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Frost and chill injury of olives
Determine water availability and quality for your proposed irrigation methods to ensure optimal grove productivity.
» frost tolerance » market attributes (oil - oil content %, extraction efficiency, flavour profile, oxidative stability; table olives - flesh:stone ratio, fruit size, flesh quality (resistance to bruising, sensory profile for the intended processing method).
Case study
Australian EVOO producer Boundary Bend has in recent years also firmly established itself in the US, gaining a successful place in the market for both its Australian EVOOs and also EVOO produced from its own US groves.
The company’s latest US grove expansion project was featured in a recent Olive Oil Times article, and is a good case study for the decision-making processes when establishing a new grove.
The project comprises two groves of 180 and 140 hectares respectively, planted in Yolo County in California’s Sacramento Valley. The region is one of the most fertile in the state and was chosen after analyses of climate and other relevant factors.
Most significantly, the groves were planted at medium-density, rather than the usual high or super-high density spacing of most new large-scale commercial groves.
The company said there were a number of reasons behind the decision, including: • the suitability of medium-density planting for all olive cultivars; • the ability to match cultivars to growing characteristics of specific sites; • the increased range of organoleptic characteristics provided by the wider cultivar range.
The new groves utilise Boundary Bend’s precision drip irrigation system, which enables optimum irrigation for both oil quality and water supply sustainability, based on monitored soil moisture levels.
The system also encompasses fertigation, again based on monitoring of the trees’ growth and development cycle, and changing needs.
Source: www.oliveoiltimes.com
In the May edition of OliveCare® News McFarlane discussed the importance of temperature monitoring in groves, given the growing significance of frost to the Australian olive industry.
He noted that frost symptoms and outcomes on olives are wideranging, and include: • damaged fruit, tip burn, leaf curl, defoliation, limb die-back and bark splitting; • lesions increase susceptibility to bacterial (olive knot) and fungal (anthracnose) infections; • increased suckering on the prevailing weather side of the tree trunk.
The most common issues, however, are to late ripening fruit from early frosts, and to spring shoots from late frosts.
Knowledge is everything
Pendleton Estate grove manager and horticultural expert Andrew Taylor said one of the biggest issues with frost is actually realising it has caused damage to your crop.
“The evidence of frost damage is not easy to find,” he said.
“The best evidence of frost damage over the winter is that you get to spring and there are no flowers on your trees. The dilemma is, though, that people don’t realise that’s what has happened.
“So this year most people have had a good harvest and next year when they don’t get a good return crop it will generally be blamed on biennial bearing. When you’re not monitoring temperatures and observing damaged flowering buds, you conclude that it’s a consequence of last year’s over-crop, or the drought or whatever, and you’ve missed the fact that there were several nights or mornings
Even light frost damage can completely ruin a crop. Image: Vera Sergeeva.
when it got too cold and the flowering buds were damaged.
“For example, in Keith they were still harvesting in late July - Pendolino, very healthy fruit, for very mild oil. And the flower buds for next year’s crop had already extended 10mm.
“The timing is very variety and location specific but the genitalia of that plant were already exposed. And how cold does it need to get to cause cell damage to that little plant? What temperature burns those buds off?
“There’s no hard and fast rule, but if buds are protruding more than 5mm you’re likely to get frost damage at -2ºC.
“The problem is that very few people have been doing data logging to get an idea of how cold it does get in their grove, whereas if you’ve got a data logger in your grove and you find you’ve got no buds left, you can ascertain the temperature which caused the damage.”
Reducing the risk
Frost fans are an increasingly common tool in very large groves and vineyards but Taylor said for smaller growers they’re not always affordable.
“If you’ve got the money to spend that may well be the way you want to go but for growers on a tight budget cultural practices are the things we need to factor in,” he said.
“They’re not going to completely negate the risk of frost, but the right grove management practices can dramatically reduce the risk.
Let it drift
“Moving air or drift reduces the freezing ability of surface water – for example, heavy dew – on small parts of plants like flower buds,” he said.
“And if not pruned properly, the trees themselves can reduce the movement of cold air in the grove as there’s no natural drift.
“So pruning your trees well and keeping a good one-metre gap between the canopy and the ground means you continue to get the natural drift across the grove, reducing the freezing effect of the frost.
“And if you’ve got a grove on a slope, and you’ve got a woodlot or shelter belts, they also need to be trimmed or pruned to enable the air to get through them. Again, anything which stops the flow of air can exacerbate the effects of frost.
Optimise tree health
“Lots of leaves on a tree - and healthy leaves – can also help protect the tree. Every plant cell is giving off a tiny amount of heat and if it’s healthy that can prevent flower buds being burned due to frost.
“Like any other major stress, a healthy tree can always survive better through a frost.”
Winter irrigation
While it’s possibly the last thing most growers would think of doing, Taylor said irrigating a frost-prone grove during winter is a smart move.
“Ground water in the middle of winter can be 14-15ºC,” he said.
“So by applying 3-4L per dripper under olive trees during frost conditions - 0 ºC or below - you’re adding latent heat to the ground, and that has to go up. Any added-in temperature you can provide can assist in reducing the risk of frost damage.
Increase soil biology
“Which brings me to the last practice, which is really important: increasing the biology in the soil under our trees and increasing the carbon content in our soil, which in turn increases the environment for biology to increase, will increase the soil temperature.
