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Agriculture is a form of primary industry that involves the cultivation of crops and pastures and the rearing

of animals to provide meat, milk, fibres and other products for humans. It was once said that Australia

‘rode on the sheep’s back’ due to the enormous economic success of our wool industry.

The importance of agriculture in Australia

Because Australia is in the unique position of being isolated from the rest of the world, Australian agriculture

is relatively free of many of the infectious diseases that affect animals and plants in other countries. This

makes our agricultural products highly sought after around the world. Stringent biosecurity measures are

in place to reduce the likelihood of disease transmission from pathogens. The introduction of new plant

diseases could potentially devastate the horticultural industry, as well as forestry and agriculture.

Australia’s main agricultural export products are listed in Table 10.6.

TABLE 10.6 Australia’s top 10 agricultural exports (by value) in 2015

MAJOR AGRICULTURAL EXPORT PRODUCTS

Beef

Wheat

Meat (excluding beef)

Wool and other animal hair

Alcoholic beverages

Vegetables

Live animals (excluding seafood)

Fruit and nuts

Sugars, molasses and honey

Barley

Total of all exports

VALUE (A$ m)

9 269

5 812

3 738

2 911

2 387

1 931

1 896

1 805

1 783

1 740

44 657

Source: Department of Foreign Affairs and Trade CC BY 3.0 AU license (https://creativecommons.org/licenses/by/3.0/au/)

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SHARE OF TOTAL (%)

19.9

12.5

8.0

6.2

5.1

4.1

4.0

3.9

3.8

3.7

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Infectious diseases in Australian agriculture

Two types of plant and animal diseases are of concern in agriculture in Australia:

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exotic (introduced) diseases such as foot and mouth disease, avian

influenza (H5N1), bovine tuberculosis, equine influenza, and virulent

Newcastle disease in domestic poultry and wild birds.

A complex interplay of three factors may contribute to the development

of infectious disease in organisms of agricultural importance (Fig. 10.42):

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host factors – susceptibility to disease, access to pathogen, concurrent

disease or poor nutrition leading to weakened immune response, drought

and heatwave stress on the host

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pathogen factors – the pathogen’s availability, its ability to transfer between

hosts, as well as virulence factors including adhesion and invasion of host

tissues, and successful establishment inside host tissues

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environmental factors – overcrowding and lack of hygiene leading to a

build-up of wastes, which provide a suitable environment for pathogen

reservoirs; a favourable environment within the host for pathogens to

establish and cause disease.

Host

endemic diseases (diseases consistently present within a country or region) such as bovine Johne’s

disease in cattle, sheep and goats, anthrax in sheep and cattle, and footrot in sheep

Pathogen

Infectious

disease

Environment

FIGURE 10.42 Factors affecting the incidence of

infectious disease in agriculture

Case study: Footrot in sheep

Footrot is an infectious disease of the hooves of sheep, goats and cattle, caused by the pathogenic

bacterium Dichelobacter nodosus. It causes painful abscesses between the toes (Fig. 10.43), lameness and

weight loss, as grazing is affected.

FIGURE 10.43

Typical appearance of

a sheep’s foot infected

with footrot

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An outbreak of footrot on a farm depends on a number of factors being present.

Pathogen factors – Dichelobacter nodosus must be present for footrot to occur.

Environmental factors – the bacterium will only survive in soil outside the host for a maximum of

4 days. Pastures that are long, dense and wet aid in pathogen survival and transfer. Temperature is

important: warm weather favours growth of the bacteria.

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Host factors – dry feet with intact tissues are not infected. Reservoirs of bacteria may form in individual

animals’ feet for years. Some dermatitis between the toes must already be present for bacteria to

invade and establish an infection. Overgrown hooves provide a suitable environment for the bacteria.

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Worksheet

The risk of

infectious disease

in agriculture

Factors contributing to the risk of infectious disease

Some of the factors that contribute to the increased risk of infectious disease in Australian agricultural

production are outlined below.

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Increased mobility of human populations

– Travellers, imported livestock and plants can carry infectious disease into Australia.

– Both cellular and non-cellular pathogens can form a reservoir in food, soil and seeds on shoes, and

in infected animals and animal products that are moved from one area to another by humans.

