High Priority Exotic Pests and Diseases of Olives in Australia

High Priority Exotic Pests and Diseases of Olives in Australia

Have you seen symptoms of the following high priority exotic olive pests & diseases of olives in your grove?

The Biosecurity Plan for the Olive Industry identifies the following high priority exotic pests and diseases of olives in Australia – are you able to recognise these?

Fortunately not yet in Australia, Olive Quick Decline Syndrome (OQDS) is a destructive wasting disease of olive trees caused by the bacterium Xylella fastidiosa.

Symptoms include leaf scorch and desiccation of twigs and branches, beginning at the upper part of the crown and then moving to the rest of the tree, which acquires a burned look.  There is no known practical treatment once a plant has been infected and it will eventually die.

The bacterium is transmitted between plants via insect vectors which feed on plant sap (such as the meadow froghopper). Spread of the disease over longer distances occurs when Xylella-infected plants are moved in trade.

Xlyella is currently spreading from the Puglia region of Italy to other Mediterranean olive production regions. The disease has also been detected in olive crops in California, Argentina and Brazil.

Xylella is also known to naturally infect many commercial, ornamental and native plants from more than 300 species, with this number ever increasing.


The National Xylella Action Plan (2019-2029)

Xylella fastidiosa is one of the most significant emerging plant disease threats worldwide, and we are fortunate that it is not yet present in Australia. The vision is that Australia remains free of bacterial pathogens of the Xylella genus and is prepared to respond should the need arise. Xylella and exotic vectors have been identified as Australia’s top National Priority Plant Pest in recognition of the potential to severely impact Australia’s plant industries and environment. Managing Xylella-caused disease is economically costly. An economic modelling study conducted by the Australian Bureau of Agricultural and Resource Economics and Sciences (ABARES 2017) estimated, that if established, the potential cost of Xylella fastidiosa to Australia’s grape and wine sector alone could range from $2.8 billion to $7.9 billion over 50 years in adjustment costs and foregone gross margins. Xylella has complex epidemiology with broad genetic diversity that enables it to colonise numerous host species, and when transmitted by common xylem sap-feeding insects, it has potential to spread vast distances.

The latest research on Xylella is reported at: Conference on Xylella fastidiosa October 2019.

Public awareness video (noting that the host range has taken off this this was produced and is now at 595 species) – https://www.science.org.au/curious/video/deadly-bacteria-threatening-plants

Click on the above links and read on

Olive fruit fly is one of the most serious economic pests of olives, but fortunately not yet in Australia.  It doesn’t harm the tree but causes considerable damage to fruit quantity and quality.

Apart from eating the olive fruit pulp, the larvae emerges to leave a hole that allows fungi to enter causing adverse changes to chemical and taste profiles of the olive making olive products inedible.

At the end of autumn both adults and larvae will be found on the olive tree.  The last larvae spend the winter as pupae in the soil. In cold winters adults and pupae will die, but in milder winters both will survive to the following spring.

The adult will then feed on nectar from blossoms and fruits of other plants, predisposing areas with lots of vegetation and fruits to the appearance of the Olive Fruit Fly.

Where summers are dry and hot >35C, the fruit fly activity and survival rate plummets (as the eggs are not viable).  So the microclimatic conditions of the grove will help to explain the year to year incidence of olive fruit fly.

In areas along the coast and mountains where there are lots of rivers, fruit trees and shade, the fly will survive the summer very well.

The female selects fruit that are not water stressed where the eggs will have the best chance of survival.  The number of holes (stings) in the fruit is a measure of fruit fly population, although not all stings will contain an egg.  After the first generation of fly has emerged, the amount of fruit stings will increase exponentially.

Olive Fruit Fly prefer table olive varieties – the thickest, sweetest and least bitter fruit.  This also applies with low to moderate crop loads where fruit will also be thicker and juicier.

Not yet in Australia, Olive moth feeds only on olives, so in endemic regions it is present in one form or another all year round.

The first (phytophagous) generation eats buds and leaves acting as a leaf miner, which can be a problem in young trees as it harms the main sprouts.

The second generation eats blossoms and affects fruit set, but because of abundant blossoms on olive this is rarely a problem.

The moth also lays eggs in the calyx of recently set fruit, and the grub penetrates the pit, spending the summer feeding on the kernel, then in September metamorphosis into an adult.

Rarely does the moth affect a high proportion of the olive crop, and the remaining fruit are able to grow more to compensate.

Control is usually chemical or by natural predators.  In hot summers with water stress the infested fruit larvae die and the affected fruit fall off.

The adult Olive bark beetle spends the summer making food galleys in the axils of small sprouts, leaves and olives causing these to dry up.  In windy conditions these weakened structures often break off, reducing production for the current and subsequent years.

Adults lay eggs in spring in damaged sap wood such as in frosted trees, and in prunings on the ground.

Adults emerge from this wood at the start of summer to start a new cycle of damage.  As adults don’t fly well the damage is restricted close to where prunings remain.

Control is by proper removal and disposal of prunings and olive tree wood.  If chemical control (pyrethoids) is used it is best when the fruit has set which is the time the adults leave the wood.

Note: fallen small fruits can be confused with Boron deficiency, and distorted shoots with Zinc deficiency.

Olive Brown Spot (Fruit rot, twig and branch dieback): (Camarosporium dalmaticum), is also described as Sphaeropsis dalmatica – Macrophoma dalmatica – Phyllosticta dalmatica – Phoma dalmatica – Phomopsis dalmatica – Camarosporium dalmaticum – Botryosphaeria dothidea – Fusicoccum aesculi

Note: Olive Brown Spot is associated with Bactrocera oleae activity, possibly with disbursal by an egg parasite: https://www.ncbi.nlm.nih.gov/pubmed/17763035

Note: The Biosecurity plan for the olive industry (Version 2.0 October 2016), is scheduled for revision in 2021/22, to take account of any new pests and diseases of olives, including the following fungal pathogens found in Australia over recent years, but not yet recorded on olives:

The following pests and diseases are now reported in Australia but not yet on olives:  Have you seen these symptoms in your grove?

