Production / Processing

Producing Prunes

Factors affecting prune production:

  1. Rainfall
    • lack of water reduces fruit size and decreases yields
    • rainfall in summer, falling as storms leads to erosion
    • good surface drainage is required
    • poor drainage leads to root damage, disease and decreased production
  2. Irrigation
    • correct irrigation is essential for profitable yields of good sized fruit
    • adequate soil moisture is necessary for rapid fruit growth in the early part of the season and during fruit-swelling before harvest
    • an established orchard should be regularly monitored for moisture and soil salinity
  3. Temperatures
    • in France, home of the d'Agen prune the temperatures are cooler
    • in California the French Improved d'Agen was developed for warmer climates
    • high temperatures can cause blue drop
    • Young has more winter chilling than MIA or South Australia
    • warm and windy spring conditions mean a greater risk of reduced fruit set
  4. Chill hours
    • refers to the amount of cold required to break dormancy
    • not enough chill in prunes produces delayed or extended flowering
    • prunes require 800-1500 chill units (Young 1160, Griffith 1000, Loxton 860)
  5. Soil
    • prunes are able to use a wide range of soils
    • depending on the rootstock, prunes can fully utilise the top 1m of soil
    • deep soils improve tree stability, but reduce the tree's ability to handle water stress
    • a pH of 6 - 6.5 is preferable
    • Young has deep red earthy soils and brown sandy soils
    • in the MIA Griffith has deep sandy soils with clay subsoil, Darlington Point has a high clay content with slow water infiltration
    • the Riverland has high pH and sandy topsoils
    • irrigation and drainage are linked to soil structure
    • wind will effect fruit set, fruit quality , water use and tree growth

Planting prunes:

  1. Site selection is influenced by
    • soil type
    • aspect
    • soil drainage
    • air drainage
  2. Wind protection
    • wind breaks are important on exposed sites
    • wind can cause economic loss: more water, reduced pollination, reduced fruit size, reduced stability of young trees, increased russet and leaf tattering and increased pre-harvest fruit drop
  3. Incidence of frost
    • depends on geographic location and local topography
    • can be controlled by soil management: weed control, compacting soil, soil moisture
  4. Land preparation
    • clear
    • rip to a depth of 45cm
    • add lime or gypsum as required
    • grow green manure or cover crop prior to planting
  5. Layout and design
    • are influenced by soil fertility, soil, moisture availability, tree vigour, soil conservation needs and irrigation design and management
    • layout types are square, hexagonal and high density hedgerow designs.
    • north - south row orientation provides most even light
    • row orientation provides best protection from wind and frost
    • row spacing depends on the size of machinery to be used (5-6.6m)
    • tree spacing depends on harvester type, soil fertility and irrigation (280 trees per hectare)
  6. Planting material
    • order trees early and communicate your needs to the nursery
    • grafting your own rootstocks can save 70% of costs
    • young nursery trees should be transported in closed containers
    • poor health of planting material can increase establishment costs and decrease profits
  7. Planting tips
    • keep graft union a constant height above soil surface
    • minimize potential weed problems
    • prevent trees from drying out
    • in wet conditions, avoid soil compaction
    • dip trees in bacterial treatment prior to planting to prevent crown gall
    • remove broken or bent roots, avoid bending during planting
    • irrigate trees immediately after planting

Varieties:

Consider the following characteristics before selecting varieties:

  1. d'Agen
    • the industry standard
    • the dominant variety in NSW and California
    • fruit is medium sized, egg-shaped with a small pit
    • firm skin stands up well to processing
    • tree is vigorous and upright
    • tree tends to alternate cropping as it ages
    • flowers are self-fertile
  2. Moyer
    • produces large fruit
    • is more difficult to dehydrate and rehydrate
  3. van der Merwe
    • flowers early, heavy cropping
    • requires different pruning and drying techniques
  4. Sutter
    • early maturing
    • average brix 28, even brix throughout tree
    • pollinated by d'Agen

Rootstocks:

  1. Myrobalan
    • 29C is suited to dryland conditions
    • 29C is shallow rooted for the first 3 - 4 years
    • 29C crops well and is resistant to root diseases
  2. Marianna
    • is more tolerant of saline water
    • shallow rooted
    • produces slightly smaller tree
  3. Buck
    • grows vigorously on light sands
    • irrigation and regular pruning essential to avoid small fruit
    • is more susceptible to dieback in wet soils