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Rise of intensive and industrial-type agriculture

– The increase in the world’s population has seen a change in the style of livestock production from

extensive pastoral systems to intensive feedlots. Feedlots carry a higher risk of disease outbreak

due to the higher stocking densities of animals. The closer animals are housed together, the more

rapidly a pathogen can spread from animal to animal or plant to plant.

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Changing patterns of land use

–

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Deforestation and irrigation practices may change the distribution of insects.

– Loss of habitat can bring bats into closer proximity to human and horse populations (Hendra and

Nipah viruses).

Climate change

– Distribution and abundance of insect vectors may change.

– Changes in rainfall patterns may favour the formation of reservoirs of pathogens in soil, plants

and insects.

– Changes in ecosystems can change availability of nutrients to plants and animals and reduce

immune responses to pathogens.

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Antimicrobial resistance

– Antimicrobials are used to treat infections in livestock (e.g. mastitis in dairy cattle, which is an

infection of the udder with environmental and faecal bacteria). The ‘off-label’ use (use in a way

that has not been officially approved) or overuse of antibiotics on farms hastens the development

of antimicrobial resistance due to rapid natural selection of resistant bacteria. Antibiotics are

also sprayed in orchards to treat plant infections. Antibiotic-resistant bacteria may be transferred

to humans through direct contact with animals, consumption of their meat or transfer of genes

between animal bacteria and human pathogens. If this happens, common bacterial infections

may no longer be able to be treated with antibiotics.

– Antimicrobials are used whenever animals are housed in high-density situations. The close

proximity of animals facilitates easier transfer of pathogens during a disease outbreak. In this

situation, animals are more exposed to their own waste products, which harbour bacteria. As well

as being used to prevent the spread of pathogens in intensive farming systems, antimicrobials such

as avoparcin and virginiamycin are used to promote growth in pigs, chickens and feedlot cattle.

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Pesticide resistance

– Insecticides, acaricides, herbicides and anthelmintics are chemicals used to manage

macroparasites and weeds on farms. Their overuse has led to the emergence of resistant forms of

parasites and weeds, making it harder to manage infectious diseases on farms.

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Loss of genetic diversity

– Genetic variation is necessary for a population to evolve in response to a disease threat. The

use of inbreeding in animals and plants, or monoculture practices in plants, can lead to reduced

resilience of a population to a new pathogenic threat.

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Increase in ‘hobby farmers’

– As Australian urban populations swell, many people seek an alternative life style or ‘tree change’.

People with little knowledge or experience of animal husbandry may be unaware of the risks of

certain practices.

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Increase in use of aquaculture as marine and freshwater animal populations decrease

– Aquaculture involves the farming of seafood. It is a growing export industry in Australia as the

world population increases and the need for a source of protein increases, but wild stocks decrease

due to overfishing, habitat loss and marine pollution. Species include fish such as salmon, tuna

and barramundi, oysters, abalone, crab, prawns and lobsters.

– Antimicrobials are used to therapeutically and prophylactically control the outbreak of bacterial

and fungal diseases due to the close proximity of organisms and the increased risk of cross-

infection. Common bacterial threats include Aeromonas spp., Pseudomonas spp. and Vibrio spp.

No antibiotics are currently registered for use in aquaculture but there may be pressure to use

antibiotics off-label.

Plant diseases of agricultural significance

A range of plants are grown commercially in Australia:

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grains – the seed of a cereal crop, such as wheat, corn and barley, for human and animal consumption

fruits and vegetables – for domestic consumption and export

fodder – food for livestock, such as oaten hay, alfalfa hay and silage

fibre – filaments or threads from plant material used for textiles, such as cotton

horticultural plants – plants cultivated for use in gardens and orchards

forestry plants – plants used in the creation and conservation of forests for human and environmental

benefits.

Australia is relatively free of most of the world’s most damaging plant pest species. However, there is

still an array of pathogens affecting Australian plants that are a threat, not only to the natural environment

but also to our agricultural industries.

Causes of infectious diseases in plants in agriculture

In the natural environment, plants are commonly attacked by pathogens. Plants have evolved unique

ways to keep these pathogens in check, such as the abscission (dropping) of infected fruits or leaves.

