Bacterial heart rot and fruit collapse of pineapple

Bacterial heart rot of pineapple and pineapple fruit collapse are 2 diseases caused by a bacterium belonging to the Dickeya species complex. The term 'species complex' is used to describe a group of very closely related organisms that cause the same or very similar diseases. Dickey spp. are motile, gram-negative plant pathogenic bacteria.

In Queensland, strains of Dickeya bacteria that have been found infecting pineapples currently belong to the species Dickeya zeae. The relationships between Dickeya species are very complicated, and the taxonomy of this group has been revised numerous times, with pineapple infecting species of Dickeya previously called Erwinia chrysanthemi.

Strains of Dickeya zeae detected in pineapples from Queensland do not seem to be associated with the severe disease symptoms reported from other countries. It is unclear how closely related strains of Dickeya zeae from Australia are to overseas strains, or to other species of Dickeya causing disease in pineapples.

Other Dickeya species have also been reported to infect pineapples in other countries.

Fruit collapse

• Fruit symptoms become visible 2 to 3 weeks before ripening.
• Fruit skin changes colour from purple to olive-green.
• Copious amounts of liquid and bubbles of rotten-smelling gas produced by fermentation are released.
• When cut open, rotten cavities can be seen within the fruit.

Heart rot

• Usually the first visible symptoms are water-soaked lesions on the white basal part of leaves in the centre whorl.
• These lesions spread (as the plant grows) into the green part of the leaf, and turn olive-green to brown in colour.
• As infection progresses, the leaves bloat, filling with a rotten smelling gas formed as the leaf tissue is fermented by the bacteria.
• Infection also travels to the 'heart' or central stem of the plant, where tissues rot.
• Infection is often restricted to the top of the stem, and plants can form new suckers from the base.

Plant stage and plant parts affected

Both vegetative growth and flowering stages of young pineapple plants (4 to 8 months after planting) are at high risk for infection. The whole plant, including fruit can be affected.

Heart rot can be confused with fungal base (butt) rots and fruit collapse with fruit soft rot pathogens. A distinguishing feature of the diseases caused by Dickeya is bloating of leaves and production of bubbles of rotten smelling gas.

Bacterial heart rot and fruit collapse diseases of pineapple are present in Brazil, Costa Rica, Hawaii, Malaysia and the Philippines.

Strains of Dickeya zeae have recently been detected in Queensland and the Northern Territory. However, symptoms seen in Queensland pineapple crops have not been as severe as those reported overseas for bacterial heart rot and fruit collapse.

Pineapple (Ananas comosus) and possibly other crop and non-crop plants.

Dickeya zeae persists in host plant debris in soil and infects host plants at the site of an injury or wound.

Following entry into the plant, the bacteria multiplies, moving through the vascular system and causing damage to several parts of the host.

The rotten smelling liquid released from diseased plants or fruit provide a secondary source of infection for plants nearby.

Fruit infection occurs through the flowers when ants or other insects transfer the infected liquid or sap from infected plants to healthy plants. Fruit infected in this way are typically symptomless until 2 to 3–weeks before ripening.

Environmental factors like high humidity, warm temperatures (25–30°C) and rain significantly enhance the impact of this disease.

In Malaysia, heart rot disease development takes from 1 to 2 weeks after initial symptoms under optimum conditions.

After harvesting, remnant host debris in soils act as a source of inoculum for the next pineapple crop. Many Dickeya spp. can survive in soil, river and irrigation water without a host for several months. These bacteria can travel for long distances, and then spread the infection to new hosts.

Not known to harm human health.

Environmental

Dickeya spp. are reported to cause soft rot in a wide range of hosts.

Economic

Most of Australia's pineapple production occurs in Queensland. In 2015–16, pineapple production was valued at $39 million, producing around 47,500 tonnes of fresh fruit for domestic consumption and 28,000 tonnes of processed fruit.

Plant diseases caused by Dickeya spp. result in significant losses of many economic crops and ornamental plants around the world. Crop losses of up to 40% have been reported for bacterial heart rot and fruit collapse diseases of pineapple in Malaysia.

Social

A severe outbreak of the diseases could significantly affect the employment and livelihood of pineapple growers, and the sustainability of the Queensland pineapple industry.

Pineapples are also a common backyard plant in Queensland. Home gardeners would also be affected.

Plants infected with Dickeya zeae may have no symptoms of the disease until 2–3 weeks before pineapple ripening. Movement of infected plant material, including plant debris and planting material poses the greatest risk of long distance disease spread.

The bacteria can also be spread in infected sap/fruit liquid in wind and wind-blown rain, on insects, soil, irrigation water and on contaminated machinery.

Regularly inspect pineapple crops for symptoms of bacterial heart rot or fruit collapse.

Look for blister-like leaf lesions, soft rot in mature fruit, and production of bubbles of rotten smelling gas.

Report suspected bacterial heart rot or fruit collapse to Biosecurity Queensland immediately on 13 25 23 or contact the Exotic Plant Pest Hotline on 1800 084 881.

There is no cure for bacterial heart rot and fruit collapse of pineapple; infected plants are infected for life.

To protect your farm from Dickeya spp.:

• use certified disease-free stock to start new crops
• keep farm and plant propagation equipment clean
• avoid sharing machinery and equipment with other growers
• plant disease resistant varieties.

Protect your farm from plant pests and diseases:

• Protect your farm from emergency plant pests
• Understand good farm biosecurity
• Visit the farm biosecurity website

Bacterial heart rot and fruit collapse of pineapple caused by Dickeya spp. is prohibited matter under the Biosecurity Act 2014.

Report suspected bacterial heart rot and fruit collapse to Biosecurity Queensland immediately on 13 25 23 or contact the Exotic Plant Pest Hotline on 1800 084 881.

If you think you have found bacterial heart rot and fruit collapse, you must take all reasonable and practical steps under your control to minimise any associated risks. This is called a general biosecurity obligation (GBO).