Bacterial diseases

>> Bacterial wilt - Ralstonia solanacearum
>> Erwinia soft rot

Bacterial wilt - Ralstonia solanacearum

Introduction

Ralstonia solanacearum is the causal pathogen of bacterial wilt, and was previously known as Pseudomonas solanacearum (Smith) Smith. It has been classified into three races based on their host range. Race 1 has the broadest host range, which includes solanaceous food crops and legumes. Race 2 of R. solanacearum is pathogenic to triploid banana and Heliconia (a relative of Musa spp.). Race 3 is restricted to potato and tomato. Surveys are few in tropical regions. In the lowland areas of the Philippines, tomato losses were 15%, eggplant and pepper 5%. Losses of 10-30% to peanut crops in central and southern China were attributable to bacterial wilt. No worldwide monetary figure is available for losses to the major food crops.

Disease Symptoms

The key symptom is wilt, being more rapid in younger plants. For tomatoes, petioles droop during the warmer parts of the day, over an extended period of days. The pith of wilted plants can be water soaked, brown or hollow. Adventitious roots form on the lower stem to compensate for blockages caused by the extensive colonisation of the bacterium in the vascular tissue. Plants may be stunted and chlorotic. Plants may wilt entirely over days under conducive conditions, such as after high temperatures. There can also be latent infection. In young potato stems, a cross-section of the vascular tissue of the lower stem and tubers (brown rot) reveals a brown ring which may extend into the pith or cortex. This disease can be distinguished from other wilt diseases by placing infected tissue, freshly sectioned, into water to observe masses of bacterial ooze streaming out. The rapid wilting also distinguishes this from fungal wilts.

Bacterial wilt in banana crops is referred to as Moko disease, a serious problem in South and Central America, the Carribean islands and the Philippines. The same pathogen is also responsible for Banana Blood wilt and fruit rot and Bugtok fruit and peduncle rot in Indonesia and the Philippines respectively. This pathogen has not yet been found in bananas in Vietnam.

There is often confusion in the differentiation of Moko disease and Panama wilt of banana, caused by the fungus Fusarium. With Moko disease, bananas at all growth stages can be infected, vascular tissue has a black to brown discolouration fingers ripen prematurely and split and the pulp is discoloured. As in other susceptible crops, when infected tissue is cut, slimy bacterial ooze is exuded.

Infected ginger crops display tuber soft rot similar to that of potato, where ooze can be exuded from freshly cut tissue. The aboveground plant parts also show symptoms of wilt.

Description of the Pathogen

This organism is a Gram negative, non-spore forming, rod-shaped, aerobic bacterium. Optimum growth is at 30-35°C and it does not grow above 41°C. In culture, colonies are cream coloured and slimy. When tissue from the base of infected stems is placed in water, ooze exudes as shown above.

Distribution within Vietnam

This disease has been recorded in all parts of Vietnam.

Host Range

Most crops belonging to the Family Solanaceae are host to R. solanacearum.
Common crops affected by bacterial wilt include eggplant, capsicum, tomato, potato, tobacco, sweet potato, banana, ginger, onion, peanut, mung bean, cashew, papaya, cassava and sesame.

Epidemiology

• The pathogen infects roots through wounds and colonises the vascular tissue causing plugging of the xylem and leaf wilting. There is often an association between nematode infection and bacterial wilt, where the nematodes create wounds in the root tissue to allow an entry point for the bacterium to infect the plant.
• The bacterium survives in the soil and can maintain infectious populations over several years. Alternative weed hosts may also play a role in survival and overseasoning. This pathogen also causes brown rot of potato tubers, where the decaying tissue of infected crops have a role in overseasoning.
• Temperature is a major determinant in the distribution of this pathogen, which is widespread in tropical, sub-tropical and warm temperate regions where the mean soil temperature is greater than 15°C.
• Wet soil increases the incidence of disease and water movement contributes to the dissemination of inoculum with water movement.
• The incidence and rate of wilting therefore increases with high temperatures and soil moisture.
• Continuous cropping of susceptible plants will also favour infection. Bacterial wilt of potatoes is more prevalent in slightly acid to acid soils, as are many soil-borne diseases.
• The pathogen is disseminated by:
• • Contaminated farming equipment, in soil on tyres and footwear.
  • Drainage water carrying inoculum through the soil.
  • Infected seed, especially in the case of groundnut.
  • Seedlings raised in infected soil, spreading the pathogen to new areas.
  • Cultural operations such as pruning.
  • Moko disease, Banana Blood wilt and fruit rot and Bugtok fruit and peduncle rot are spread mechanically, by insects and on infected plants.