Field symptoms of the defoliating Verticillium wilt syndrome in ‘Arbequina’ olives: A, a 2.5-year-old tree completely defoliated; B, defoliation of twigs and branches in sectorial pattern; C, extensive defoliation of 4-year-old tree (note abundant fallen green leaves on soil under the tree canopy); and D, dark brown xylem discoloration in transverse section of a trunk from a Verticillium wilt–affected olive tree.

The Verticillium Wilt fungus affects the vascular system starting in the soil from spores close to a root.

Note: a whole branch is usually affected (indicating the infection is moving up the tree xylem from one or more of the infected roots. This wood should be removed and burnt

The fungus infests the tree from the roots to the leaves, and dead leaves fall to the ground containing spores to continue the cycle of infection.

Damp clay soils with high temperature are especially favourable for infection, such as in irrigated groves.  Soils high in organic matter are resistant to Verticillium due to the presence of antagonistic microorganisms

There is no way to eradicate the infection from a tree, although the tree can be nursed along with reduced watering and nitrate fertilizers, pruning and balanced nutrition.

Plant resistant olive varieties and avoid planting olives in soil that has grown susceptible plants – vegetables and especially cotton, and don’t over water trees.

Ref: https://apsjournals.apsnet.org/doi/pdfplus/10.1094/PDIS-06-11-0496

Ref: https://imafungus.biomedcentral.com/articles/10.5598/imafungus.2018.09.02.07

About the pathogen

  • Cytospora species have been reported as destructive plant pathogens causing canker diseases, and as saprobes that colonise the wood of dead or dying trees.
  • Cytospora species have traditionally been considered as secondary pathogens, invading stressed and wounded trees.
    Canker diseases in general can cause death of young trees, but canker diseases increase in prevalence as orchards get older which can impact on yield, the life span of trees and production costs.
  • Cytospora canker, caused by Cytospora sorbicola, is reported to occur in in Russia and the USA.
  • Species-level identification has been difficult based on morphology alone. Species identification based on multi-region sequence data has only recently been published. Therefore, the impact of this particular species may be difficult to quantify.

Cytospora canker found in Australia

  • Cytospora canker, caused by Cytospora sorbicola, was detected on an isolated commercial property in Nannup, Western Australia, in November 2019. Samples were taken from two plum trees exhibiting dieback during a routine industry survey of stone fruit orchards.
  • Cytospora species are plant pathogens which cause dieback and canker diseases on a wide range of hosts including fruit and nut trees and plantation timbers.
  • The WA Department of Primary Industries and Regional Development (DPIRD) sampled plums, nectarines, peaches and apricots on the property. Cytospora canker was not detected in any other stone fruit sampled.
  • DPIRD reported that the two infected plum trees were alive with healthy growth in the lower regions. New leader branches were unaffected, indicating the affected plants have not been significantly impacted.
  • Dead leader branches will be removed from the two trees and destroyed to minimise any further release of spores from infected material. The grower has increased biosecurity measures with guidance on general management information for canker diseases of fruit trees.
  • Surveillance of the property will continue. There are no neighbouring properties growing horticultural crops, and there is minimal movement of orchard machinery and equipment, or plant material (except fruit) from the property.
  • Fruit is not considered a pathway for Cytospora canker based on its primary association with trunk and stem diseases.
  • The plum trees were purchased in July 2018 from a Victorian nursery which was surveyed and no evidence of the disease presence was found.
  • This is believed to be a first record of Cytospora sorbicola in Australia, noting the Cytospora genus has recently been revised in 2017–18 with several new species described.
  • The detection of Cytospora sorbicola is being considered by the Consultative Committee for Emergency Plant Pests (CCEPP) to determine if it is an Emergency Plant Pest.
  • The CCEPP is determining capacity to investigate possible misidentifications of historical specimens of leucostoma, Cytospora spp. and Diaporthe spp. This will to help to determine the current distribution of Cytospora canker in Australia.

Biosecurity and reporting

  • Growers should have in place on-farm biosecurity measures to protect their crops from pests and diseases. More information is available at com.au.

Everyone has an obligation to report unusual plant pests and diseases.  You can do this by phoning the national Exotic Plant Pest Hotline on 1800 084 881.  This will put you in contact with your state or territory department of agriculture or primary industries.

Disease symptoms observed on olive trees. (a) Wilt and die-back. (b–d) Sub-cortical browning as longitudinal streaking from young (12 years old) and adult (26 years old) trees. (e) Cankers and brown streaking on branches. White arrow, region from which Pl. richardsiae was frequently isolated. (f, g) Microscopic features of Pl. richardsiae . (f) Conidi- ophores and conidiogenous cells. (g) Brown, globose, and hyaline, cylindrical conidia. Scale bar in f, g = 10 μm. 

Neofusicoccum parvum is associated with general olive decline symptoms similar to Verticillium Wilt observed on olives in Puglia Italy.  It is also recorded on a broad range of hosts in Australia including Olea africana in NSW (collected 2013)

The symptoms comprised a general decline of the trees beginning with foliar browning and leaf drop, wilting of apical shoots, die-back of twigs and branches, and brown streaking under the bark of the trunk, branches and twigs. In more advanced stages of the disease necroses and cankers were observed on the bark.