depending upon the season. Growers planning on applying dormant oil should do so in mid February. Good results have also been reported in Kern County with mid to late January applications.
A three-year oil timing test was performed several years ago with Chris Wylie at Agri-World (Madera) in which he faithfully applied 6 gallons of Volck® oil (originally refined by Chevron Chemical, but discontinued by Valent in 2006) weekly from January 17 to March 7. Volck® was a 476 oil, which indicates the temperature in 0 F at which 50% of the oil “cracks” off from the refinery’s distillation tower.
Weekly ratings of the Agri-World trial by Chris and I during bud push and bloom confirmed that in the Madera area (one of the higher chilling pistachio regions), the best oil response (uniform and coincidental leaf out and bloom of male and female flowers) occurred from applications during the first two weeks in February. Treatment after mid-February was not as consistent or advanced.
Waiting until mid-February also allows the maximum accumulation of “natural chilling” from winter temperatures below 450 F. Volck® has now been replaced with oils similar in their 50% distillation rating, but the carbon-based molecules comprising the oil have less range in molecular weight.
This results in a slightly faster rate of breakdown on the plant tissue, which may affect their insecticidal and rest breaking efficacy. Britz/Simplot is one company marketing a “Volck®-like” oil registered for pistachios. Its 50% distillation temperature is 4700 F and I have confirmed its efficacy and safety on pistachio.
Other oils researched and registered for pistachios include oils with 415 and 4400 F distillation temperatures. Oils “cracking” from the distillation refining tower at these lower temperatures are lighter in molecular weight. Lighter weight oils provide less risk of phytotoxicity (injury to plant tissue).
My research with oil over 12 years indicates pistachio is quite tolerant of dormant oil application. However, this is NOT to say oil damage cannot occur. I witnessed true oil burn for the first time on six-year-old trees in 2005 in Tulare County.
It was isolated in one area of the block and the adjacent row showed no symptoms. The cause was never determined. The injury was limited to loss of vigorous, oneyear-old wood in the tops of the canopies. Excessive vigor and low carbohydrates was suspected.
My research also indicates oil with higher 50% distillation temperatures (470 oil) provides better rest breaking effects than lighter oils when the chilling hours are less than 700 hours. Such a test was performed at Tejon Farming (base of the Tehachapi’s) during the 2003 season where only 550 chilling hours were recorded.
A 470 oil applied in mid-February resulted in 50% bloom on April 10 compared to May 1 for the untreated trees. Trees treated with a 415 oil were about five days behind those treated with 470 oil. Unfortunately, yield data collection was not possible, but needless to say, Tejon now regularly uses oil!
Oil is not for everyone! It is a TOOL with many factors affecting its performance! It cannot put buds on trees! Nor can it overcome deficit irrigation, which significantly limits the tree’s productive capacity from low carbohydrates and insufficient fruit wood.
Oil applied at sprayer speeds too fast for optimal coverage, improperly timed, applied to stressed trees, used at too low a concentration, applied by tractor drivers who miss rows, or used in an area with potential spring frost can easily negate any benefits of use. Oil should NEVER be used if it is MILKY in color. This means the emulsifier has broken off (separated) from the oil, and burn to the pistachio trees is guaranteed.
Protect yourself by never transferring oil from the vessel it is delivered in, unless you have one dedicated to oil alone.
