INTEGRATED MANAGEMENT OF POTATO bacterial wilt
Dr. Sylvie Priou and MSc. Pedro Aley
International Potato Center (CIP), Apartado 1558, Lima 12, Peru.

THE PATHOGEN

Ralstonia solanacearum, previously called Pseudomonas solanacearum, is the causal agent of the disease known as potato brown rot or bacterial wilt (BW). The bacterium affects over 200 species, especially tropical and subtropical crops families, the most susceptible crops being potato, tomato, tobacco, eggplant, pepper, banana and groundnut. Race 3 (Biovar 2A) strains, that affect mainly potato and occasionally tomato and other solanaceous crops and weeds, are the most common in higher elevations of the tropics (up to 3,400 m. a. s. l.). At lower elevations in the tropics, Race 1 strains are prevalent and affect a wide range of crops and weeds.

SYMPTOMS

• Foliage symptoms: Unilateral or total wilting of potato

Above ground symptoms of BW include wilting (similar to lack of water), stunting and yellowing of the foliage. The browning of vascular bundles may be seen when the cortex is peeled. Characteristic too, is the initial wilting of only part of the stems of a plant, or even one side of a leaf or stem. If disease development is rapid, the entire plant wilts quickly, without yellowing. Alternatively, the diseased stem can wilt completely and dry up, while the remainder of the plant appears healthy.

• Tuber symptoms: Bacterial exudate in tuber eyes

External symptoms on the tuber are visible at harvest when infection is severe. Bacterial ooze collects at tuber eyes causing soil to adhere to the secretions.

Field diagnosis:The vascular flow test

Diagnosis in the field can be easily accomplished by cutting a piece of stem 2-3 cm long from the base, and suspending it in clear water in a glass container. The cut stem can be held with an opened paper clip to maintain a vertical position. Within a few minutes, the smoke-like milky threads exude from the cut stem (Figure 1).

           

Figure 1

Bacterial exudate from vascular ring

Tuber symptom is often described as brown rot. Cut tubers show a pus-like slime coming out of the vascular ring with a slight squeezing or it may exude naturally (Figure 2). In advanced stages of the infection, tubers exhibit brownish discoloration of the vascular ring,

     

Figure 2

DISSEMINATION

Infected seed tubers are the main means of dissemination of R. solanacearum (particularly for race 3 strains). Disease development depends on temperature and humidity, temperatures above 25º C favor bacterial muliplication and thus wilt development. In cool conditions, such as tropical elevations above 2500 m, infected but symptomless plants may harbor the bacterium and then transmit the pathogen to progeny tubers as latent infection, leading to severe disease outbreaks when grown at warmer locations. The pathogen can survive in soil (mostly on plant debris) and in the rooting system and rhizosphere of many hosts (weeds, other host crops, potato volunteers). The pickaxes, hoes and ploughs we use at planting, hilling and harvesting can also carry the disease to healthy fields, because they are in contact with the infested soil. Also infested soil adhered to farmers’ feet or animal hooves also contribute to disease dissemination. Bacterial entrance into potato roots is facilitated by wounds made by tools during post-emergence cultivation, and by nematodes and insects in the soil.

Bacterial wilt can also be carried in the sacks where we put infected tubers. When we put healthy seed tubers in those sacks, they become infected.

When surface runoff water after rainfall and irrigation water passes through a diseased field, they can spread the disease to a healthy field.

CONTROL

Integrated management

There is no effective chemical control, thus BW incidence can be reduced only if various control components are combined. An integrated disease management (IDM) approach can lead to significant reduction, or even eradication of BW.

Preventive measures

Use of BW-free seed is the most important component of BW IDM. In seed certification schemes, no bacterial wilt must be tolerated during the growing season. For seed production, only BW-free seed tubers originating from disease-free areas must be used, and where no apparently healthy tubers from lower BW-endemic areas are planted, preferably certified seed or from a seed company or known and trusted seed-grower association.

