Fire blight

A single outbreak of fire blight in Michigan led to the death of 220 000 apple trees and estimated losses of USD 42 million in 2000. This is not a disease we want in South Africa. By Anna Mouton.

Fire blight is a bacterial disease caused by Erwinia amylovora. Most bacteria in the genus Erwinia are plant pathogens.

Fire blight is an example of a bacterial vascular wilt. Erwinia amylovora enters the vascular tissue of the plant — the xylem — and hinders the movement of water and nutrients. Lack of water leads to wilting and the death of affected plant parts. Erwinia amylovora also damages the walls of xylem vessels. This allows the bacteria to invade and lay waste to surrounding tissues.

Why does fire blight matter?

The fire-blight bacterium infects plants in the rose family, including pome and stone fruit, and ornamentals such as cotoneaster and firethorn. It has a financial impact on apple and pear production — fire blight is considered the main limiting factor for pear production in the eastern United States.

According to a 2003 publication by plant pathologist Jay Norelli and colleagues, the total annual cost of fire blight in the United States is thought to be more than USD 100 million.

An example of the impact of fire blight in Europe is the 1997 outbreak in the Po valley of northern Italy. Pears were the most-affected fruit type and approximately 500 000 trees were destroyed. Nearly two-thirds were 4–9-year-old trees. The scary thing is that the first case of fire blight in this region was only in 1994.

Erwinia amylovora does not occur in South Africa. If it were to establish here, it could prove devastating to apple and pear production. Fire blight is also regulated by many countries, including China, and its presence would have implications for market access.

How do I recognise this disease?

Fire blight kills flowers and new growth and may kill trees by girdling large branches and trunks. Infected tissues often ooze cloudy liquid. The ooze distinguishes fire blight from fungal infections that may have otherwise similar signs.

Infected blossoms appear water-soaked before shrivelling and dying. Leaves on the same spur or nearby stems blacken, shrivel, and die. Dead leaves tend to cling to stems. As wilting usually starts at the tip, affected stems are often hook-shaped.

Immature fruit can also become infected. The fruit appear water-soaked, then shrivel and die. Dead fruit tends to remain attached to the tree.

Fire blight causes cankers in larger branches. The bacteria can move downward through the vascular system and can eventually reach and kill the rootstock — this is called root blight.

M.29, M.26, and M.9 often develop root blight even though the scion appears normal. M.7 can contract blight when its suckers become infected. Root blight kills most infected trees within three years of infection.

Diagnosis of fire blight requires the laboratory culture and identification of Erwinia amylovora.

How does fire blight spread?

Erwinia amylovora does not reproduce in the environment or on the surface of healthy plants. However, it can survive on plant surfaces for a few weeks. The bacteria overwinter in cankers and are released in spring when the cankers become active and start to ooze.

Insects such as flies and ants love the sticky, sweet, bacteria-rich ooze — they help to spread the disease to blossoms. Ooze is also splashed about by rain and dried ooze is carried by the wind. Bacteria have been shown to survive in dry ooze for more than a year.

Fire-blight bacteria can multiply on the stigma — the part of the flower which receives pollen — and in the nectar. Infections of blossoms are promoted by temperatures of 18–30 °C accompanied by rain. Pollinators and other flower visitors keep the disease spreading.

Infections starting in flowers spread through the plant to the spur and then to other plant tissues. Bacteria can also invade immature shoots and young leaves through small wounds resulting from insect damage or bad weather. Outbreaks of fire blight are often associated with summer storms.

Several commonly grown ornamental plants, including cotoneaster and firethorn, can be a source of fire-blight bacteria that can potentially be transmitted to apple and pear trees.

Where did it come from?

Erwinia amylovora seems to have evolved on native American plants. Fire blight was first observed on apples and pears in the northeastern United States in the eighteenth century. It took another hundred years for scientists to figure out that it was caused by bacteria — in those days no one knew that bacteria could cause plant diseases.

Fire blight made its way to New Zealand in 1919. The source is believed to have been infected nursery trees. The disease is still present in New Zealand but has so far not established in Australia.

The first outbreak of fire blight in England was reported on pears in 1958. The bacteria were probably introduced with infected plant material or on contaminated fruit boxes. By 1966 the disease had spread to continental Europe. Fire blight usually initially established on pears and then spread to other hosts.

At present, fire blight is entrenched throughout North America and much of Europe. It is widespread in Israel. In Africa, it is present in Algeria, Egypt, Morocco, and Tunisia. Erwinia amylovora has not yet managed to successfully invade China, Japan, India, Taiwan, or South Africa, and is not known to occur in South America.

Control of fire blight

The only truly effective way to control fire blight is to stop it from entering a country in the first place. Control measures have not proved very effective at preventing establishment once the bacteria reach a new region. Strict phytosanitary measures for the importation of plant material — especially for propagation — from affected countries are essential.

Cultural practices help to control the disease in areas where fire blight occurs. These include reducing tree vigour and chemically controlling shoot growth. Affected parts of trees should be pruned out as soon as possible.

Attempts to develop resistant cultivars are ongoing. Some of the Geneva rootstocks have been selected for fire-blight resistance. Other potentially tolerant rootstocks include the B.9 — B for Budagovsky — rootstock and certain Japanese rootstocks.

Disease-resistance studies have been conducted by various research groups in different countries. Results have been variable — tree and environmental conditions play a role in the disease-resistance rating assigned to cultivars and rootstocks.

Chemical control relies on either copper-containing compounds or antibiotics. Streptomycin is the most commonly used antibiotic for fire blight. Warning systems are available for certain countries and can help inform spray applications by identifying risk periods.

Unsurprisingly, Erwinia amylovora has evolved resistance to streptomycin in several countries. Antibiotics are also subject to the same increasing regulatory pressures as other agrochemicals — so chemical control of fire blight is likely to become more challenging still.

Promising results have been obtained in trials with biocontrol agents for fire blight, but consistent control has proven challenging. Biocontrol is therefore not widely practised.