Nutrition:

  1. Poor tree performance can be caused by:
    • lack of pruning
    • physical injury
    • deep root cultivation that causes root damage
    • poor soil drainage
    • pests or diseases
    • shallow soil or low moisture
  2. Indicators of crop fertilizer needs
    • annual shoot growth
    • leaf size and colour
    • crop load and fruit size
    • leaf and soil analysis
    • orchard history
  3. Soil pH
    • 6.0 to 6.5 is recommended
    • if necessary lime or dolomite can be incorporated into the soil to maintain pH
  4. Nitrogen
    • trees deficient in nitrogen have small pale green leaves and reduced shoot growth
    • young trees require 120 to 180g N/tree per annum
    • bearing trees require 500 to 600g N/tree per annum, dependent on crop load
    • nitrogen fertilizers should be applied when roots are actively growing in early to mid spring
    • nitrogen applied in late summer to autumn in the tree is stored by the tree for growth in the following season
  5. Phosphorus
    • phosphorus deficiency may cause poor root development, stunted growth and delayed maturity
    • phosphorus should be applied before planting (superphosphate 250kg/ha)
  6. Potassium
    • low levels of potassium may cause pale foliage, rolling of individual leaves and 'potassium leaf scorch'
    • sandy soils may require routine applications of muriate of potash, sulphate of potash or potassium nitrate
  7. Application of fertilizers
    • broadcast
    • fertigation
    • foliar sprays
  8. An integrated approach to plant nutrition
    • a soil analysis should be carried out before planting and regularly in bearing trees
    • leaf analysis enables fertilizer programs to be adapted to season and crop load, prune leaves should be sampled mid-January to mid-February

Irrigation:

For best production there needs to be optimum balance between water and air in the root zone. Soil moisture should be monitored regularly.

  1. Prune tree growth stages
    • dormancy - irrigation not required
    • pre-budburst - if winter has been dry, some irrigation may be required
    • budburst to tip-hardening - rapid cell division, adequate soil moisture required
    • tip-hardening and stage 2 - deficit irrigation strategies can be used
    • pre-harvest - water stress will reduce fruit size
    • post-harvest - soil moisture management is required to ensure next season's development
  2. Irrigation systems
    • flood and furrow
    • sprinklers
    • drip irrigation

Pruning:

  1. long
  2. medium
  3. delayed

Further information on pruning can be found in the Prune Manual

Blue Drop

Thining:

Fruit size can be regulated by fruit thinning.

Protecting the crop

Further information can be found in the Mite Management Manual or The Orchard Plant Protection Guide for Deciduous Fruit in NSW

  1. Prune diseases:
    • prune rust - a fungal disease that affects leaves, infection occurs when moisture and temperature conditions are favourable, control is reliant on protectant fungicides
    • brown rot - fruit is the main site of infection but can affect twigs and blossom, insects can be vectors of the disease, fungicide application at blossom will help in control of disease on fruit
    • bacterial canker - trunks, branches, shoots, buds, flowers, leaves and fruit can be affected, it is spread by rain, sprinklers,insects and pruning implements
    • shot hole - the fungus requires moisture for infection
    • crown gall - is caused by a bacterium, infections are initiated at wounds
  2. Viruses found in prunes:
    • prune necrotic ringspot virus
    • prune dwarf virus
    • chlorotic leafspot virus

    Split Fruit

  3. Insect and mite pests:
    • Apple leafhopper
    • Bryobia mite
    • Carpophilus beetle
    • Fosted scale
    • Fruit-tree borer
    • Light brown apple moth
    • Plague thrips
    • Prune mite or peach silver mite
    • Queensland fruit fly
    • San José scale
    • Two-spotted mite
    • Western flower thrips
  4. Prune disorders
    • pre-harvest drop - can be caused by high temperatures, poor soil fertility, poor irrigation, lack of pruning, failure to control pests
    • scab - caused by strong winds during the first 12 weeks of prune development
    • fruit cracking - caused when moisture stress is followed by irrigation or heavy rainfall
    • salinity - caused by saline irrigation water or the presence of a saline watertable in or just below the root zone