The original planting material used in clonal (pre-basic and basic seed tubers) or positive selection multiplication scheme must be tested for latent infection with R. solanacearum (See below). In endemic areas where healthy seed tubers are hardly available, an alternative to planting seed tubers is the use of true potato seeds (TPS) or clean cuttings obtained under controlled growing conditions from micropropagation.

Potato for seed production must be planted in a BW-free land, i.e. where no BW was found in previous potato crops or where potato has not been planted for many years, or in land recently put into production.

To prevent further contamination of the plot the application of various sanitation and cultivation practices are necessary:

• We must work first in BW-free fields.
• After having worked in an infested field, prior to beginning work in the BW-free plot, tools must be disinfected. They can be washed at least with water at home or in buckets in the field (not in irrigation channels) and if possible washed with water with a 0.1% solution of sodium hypochlorite¹ (bleach).
• Since the BW pathogen can disseminate in soil adhering shoes or animal hooves, before entering the plot soil must be removed from those o washed with water. Alternatively farmers can step on quicklime or stove ashes placed on a plastic bag or a ditch (10 cm-deep) dug at the entry to the plot.
• The bags and crates used to take the seed and the cloth wrapper used in planting must be washed with detergent.
• Water is another element that must be taken into account in prevention, keeping irrigation or surface runoff water from rainfall from diseased plots from entering the healthy plot. This can be avoided by building drainage ditches.
• Tuber collection after harvesting is also a useful practice to prevent and avoid many animals, mainly pigs and horses or donkeys, from entering the plots and feeding on the tubers.
• Fencing the plot is an alternative (though a costly one) to prevent animals from entering.

Detection of latent infection in seed tubers for the prevention and control of BW

The classical method of detecting tuber infection consists of incubating tubers for 3 to 4 weeks at around 30°C and observing oozing from the eyes or stolon ends, or cutting the tubers to observe oozing from the vascular ring. However, this method is time and space consuming and may not reveal low infection rates or rots due to other pathogens may develop. Because of this, CIP developed a simple, sensitive, quick and low-cost technique to detect latent infection in tubers that is suitable for seed testing: post-enrichment NCM-ELISA (Enzyme linked immunosorbent assay on nitrocellulose membrane). The kit is being distributed to seed programs worldwide for seed quality testing, seed certification or research (varietal evaluation for resistance to BW, development of control components, for studies on disease epidemiology e.g., survival and spread of the pathogen).

What must be done in a plot with BW

Sanitising and cultural practices are intended to prevent or control survival and dissemination of the pathogen. The same measures are normally used to control other potato diseases and pests.

Wilted potato plants must be uprooted from the field as soon as they appear, to prevent them from transmitting BW to surrounding plants. BW-diseased plants must be uprooted with the surrounding tubers and soil, taking them out of the plot in a plastic or burlap sack to avoid dropping contaminated soil.  Then, the plants must be buried outside the plot and far from irrigation channels.

Uprooting weeds is recommended because the bacterium survives in many species of weeds.

Upon harvesting, we must collect harvest remains and give them to the animals. We collect rotten tubers and bury them outside the plot, far from irrigation channels.

Alternatively, cultural practices can be reduced to prevent damaging the roots of the plants, which would allow easy entry of bacteria, for instance, make just one hilling during the vegetative period.

After each cultural practice, tools and sandals or shoes must be washed to prevent transmission to other potato plots.

Likewise, irrigation water from an infested field must be prevented from flowing into neighbouring fields.

What must be done in a plot where BW occurred

Crop rotation is one of the most effective components of BW integrated management that, added to adequate management and careful performance of cultural practices, enables to eliminate bacteria from infested soil and thus recover the soil for potato growing. Only healthy BW-free seed must be planted thereafter.  

However, crop rotation shall be an effective practice only if the following components are also applied:

• During potato harvest and prior to rotating, we must collect harvest remains and rotten tubers, bury them outside the plot and far from irrigation channels, or feed them to the animals.
• During crop rotation, we must take into account that host weeds and volunteer plants must be uprooted, since they can maintain or increase bacteria in the soil.
• Likewise, we must take care that the water we use to irrigate the crop does not come BW-infested plots, or that surface runoff water from rainfall does not run down slope an infected plot. In this case, building drainage ditches is required.
• Not only the tools, but also our shoes must be cleaned.

