Symptoms and management of Olive diseases and disorders

Barbara Hall and Dr Len Tesoriero  

  • A: South Australian Research & Development Institute, Plant Research Centre, Hartley Grove  Urrbrae  SA  5064
  • B: NSW department of Primary Industries, Elizabeth Macarther Agricultural Institute, Menangle NSW  2568.

Olives have been grown in Australia almost since first settlement.  However, the recent expansion of plantings in the last decade has bought olives into new areas, with new growers, and with a host of disease problems occurring, particularly in the younger establishing plantations.

Many different organisms have been recovered from these plants, however, only some are known to cause diseases of olives.  It is possible that these diseases have been around for many years, but are only becoming noticed because of the increased interest in olives as a commercial crop.   

Below are common symptoms of known diseases and disorders of olives in Australia, with some current suggested management strategies if known.

Disease:  Peacock spot (Spilocaea oleagina)

  • Also known as Olive leaf spot and Bird’s-eye spot, Peacock spot develops with high humidity and rain. This disease occurs only sporadically, particularly when wet weather occurs in spring.
  • Symptoms:  It first appears as small sooty blotches on the leaves that later become muddy green to black, often with a yellow halo. Often the leaves drop prematurely.
  • Management:
    • Prune to open the canopy for improved airflow
    • Reduce nitrogen use to prevent excessive canopy growth
    • Avoid excessive irrigation
    • Copper can be applied before the start of spring or autumn rains. Ensure good coverage of leaves.

Cercospora leaf mould (C. cladosporioides / Pseusocercospora cladosporioides)

  • Often occurs together with peacock spot.
  • Symptoms:   The first signs are grey blotches on the underside of the leaves, the top of the leaves will yellow, and defoliation occurs.
  • Management:  Can be managed the same way as peacock spot.
Photo courtesy Researchgate

Verticillium wilt (V. dahliae)

  • This is a soil borne fungus, which affects the roots and attacks the vascular tissue of the tree.
  • Symptoms:   Initially one or more branches will yellow and wilt, usually early in the growing season, however, the tree will eventually die.
    • Management:  No fungicides are registered to control this disease and once trees are planted it is difficult to manage.  Management is based on avoidance of the fungus and reducing inoculum level in the soil.
      • Use disease free planting material
      • Avoid planting into ground previously planted to alternative hosts of the fungus eg cotton, stone fruit, Brassica’s, potatoes and tomatoes.
      • Avoid inter-row cropping with susceptible plants, eg clover
      • Avoid soil movement from infected areas to non-infected areas
      • Reduce inoculum levels before replanting by keeping the soil weed free and growing resistant plants eg grasses for several years.  fumigation will also reduce inoculum.
Phytophthora Root Rot Olives photo courtesy Australis Plants

Phytophthora root rot (Phytophthora spp.)

  • 7 different species have so far been identified as causing problems with olives, usually where excessively wet soils, clay-panning or poor drainage was a problem.
  • Symptoms:   Causes root rots, stem and crown cankers. Leaves wilt, yellow and may drop. Trees may die suddenly or slowly decline over several years.
  • Management:
    • Avoid waterlogging and excessive irrigation
    • Avoid soil movement from infected areas to non-infected areas
    • Treat with Ridomil Gold (metalaxyl-M). Fungicides do not eliminate Phytophthora from the soil and treatment must continue.

Charcoal root rot (Macrophomina phaseolina)

  • Unlike Phytophthora, this fungus appears to like drier soil conditions, particularly where plants have been water-stressed during summer.
  • Symptoms:  Causes a root rot. Affected roots have typical black speckles on their surface.
  • Management:  Avoid water stressing plants

Bacterial stem cankers and dieback (Pseudomonas syringae, Xanthomonas campestris, Ralstonia solanacearum)

  • These bacteria were likely to have entered plants through pruning wounds or where frost/cold injury had caused stem tissue to crack or peel.
  • Symptoms:  vary from slow decline of trees and tree death to localised cankers around wound sites.
  • Management:
    • Avoid wounding trees, as this acts as an entry point for bacteria.
    • Copper can be used as a protectant but is not able to eradicate established infections.

Nematodes

  • Root knot nematode (Meloidogyne sp.), Citrus Nematode (Tylenchulus semipenetrans) and Root lesion Nematodes (Pratylenchus spp.) have been found affecting olives.
  • Symptoms:   Symptoms vary from unthriftiness to stunting and leaf yellowing. Root galling is found with root know nematode.
  • Management:  If nematodes are suspected, soil can be tested before planting to determine what nematodes are present and theirlevels. If levels are high, fumigate soil before planting or bare fallow or crop to resistant cover crops for as long as possible to reduce levels.