Control

• Control practices tend to provide alternatives to pesticides, by way of soil amendments, biological control and cultural practices. Many of these techniques are still being researched and rotation with non-susceptible crops or a fallow period between crops seems to be the most feasible.
• Chemical control is generally not feasible.
• Cultural practices include:
• • Rotation with maize or rice for > 2 years.
  • Using healthy seed / certified seed and seedlings, stored under dry cool conditions
  • Planting in cooler conditions, or using resistant rootstock when planting in summer
  • Maintaining clean machinery may aid in the minimising the spread of the disease.
  • Deep burying of inoculum with tillage, or repeated disk ploughing of dry soil to expose inoculum to the sun can be performed as a control practice.
  • Fallows of 18-months have been shown to control the bacterial wilt of banana, with a shorter period for dry soils.
  • All rhizome tissue and weeds must be removed for an effective fallow. Since major cause of disease spread in bananas is through the replanting of infected suckers, new plantings should be started with tissue cultured plants to restrict the spread of the pathogen.
  • Selecting resistant cultivars of each crop
  • Biological control with bacterial antagonists or avirulent strains. However this measure is generally useful only under greenhouse conditions.

Wilt symptoms on tomatoes Infected stem tissue

Infected stem tissue

Bacterial ooze exuding from freshly cut stem tissue placed in water

Culture shot of Ralstonia solanacearum

Erwinia soft rot

Introduction

The Erwinia genus is generally responsible for soft rots in potatoes, Brassicas, bananas and other succulent plants, boll rots in cotton and blackleg in potatoes.

Disease Symptoms

Initial infection of Brassicas occurs on the outer petioles that are in contact with the soil. Petioles become water-soaked and the bacterium progressively colonises the head, often causing complete head rot. Infection can also take place through the main root, then into the stem and head of the plant. Insects can vector the bacterium, so infections may have arisen from the site of feeding or wounding from the insect.

Bananas being propagated from freshly cut rhizome tissue or suckers may show poor sprout emergence, stunting and yellowing. Incidence is highest in wet humid areas where the material has been treated with hot water prior to planting.

Potatoes are susceptible to tuber rots caused by E. carotovora (Jones) Bergey et al., where soft, depressed areas around lenticels of the potatoes develop; E. atroseptica (Van Hall) Dye, or blackleg, where stems of infected plants exhibit an inky black decay; and, E. chrysanthemi which is responsible for the rapid breakdown of tubers within one week of planting. The tuber develops a soft, watery smelly rot with only the skin left intact. Carrots can exhibit the same symptoms as potatoes.

Bacterial stalk rot of maize is caused by E. chrysanthemi, where collapsed twisted stalks are a good indication of this disease.

Soft rot of onion and spring onion can occur, where the fleshy infected bulb tissue is watersoaked an pale brown, softening as the infection progresses. The foliage may become chlorotic and wilt.

Description of the Pathogen

E. carotovora is cultured on Stewarts MacConkey-pectate media, where it forms craters in the media. The organism is rod-shaped, Gram negative, approximately 0.7-1.5um long and has peritrichous flagella. They are non-spore-forming and facultatively anaerobic. The soft rot pathogens can be differentiated with biochemical tests.

Distribution within Vietnam

This disease has been recorded in all parts of Vietnam.

Host Range

Hosts of this pathogen include potato, lettuce, Brassicas, bananas, onions, maize and succulent plants. Head and rhizome rots of banana are caused by Erwinia carotovora subsp. carotovora and E. chrysanthemi. Boll rot of cotton can be caused by E. herbicola thought to be disseminated by stinkbugs. A reddish brown discolouration of lint cells of outwardly unblemished immature bolls can occur. Soft rot of cucumber and melon also occurs but as a post-harvest disease.

Epidemiology

The soft rot pathogen, Erwinia carotovora subsp. carotovora can be disseminated into new infection sites by irrigation water or an insect invasion of the crop. All plants in the field can be infected and the disease is likely to develop on produce in transit and in storage. This is particularly so for lettuce and potatoes.

The pathogen survives in the soil and enters the host through hydathodes, lenticels or wounds. Infections in banana usually arise from wounds to the rhizome. Enzyme secretion causes a breakdown of cell walls.

Colonised tissues become water-soaked and secondary organisms cause decay.

Soft rot increases when the humidity is high, soils are moist and temperatures are above 22°C. Diptera spp. are known insect vectors.

The disease is more prevalent in the wet seasons, and where excess nitrogen has been applied.

Control

• Avoid wounding plant tissue during cultivation, harvest, packing and storage.
• The primary inoculum in blackleg of potato is the seed tuber, and there are a number of control measures effective in reducing disease severity. Seed tubers, especially those that have been cut should only be planted in well-drained soil. The same recommendations apply to freshly cut banana tissue. Avoid washing seed potatoes and take care not to damage them prior to planting.
• By avoiding excess irrigation, the soil remains aerobic thus non-conducive to the growth of the pathogen.
• Removing piles of discarded vegetables will prevent a potential inoculum source from which insect vectors can transmit the disease. Control the insect vectors.
• Use resistant varieties.
• Frequently clean and disinfect farm equipment. Use sterilised knives and trimming equipment especially in the case of bananas.
• Store produce in conditions favourable to the rapid healing of injured tissue.
• For post-harvest soft rot management in potato, avoid excess moisture prior to harvest, harvest only when tubers are mature and the temperature is below 20°C, and cool to below 10°C as quickly as possible. Ensure good ventilation to minimise carbon dioxide accumulation and moisture build up. Use only clean water to wash the potatoes, if washing is necessary. They should be packed in aerated containers for storage or marketing.