When plants are grown for horticultural or agricultural purposes, they are grown in higher densities,

and pathogens that were not considered a problem previously may become significant. If soil pH,

nutrient balance and water availability are not optimal, the stress caused to plants can reduce their

natural ability to inhibit pathogen invasion and growth. Many of these pathogens gain entry through

natural openings in the plants, such as stomates, or wounds caused by insect bites or other damage

due to hail, wind or pruning.

The majority of plant infectious diseases are caused by the types of pathogens described below.

Fungi

Fungi are by far the most common cause of plant disease. Terms such as

‘rust’, ‘smut’, ‘blight’ and ‘mildew’ are used to describe fungal diseases in

plants. Some have colourful names such as gummy stem blight and white

blister (Fig. 10.44). Reservoirs of fungal spores exist in contaminated seeds,

farm machinery, soil and nearby weeds, and are generally transmitted

by wind, water and contact with the reservoirs through normal farming

operations. Fungi enter plants through their stomata or any other opening

caused by mechanical damage to the plant, such as pruning and insect

bites. They damage the plant by destroying conducting tissues and

absorbing nutrients from the plants.

FIGURE 10.44 White blister disease looks like

large white blisters on the leaves.

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Insects and mites

Insects and mites not only cause direct damage to plant tissue, but may

also act as vectors for other pathogens. Examples are aphids, fruit fly, citrus

leaf miner and mealybugs. The citrus leaf miner is a moth that lays its eggs

under the leaf of the citrus plant. The larvae hatch and burrow through

leaves, leaving characteristic ‘mines’ (Fig. 10.45), and the leaves then twist

and curl. Young plants are most at risk as their growth can be severely

inhibited.

Bacteria

FIGURE 10.45 The characteristic tunnels made by

the citrus leaf miner

Reservoirs of pathogenic bacteria may occur in soil, weeds and seeds.

Humans can also harbour bacteria on their hands and equipment from

previous work with a contaminated crop of plants. However, bacteria only

multiply and spread when certain conditions are met. These include humid,

warm weather, overcrowding of plants, inappropriate soil conditions (water,

nutrients, pH and salinity) and poor air circulation. Examples of bacterial

diseases (Fig. 10.46) include black rot of brassicas, bacterial canker of

tomatoes and bacterial blight of peas. Pseudomonas spp. are particularly

common bacterial pathogens of plants as they are capable of tolerating a

wide range of conditions.

FIGURE 10.46 Bacterial disease causes the tissues of

the plant to rot, change colour and become slimy

Nematodes

Thousands of nematode species live in soil but only a few act as plant

pathogens. An example is the root knot nematode (Fig. 10.47), a pathogen

of agricultural significance, particularly for tomato growers. The nematode

attacks plant roots, creating galls and lumps. The plants subsequently wilt,

turn yellow and die. The eggs of these nematodes can persist in the soil

for a year and reinfect the next crop. The infestation can be dealt with by

repeated cultivation of the soil and exposure to the sun, combined with

removal of residual root material after harvesting to reduce reservoirs of

the eggs.

FIGURE 10.47 Nematode eggs are clearly visible

on these soybean roots.

Viruses

Plant viruses are obligate intracellular parasites and are less well understood

than animal viruses. The first to be discovered was the tobacco mosaic

virus, which infects tobacco plants and produces a mottling pattern on

the leaves. The tomato mosaic virus and the pepper mild mottle virus are

other examples (Fig. 10.48). All these viruses are stable in the environment

and can persist in plant material left over after cropping. They can also

form a reservoir on contaminated equipment. Increased plant densities

and frequent handling of plants by humans appear to play a role in its

transmission.

FIGURE 10.48 The mottled appearance of a

tomato leaf infected with tomato virus

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Phytoplasmas

Phytoplasmas are related to bacteria but do not have a cell wall. They are

transmitted from plant to plant by insect vectors, and inhabit phloem tissue.

They are pathogens of agricultural importance because they have been

reported in plants such as tomatoes, strawberries, grapes and pumpkins

(Fig. 10.49).

