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Bacterial wilt of potatoes

Farmnote 84/88 [Replaces Farmnote 40/82]

[Reviewed July 2005]

By Rob Floyd, Plant Pathologist, Division of Plant Industry and Neil Delroy, Research Officer, Division of Horticulture

Bacterial wilt of potatoes (Pseudomonas solanacearum) is a destructive disease most active during the summer months with high temperatures and abundant moisture.

The disease can attack potatoes at all stages of growth and will remain infective indefinitely. The disease is easily spread, but difficult to control. The method of handling infected crops will largely determine the likelihood of further spread of the disease.

Bacterial wilt is important to the potato industry and any suspect crops must be reported to Agriculture Western Australia.

Symptoms of the disease

Field symptoms are usually similar to those of other wilt diseases, including black leg, and to insect damage. Also, wilting is usually only seen during high temperatures and may be confused with localised water stress.

Check on potato plans regularly to determine the cause of any suspicious wilting before bacterial wilt is confirmed.

Plant symptoms

Wilting of single or isolated plants during hot weather.
Brown colouration of the inside of stems and roots.
Breakdown of tissues and development of a creamy-white bacterial slime which can be squeezed out of the tissue.

Tuber symptoms

Weeping of slime from the eyes of the tuber, commonly known as ‘sore eye’. This can be disguised by a plug of soil when the tuber is first dug.
Discolouration of a ring of tissue about 4 to 5 mm below the skin.
Slime developed in discoloured tissue which can be squeezed out as beads of pus.

Photo: Internal symptoms include darkening of the vascular ring
Internal symptoms include darkening of the vascular ring. Creamy white pus can be forced from the discoloured tissue when the tuber is squeezed

Photo: External symptoms on the tuber
External symptoms on the tuber first show as 'weeping or sore eyes' (arrowed)

Action

As the disease is regarded as important to the potato industry and the consequences to the individual are so severe, it is essential to report immediately any suspicious plants or tubers to Agriculture Western Australia.

In most cases the symptoms are caused by another problem. Prompt reporting of symptoms can allow the condition to be identified and treated. If, however, bacterial wilt is confirmed immediate control measures must be taken to contain the disease.

Control on your property

If bacterial wilt is confirmed on your property the following measures must be taken.

Restrict access to the paddock to prevent spread from the affected crop on machinery or by stock.
Harvest the crop as soon as practical to reduce development of the bacteria.
Hold the harvested tubers in a safe shed for 14 days or as instructed to allow development of the disease before attempting regrading and sale.
Use secondhand bags in the field where harvesting into bags is needed.
Where sale of the graded tubers is considered to be practical and safe, then specially marked bags which can be destroyed after use may be needed.
Sterilise, with an approved material, all bins, machinery and handling areas, including sheds, which have come into contact with the affected crop. Where soil adheres to any surface it must be removed by washing and any clods broken down to allow the sterilant to reach all of the material. All wash water must be collected into a safe sump to prevent it contaminating new areas. A wash with hypochlorite solution should be followed with a water wash to reduce corrosion of metal.
Quarantine affected paddocks to control the disease and growth of all potatoes and related crop and weed plants, such as tomato. Nightshade must be controlled to prevent carryover of the disease in the field.

General prevention

Bacterial wilt is most commonly transferred from property to property by the seed. As far as can be determined, all local outbreaks have been caused by use of diseased seed.

While all care is taken during seed crop inspection, to detect the presence of bacterial wilt, the inspections are often during the cooler months when the disease is least likely to be seen in the field. No guarantee can be given that even with approved seed there will be no risk of contamination; however, the risks are less than when table stock is planted. Check all seed before planting for the presence of any suspicious symptoms.

Cutting the seed increases the spread of bacterial wilt from infected tubers. Frequent sterilisation of cutting knives in 2 per cent sodium hypochlorite solution is a worthwhile precaution during cutting.

When borrowing any potato machinery, thoroughly clean it down and sterilise it with a hypochlorite solution for control of bacterial wilt and other soil borne diseases.

Sterilisation

The recommended material for sterilisation of equipment and machinery is a solution of sodium hypochlorite. The commercial product contains 12.5 per cent available chlorine and is diluted by adding 1 L of product to 12 L of water for a 1 per cent solution (or 8 L in 100 L water). As the chlorine is lost when in contact with soil, very dirty machinery may need a second wash with the solution to remove softened clods.

Other chemicals such as methyl bromide may be used to fumigate sheds and machinery, but this can only be carried out by a licensed operator.

Summary of legislation

Several Acts cover the effect of bacterial wilt and these are designed to protect your industry. Some points from these Acts are quoted below.

"Every occupier of any orchard in which any disease appears shall, within 24 hours after first discovering or becoming aware of its presence, give written notice thereof to an inspector or to the Chief Executive Officer, Agriculture Western Australia."

For the purpose of the Act this includes:

"Any land used for the purpose of growing or cultivating plants and includes any garden, farm, vinery, vineyard and hothouse and any place where any plant is cultivated or where any plant, which has been cultivated is growing."

Imported goods

Apart from a number of general disease prevention requirements applicable to all plant material imports there are specific requirements related to bacterial wilt and potato spindle tuber viroid. These are as follows:

Potato machinery, equipment and containers - bacterial wilt and spindle tuber viroid

To be certified as:

originating from a property which has been free from bacterial wilt and potato spindle tuber viroid for the preceding five years and has not been used on any other property during the preceding three years; and
having been steam cleaned under the supervision of an officer of Agriculture Western Australia in the originating State.

Seed importation is also covered by similar regulations to protect the local industry from infection from other States.

Potatoes - potato spindle tuber viroid, golden nematode (Globodera rostochiensis), bacterial wilt or brown rot, Irish blight (Phytophthora infestans)

To be certified that:

they were grown under an approved pathogen testing scheme and are contained in sealed new bags or packages bearing legible details of the State or Territory of origin and the relevant certification scheme; or
1. they were grown in a district where potato spindle tuber viroid and golden nematode are not known to occur; and
2. they were grown on a property situated at least 20 km from a known outbreak of the disease bacterial wilt or brown rot detected within the last five years; and
3. they were not harvested, cleaned, washed, graded or packed with equipment or in premises with or in which potatoes, grown within 20 km of a known outbreak of the disease bacterial wilt or brown rot detected within the last five years, have been handled. Alternative conditions for premises may be approved by the Chief Executive Officer; and
4. bulk bins have been cleaned of soil and plant material and treated immediately before filling by thoroughly spraying with a 1 per cent solution of formaldehyde.
further to the best of the inspector's knowledge they are free from: Pseudomonas solanacearum (bacterial wilt or brown rot) and Phytophthora infestans (Irish blight).