Anthracnose (Colletotrichum acutatum)

  • This disease needs wet conditions with high humidity. It affects fruit close to harvest.
  • Symptoms:   Causes soft circular rots on the fruit, usually on the shoulder, and at high humidity produces an orange slimy mass of spores on the fruit surface.
  • Management:
    • Prune to aerate the canopy
    • Copper can be applied as a protectant, and there is also a permit for Amistar, which should also be applied as a protectant.
Apical End dessication courtesy of DPI

Apical end desiccation

  • Also referred to as “soft nose”, this condition is apparently caused by sudden changes in temperature and humidity. It has also been associated with nutrient deficiencies such as Calcium and Boron.
  • Symptoms:   The apical end of the fruit shrivels, mostly seen near harvest. The internal flesh and pip may be blackened.  Sometimes secondary fungal rots infect the shrivelled end.
  • Management:
    • Ensure the nutrient status of the tree is adequate, particularly with regard to boron and calcium
    • Increase organic status of soil and encourage good root growth in young trees to maximise access to nutrients
    • Applying fungicides does not improve this condition Several other diseases are found on olives, however, these are generally minor and often only of significance to young or damaged plants.
Olive Knot courtesy of DPI

Olive Knot (Pseudomonas savastanoi)

  • This bacterial disease has been found in several properties in South Australia and 1 in Victoria, all on cv Barnea. All cultivars are susceptible, and damage can be severe when weather favours disease. Bacteria survive in the galls and are readily spread by water at all times of the year, infecting at wounds (eg leaf scars, pruning wounds).
  • Symptoms:   Rough galls or swellings about 0.5 to 2 inches in diameter occur on all parts of the plant and small shoots may be defoliated and killed.  Olive knot reduces productivity by destroying twigs and branches but does not kill trees.
  • Management:
    • Prevent wounding to reduce chances of infection
    • Apply copper at post harvest and spring
    • Avoid pruning or harvesting in wet weather
    • Avoid over fertilising
    • Remove alternate hosts
    • Pruning infected trees to remove the disease, burning the prunings
    • Practice good hygiene to minimise spread

 

Several other diseases are found on olives, however, these are generally minor and often only of significance to young or damaged plants:

  • Rhizoctonia root rot (Rhizoctonia spp.). Rhizoctonia has been consistently recovered from browned and rotted roots of young plants. Above ground symptoms include tip death, defoliation or death. While Rhizoctonia has been identified in roots of mature plants, it does not seem to cause a problem in otherwise healthy trees.
  • Stem cankers (Botryosphaeria sp.). This fungus is occasionally detected on branches of trees, resulting in yellowing of foliage above the affected area. In Western Australia, the same fungus has been detected on apple and stone fruit trees, which may be responsible for cross- infection of nearby olives. This fungus can also be a secondary coloniser of dead wood.
  • Minor root rots (Pythium spp., Fusarium spp.). These fungi are common in all soils, but are more prevalent in wet, poorly drained areas. They are not considered to be a major problem with mature trees, but will seriously affect young trees and those weakened by other stresses.
  • Fruit Rots (Botryosphaeria sp., Alternaria spp. & Coleophoma oleae). Usually occur on fruit already damaged by other causes, particularly in wet and humid weather. Avoid damage to fruit.
  • Crown Gall (Agrobacterium sp.). So far this has only been detected in potted nursery stock, but could be serious if it establishes in the field.

Acknowledgements

Much of this information was obtained through commercial diagnostic samples submitted to laboratories in NSW & SA. Many of the tests undertaken in NSW were funded by the Australian Government and the Southern Highlands Olive Growers Association through the RIRDC project (Principal Investigator, Robert Spooner-Hart, UWS, Richmond): Sustainable pest & disease management in Australian Olive production. We wish to thank the staff of the diagnostic laboratories for undertaking much of the laboratory work, Dr Michael Priest (Mycologist, OAI) for confirming fungal species identifications, Dr Ric Cother and Dorothy Noble (Bacteriologists, OAI) for bacterial identifications and Dr Andre Drenth (QDPI, Indooroopilly) for confirming species identifications for Phytophthora. Growers, horticulturists and crop consultants are acknowledged for the supply of diagnostic samples.

Useful references:

Books:

  • Ferguson, Louise, G. Steven Sibbett and George C. Martin. Olive Production Manual. University of California, Division of Agriculture and Natural Resources: Publication 3353. 1994
  • M. L. Flint, & M. Brush Pest Management Guidelines for Olives. University of California, Division of Agriculture and Natural Resources,
  • Manuel Civantes Lopez-Villalta. 1999. Olive Pest and Disease Management. IOOC

Web sites:

More about Dr Len Tesoriero

More about Verticillium Wilt of olive

 

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