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Barrie Sheerman/Alamy Stock Photo

Nigel Cattlin/Alamy Stock Photo

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Case study: Panama disease of bananas

Bananas are the largest horticultural industry in Australia

and a best-selling product in our supermarkets. Most banana

plantations are in North Queensland, which produced 95% of

Australian bananas in 2014–15. In March 2015, the pathogen that

causes Panama disease of bananas was detected on a property that

grew Cavendish bananas in the Tully Valley, North Queensland.

Cavendish bananas are the main type grown in Australia. Panama

Tropical Race 4 disease is caused by the highly contagious fungus

Fusarium oxysporum. It causes yellowing and wilting of leaves and

splitting of stems (Fig. 10.50). The conducting tissues are damaged

and so the plant is starved of water and food. The disease is

spread through root-to-root contact and contaminated soil from

machinery and shoes.

Farming operations ceased and the affected property was sold

FIGURE 10.49 Symptoms of phytoplasma infection vary; they

include yellowing, stunting and ‘witches’ brooms (many small,

distorted shoots growing clumped together).

to the Australian Banana Growers Council, because the fungus

contaminates the soil permanently, and remains a biosecurity risk. No other plants can be grown

commercially on this property for this reason. The perimeter fences were reinforced, all banana plants

were destroyed and stabilising ground cover was established to prevent soil runoff into neighbouring

properties and waterways. Strict quarantine rules enforced by

Biosecurity Queensland have contained the outbreak to one farm

so far. During the outbreak, the price of bananas skyrocketed in

Australian supermarkets.

Abiotic factors that cause disease

At the start of this chapter, you read that disease arises from

an imbalance between the pathogenicity of the agent and the

defences of the host. Abiotic factors play a major role in setting

a plant up for invasion by a pathogen. These factors include any

major alterations in:

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temperature variation

light availability

chemical agents (natural and synthetic)

water quantity and quality

nutrient availability in soils.

As with infection of animals, for infectious diseases of plants to

exist, there must also be a chain of infection.

Effects of infectious diseases in plants

The effects of infectious disease in plants can be considered at three

levels:

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biological effects on the individual plant

social and economic effects on the farmer

social and economic effects on Australia’s economy.

These effects are discussed on the following page.

FIGURE 10.50 Stem of a banana plant with Panama disease.

This cutaway of the stem shows the most characteristic

disease symptom–the reddish-brown discolouration of the

water-conducting tissues

Weblink

Panama disease

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Biological effects of pathogens on individual plants

Some of the ways in which pathogens disrupt the normal operation of plant tissues are summarised in

Table 10.7. These symptoms often occur in combination.

TABLE 10.7 Effects of pathogens on plant tissue

SYMPTOM FEATURES

Death of

plant

Plants lose their ability to balance water

uptake with water loss through diseased

conducting tissue, or the ability to produce

food (photosynthesise) because of loss of

photosynthetic tissue.

SYMPTOM

Destruction

of tissues

(necrosis)

FEATURES

Cell death can be caused directly through

pathogen attachment and invasion of cells or

indirectly though the effects on photosynthetic

and conducting tissues.

FIGURE 10.51 Jarrah forest infected with

‘dieback’ fungus, Phytophthora cinnamomi

FIGURE 10.52 Spots of necrosis on the leaves

of a hazelnut tree, caused by the bacterium

Xanthomonas arboricola

Abnormal

growth

Normal plant growth is regulated by a series

of complex interactions between hormones

(trophic factors) and cells. Disease processes

may interfere with the production, distribution

and action of hormones. Tumour-like galls are a

common sign of infectious disease.

a

Discolouration of

tissues

Leaves may turn yellow (chlorosis), indicating

a problem with the production of chlorophyll.

Mosaic patterns are common with viral infections.

FIGURE 10.54 Peanut leaf with concentric ring

spots caused by peanut mottle virus

Wilting

b

Wilting happens when the plant loses more water

than it takes up from the soil. Causes include root

damage, and damage to or interference with

conducting tissues.

FIGURE 10.53 Gall wasp infestation in a citrus

tree: a wasp larvae emerging from a gall;

b wasps secrete growth-regulating chemicals

that cause abnormal swellings (galls) to develop.

FIGURE 10.55 Wilt in a tomato plant caused by

the bacterium Ralstonia solanacearum

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Michael Pez, Western Australian Department of

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