Susceptible crops must be avoided, such as all solanaceous plants (potato, tomato, bell pepper, South American hot peppers, eggplant, tobacco), because BW remains in the plot and will cause even greater damage. If cereals (corn, wheat, barley), grass crops or other non-susceptible crops such as sweetpotato, cabbage, cauliflower, squash, celery, leek, onion, carrot, pea, bean, and others are used to rotate, BW does not develop and does not survive in the roots of such crops.

Rotation with at least 2 non-susceptible crops is recommended when BW incidence in the previous potato crop was below 20%. Such 2 non-host crops may be planted for 1 or 2 years, depending on irrigation water availability. If BW incidence was over 20%, rotating requires more than 3 crops, according to the incidence percentage.  Thereafter a tolerant or moderately resistant variety can be planted (See below)

To keep or improve soil fertility, during rotation we must change different crops from one planting season to the other, and alternate them with legumes.  

The soil can be ploughed and exposed to sunlight or to frost during the warmer or colder months, depending on the area. This practice may be repeated several times depending on financial and human resources available for labor.

Nematode control

Since the root knot nematode (Meloidogyne incognita) facilitates the penetration of the bacteria in the roots that increases BW incidence, nematodes should be controlled by the means of rotation with non-host crops and manure applications.

Host resistance

Planting of resistant varieties is the most effective way of controlling a disease, however, until now no highly resistant materials exist. The past CIP breeding program produced advanced potato clones with moderate levels of resistance to plant wilt that can give high yields in the presence of the disease, but they harbor tuber infection. Latent infection is the main cause of spread of the disease and this type of resistance is not effective if infected seed tubers are planted and when agro-ecological conditions are too favorable to the pathogen such as high temperatures, high soil humidity, presence of nematodes. Furthermore resistance has been found to be strain-specific so clones have to be evaluated in location where they will be used.

Therefore, in the absence of better sources of higher levels of resistance, national programs should continue breeding and/or selecting for BW resistance from CIP advanced and local materials but taking into account plant wilt and also tuber (visible and latent) infection rates and comparing them to locally-grown varieties in order to determine risks of disease spread.

The use of the moderate levels of resistance that are available, however, can make a huge impact on ware potato production and food security in BW-endemic areas only if BW-free seed can be provided.

¹ Example: mix 100 cc of household bleach that comes at 6% sodium hypochlorite in 6 litres of water.

Useful references

Allen, C., P. Prior and A.C. Hayward (Eds.), 2005. Bacterial Wilt Disease and the Ralstonia Solanacearum Species Complex. APS Press, St. Paul, Minnesota USA, 510 p.

Bentley, J., A. Paz, G. Juanes, J. E. Martínez, H. Equise, J. L. Quiruchi, R. Rioja, O. Barea, R. Salinas & G. Thiele, 2003. El Taller Comunitario y la Radio, una Experiencia en Bolivia. LEISA Revista de Agroecología 19:42-45.

Bentley, J.W., S. Priou, P. Aley, J. Correa, R. Torres, H. Equise, J.L. Quiruchi and O. Barea, 2005. Method, Creativity and CIALs. International Journal of Agricultural Resources, Governance, and Ecology, 5(1): 90-105.

Elphinstone, J. G., Hennessy, J., Wilson, J.K. and D. E. Stead. 1996. Sensitivity of different methods for the detection of Pseudomonas solanacearum (Smith) in potato tuber extracts. EPPO/OEPP Bulletin 26: 663-678.

French, E.R. 1994. Integrated control of bacterial wilt of potatoes.CIP Circular 20: 8-11.

French, E.R. 1994. Strategies for integrated control of bacterial wilt of potatoes. In Bacterial wilt: the disease and its causative agent Pseudomonas solanacearum (eds. Hayward, A. C. and Hartman, G. L.) p. 199-207, CAB International, Wallingford (Reino Unido).

