Potatoes - Bacterial Wilt
Note Number: AG0314
Published: June 1995
Updated: April 2011
Bacterial wilt is one of the most destructive diseases of the potato, which has a very wide host range. On potato, the disease is also known as brown rot, southern wilt, sore eye or jammy eye.
Bacterial wilt of potato is generally favoured by temperatures between 25°C and 37°C. It usually does not cause problems in areas where mean soil temperature is below 15°C.
Under conditions of optimum temperature, infection is favoured by wetness of soil. However, once infection has occurred, severity of symptoms is increased with hot and dry conditions, which facilitate wilting.
Bacterial wilt is a serious problem in many developing countries in the tropical and sub-tropical zones of the world. It is usually found between the latitudes 45°N and 45°S. It has been recorded in all Australian states except Tasmania.
Bacterial wilt is responsible for causing considerable losses to the potato industry where the disease exists.
In the south east of Victoria, it has caused considerable losses in the past to the potatoes planted mainly in the swampy areas. However, the threat of the disease is potentially significant to the seed potato industry. Some importing states and countries regard bacterial wilt in the same light as black wart, ring rot, and potato cyst nematode (PCN) and ban imports from areas known to be infected.
The disease can cause total loss of a crop and prevent the use of land for potato production for several years.
Bacterial wilt is caused by a soil-borne bacterium named Ralstonia solanacearum (earlier known as Pseudomonas solanacearum). Based on the type of host plants it attacks it is divided into three races, and based on its biochemical properties it is divided into four biovars. The most widespread strain in Australia is race 3/biovar ll. This strain is known to occur in New South Wales, Queensland and it primarily attacks potato. Two other strains, which attack other hosts besides potato, are confined to the Northern Territory and Queensland.
Bacterial wilt attacks more than 200 species. These include economically important hosts such as tobacco, potato, tomato, eggplant, pepper, banana, peanut and beans. Thorn apple and nightshade are two common weed hosts that are attacked by the disease.
Typical symptoms are wilting, yellowing and some stunting of the plants, which finally die right back. Wilting is first seen as a drooping of the tip of some of the lower leaves similar to that caused by a temporary shortage of water. At first only one branch in a hill may show wilting. Affected leaves later become permanently wilted and roll upwards and inwards from the margins. The wilting then extends to leaves further up the stem and is followed by a yellowing of the leaves. This yellowing, wilting and in-rolling of the leaves makes diseased plants very obvious, especially when surrounded by healthy plants. The leaves finally turn brown and fall off, beginning at the base of the stem and continuing upwards.
Symptoms in the tuber are very specific: brownish-grey areas are seen on the outside, especially near the point of attachment of the stolon. Cut tubers may show pockets of white to brown pus or browning of the vascular tissue which, if left standing, may exude dirty white globules of bacteria. As the disease progresses bubbly globules of bacteria may exude through the eyes; soil will often adhere to the exuded bacteria, hence the name 'sore eyes' or 'jammy eyes'.
Potato wilt bacterium is a soil-borne organism primarily inhabiting the roots. It enters the root system at points of injury caused by farm implements, nematodes and by other means. It is spread by irrigation water, flood waters and contaminated soil.
The wilt bacterium is able to survive for periods up to 2-3 years in bare fallow soils, and for longer periods in soils cropped to non-solanaceous crops.
Infected seed is an important method of dissemination, both locally and over considerable distances. It is not the heavily infected tuber that is the problem since these generally rot away, only maintaining contamination of the land in which they were grown. However, slightly infected tubers, which show no visible symptoms, pose a serious threat of spreading the disease to new areas. Self-sown potatoes are extremely difficult to eradicate and, if a paddock is infected, the disease may remain in it for five or six years after the initial outbreak.
Bacteria can also be spread to clean tubers from an infected seed-cutter. There is also a real danger of infection if secondhand bags are used or if half tonne bins have held infected potatoes. Growers should be aware of these risks and take precautionary measures.
Bacterial wilt is difficult to control (or eradicate) because of the soil-borne nature of its causal organism. Therefore, following options should be considered in managing the disease.
Minimising the occurrence
- Adopt rotations with pastures, cereals and non-solanaceous crops for periods exceeding five years.
- Use of certified seed from reliable sources. Exclusion of the disease may be exercised by quarantine or other legislative measures. For example, Tasmania, which so far has not recorded bacterial wilt, is very careful to import only healthy seed. New Zealand and South Africa ban the importation of seed from areas known to have the disease. Other measures of control include:
- Planting in areas where bacterial wilt has not occurred previously.
- Control self-sown potatoes.
- Control weed hosts such as nightshade, thorn apple, Narrawa burr around dames, along channels and in the paddocks after cropping potatoes.
- Avoid deep ploughing – the organisms survive in the deep, cool layers of soil.
- Irrigation water should never be allowed to run freely over or below the soil surface. It should never be allowed to return to the dam or stream from which it is pumped, nor to any other irrigation source.
- Regular crop inspection for disease symptoms and remove and destroy diseased plants, tubers and immediate neighbours.
- Use stock to clean up chats, discarded tubers and crop debris, but do not allow the stock back onto clean paddocks.
- Do not return potato waste, e.g. oversized, misshapen and diseased tubers to paddocks.
Minimising the spread
- Machinery taken onto a diseased paddock should be left on the paddock while it is being worked.
- Machinery removed from the paddock should then be washed clean with a disinfectant solution in a dedicated area for equipment wash down.
- Use high-pressure wash to clean machinery, sheds etc to remove soil adhering to any surfaces.
- Clothing and boots of people working in the paddock should be exchanged for clean items when leaving the paddock, or else boots should be washed in a suitable disinfectant.
- After harvest, all diseased and discarded tubers should be collected and buried at least one metre underground.
- On no account should any of the produce from a diseased crop be kept as seed.
- Load and unload vehicles only in designated areas with sealed or hard ground or bare paddocks away from potato paddocks.
- Choose transport roots that minimise travel through potato paddocks and regions.
- If second-hand bags or half tonne bins have been used to hold potatoes, these should be thoroughly washed and disinfected before being used again. Bags should be disinfected or discarded.
- Ask visitors, contractors and workers to wear overalls, gumboots and overshoes on the property.
Compendium of Potato Diseases (2001) ed. by Stevenson, W.R. and et al, American Phytopathological Society, St. Paul, MN, USA.
Mulder, A. and Trukensteen, L. J. (2005) Potato diseases, 2508 AC Den Haag The Netherlands.
Potato Health management (2008) ed. by Johnson, D.A. American Phytopathological Society, St. Paul, MN, USA.
Contact/Services available from DPI
Correct diagnosis is essential for effective pest and disease control. A commercial diagnostic service is available at the DPI Knoxfield.
For further information, phone Crop Health Services on (03) 9210 9222 or fax (03) 9800 3521.
For further information on registered chemicals, phone DPI Customer Services Centre on 136 186.
This Agriculture Note was developed by Roger Osborn, DPI Victoria in June 1995.
It was reviewed by Dolf deBoer, BioSciences Research and Neville Fernando, Farm Services Victoria in April 2011.