“In short, more heat is radiated from healthy, active soil during cold frost conditions than from sand, so while it doesn’t remove the risk it will assist our groves to combat frosts.”
Resources
NSW DPI Citrus Development Officer Steven Falivene gave a presentation on frost management in olives at the AOA National Olive Conference in Albury in 2019.
Don’t Get Burned - How to Manage Frost provides an overview on how frosts events occur, and more information on the on-farm practices that can be implemented to reduce the intensity of frosts.
The presentation also included information on the thresholds for olive frost damage#2, as follows:
Frost damage is a time and temperature relationship - fruit/shoots need time to decrease in temperature to reach freezing point: • olive fruit can be damaged below -1.7ºC • young olive trees and branches can be killed below -5.5ºC and • mature trees can be killed at temperatures below -.5ºC
Note: these are not precise numbers because the damage varies according to the specific temperature at ground level around the tree, the duration of the cold spell, the olive variety, the age of the tree, and whether the trees have had a chance to harden off.
Additional grove management practices discussed include: • mowing high grass adds 1ºC • bare soil allows sun to reach soil, which will store sunlight heat better than dry soil; keeping soil bare and moist can add 1ºC • water over trees can also assist, as the temperature of water adds heat and wet ice around fruit prevents fruit temperature falling below 0ºC.
Falivene also covered mechanical aids including frost fans, helicopters and burners, and gave the following advice on planning and monitoring: • identifying frost risk areas on your property - both before you plant and when considering mitigation measures - is a no-brainer, as is choice and positioning of varieties. Some varieties are less frost sensitive and early ripening varieties should be planted in higher risk areas: frost monitoring is also crucial, due to the need to identify affected fruit and harvest/process immediately to protect oil quality.
Examples were also provided examples of how to carry out an economic analysis of frost prevention aids in your grove.
Frost and EVOO quality
Modern Olives Laboratory Manager and sensory expert Claudia Guillaume also presented at the 2019 Conference on the Impact of Frost on EVOO Quality Parameters:
“Frost is one of the most important weather-related hazards for the Australian olive industry,” Guillaume said.
“Early frosts will normally affect the fruit, leading to significant changes in the chemical and organoleptic characteristics of the oils. Testing of quality chemical parameters in particular showed significant differences in oils produced from frost-damage fruit.
“Those chemical parameters were not significantly different in the oil produced from fruit immediately after being frosted, however sensory characteristics and the biophenols profile showed significant changes even with oils produced within a short time after the freezing event.
“Those changes became more evident with the oils produced at increasing timeframes from the moment of fruit freeze.”
Guillaume’s presentation provides an overview of the research
While flower damage is often hard to identify, the effects of heavy frost on fruit are very obvious.
Cultural practices to reduce frost damage 1. prune the canopy open 2. improve and maintain optimum leaf health 3. mow grass and control weeds under trees 4. manage and utilise irrigation on the coldest nights – water under and over trees 5. mulch all prunings and introduce biology by way of compost to improve soil biology
Frost-damaged fruit is not pretty – or usable!
conducted, which studied the quality parameters of at three varieties of fruit - Frantoio, Barnea and Picual – at four stages: before frost, immediately after a frost event, two weeks after a frost event and four weeks after a frost event.
The tests carried out included FFA, PV, UV coefficients, IND and Biophenols profile, along with sensory analysis.
Stand-out results include: • Picual: FFA levels almost trebled in oils produced four weeks after a frost event, while IND (induction time, an indicator of shelf life) decreased by seven months and PPH were a quarter of the original content; • Frantoio: FFA levels doubled in oils produced two weeks after a frost event and were outside of specifications in oils produced four weeks after a frost event, while IND decreased by nine months and
PPH decreased by three-quarters in oils produced four weeks after a frost event and PV was outside of standard specifications in oils produced two weeks after a frost event. • Barnea: FFA levels doubled in oil produced four weeks after a frost event, while IND decreased by seven months, PPH decreased to one-fifth of the original and PV was outside the standard specifications.
Most importantly, all oils registered significant sensory defects immediately after frost and those produced two weeks after a frost event were classified as Lampante (not fit for human consumption).
This is just a cherry-pick of the wealth of greater detail in both presentations. The slides and videos of both are available for viewing on the OliveBiz website - www.olivebiz.com.au. From the home page go to Events – Conference – 2019 National Conference & Trade Exhibition Presentation Videos and Powerpoints – Presentations Day 1.
Plant protection products
Another option in the grower toolbox are agricultural plant protective products. Formulated to increase a plant’s natural resistance by activating the plant’s own defence system, these products aid recovery from a stress event (including frost) and act as a key plant ‘protection’ mechanism.
They can be applied either prior to or following a stress event and contain nutritional elements for energy, photosynthesis and cell integrity.
They are also said to assist in drought conditions, as well as with pest and disease incursions.
Poor siting of this grove means the road embankment acts as a dam, holding back the cold air and leading to frost-damaged fruit in most years.
#Ref: ‘Organic Olive Production Manual’ by Paul Vossen, (Chapter 1: Site, varieties, and production systems), available for purchase through various on-line booksellers. #2Ref: www.oliveoilsource.com/page/frost-prevention.
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