French, E. R, Gutarra, L., Aley, P. and J.G. Elphinstone, 1995. Culture media for Ralstonia solanacearum isolation, identification and maintenance. Fitopatología 30: 126-130.

French, E.R., Anguiz, R. and Aley, P. 1997. The usefulness of potato resistance to Ralstonia solanacearum, for the integrated control of bacterial wilt. In Bacterial wilt disease: Molecular and ecological aspects (eds. Prior P., Allen C. and Elphinstone J.) pp. 381-385. INRA edn, Springer Verlag, Berlin (Germany).

Hayward, A.C. 1991. Biology and epidemiology of bacterial wilt caused by Pseudomonas solanacearum. Annu. Rev. Phytopathol. 29: 65-87.

Hayward, A. C. and G.L. Hartman (Eds). 1994. Bacterial wilt: the disease and its causative agent Pseudomonas solanacearum. 259 p. CAB International, Wallingford (Reino Unido).

Lemaga, B., R. Kakuhenzire, B. Kassa, P.T. Ewell, and S. Priou, 2005. Integrated control of potato bacterial wilt in Eastern Africa: The experience of African Highlands Initiative. pp. 145-157. In: Bacterial Wilt Disease and the Ralstonia solanacearum Species Complex. C. Allen, P. Prior and A.C. Hayward (Eds.). APS Press, St. Paul, Minnesota USA, 510 p.

Martin, C. and E.R. French. 1985. Bacterial wilt of potato: Pseudomonas solanacearum. Technical Information Bulletin 13, 8 p. Centro Internacional de la Papa, Lima, Perú.

Prior, P., Allen, C. and J. Elphinstone (Eds). 1998. Bacterial wilt disease: molecular and ecological aspects. 447 p. INRA edn, Springer Verlag, Berlin (Alemania).

Priou, S., L. Gutarra and P. Aley, 1999. Highly sensitive detection of Ralstonia solanacearum in latently infected potato tubers by post-enrichment ELISA on nitrocellulose membrane. EPPO Bulletin / Bulletin OEPP 29: 117-125.

Priou, S., Gutarra, L., Fernandez, H. and P. Aley. 1999. Sensitive detection of Ralstonia solanacearum in latently infected potato tubers and soil by postenrichment ELISA. CIP Program Report 1997-98, p. 111-121, International Potato Center, Lima, Perú.

Priou, S., Aley, P., Chujoy, E., Lemaga, B. and E.R. French. 1999. Integrated management of bacterial wilt of potato. CIP slide training series (57 slides and a 30 page-guide in English and Spanish). International Potato Center, Lima, Perú.

Priou, S., R. Torres, A. Villar, L. Gutarra and F. de Mendiburu, 2001. Optimization of sample size for the detection of latent infection by Ralstonia solanacearum in potato seed tubers in the highlands of Peru. Potato Research 44: 349-358.

Priou, S., C. Salas, F. de Mendiburu, P. Aley and L. Gutarra, 2001. Assessment of Latent Infection Frequency in Progeny Tubers of Advanced Potato Clones Resistant to Bacterial Wilt: A New Selection Criterion. Potato Research 44: 359-373.

Priou, S., O. Barea, H. Equise & P. Aley, 2004. Capacitación e investigación participativa para el manejo integrado de la marchitez bacteriana de la papa. Experiencias en Perú y Bolivia. CIP-PROINPA-DFID; Centro Internacional de la Papa (CIP), Lima, Perú, 85 p.

Priou, S., P. Aley, and L. Gutarra, 2005. Assessment of resistance to bacterial wilt in CIP advanced potato clones. pp. 261-267. In: Bacterial Wilt Disease and the Ralstonia solanacearum Species Complex. C. Allen, P. Prior and A.C. Hayward (Eds.). APS Press, St. Paul, Minnesota USA, 510 p.

Priou, S., L. Gutarra and P. Aley, 2006. An improved enrichment broth for the sensitive detection of Ralstonia solanacearum (biovar 1 and 2A) in soil using double-antibody sandwich enzyme-linked immunosorbent assay. Plant Pathology 55: 36-45.