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Foliage: the first visible symptom is a wilting of the leaves at the ends of the branches during the heat of the day with recovery at night. As the disease develops, a streaky brown discoloration of the stem may be observed on stems 2.5 cm or more above the soil line, and the leaves develop a bronze tint. Epinasty of the petioles may occur. Subsequently, plants fail to recover and die. A white, slimy mass of bacteria exudes from vascular bundles when broken or cut.
Tubers: external symptoms may or may not be visible, depending on the state of development of the disease. Bacterial ooze often emerges from the eyes and stem-end attachment of infected tubers. When this bacterial exudate dries, soil masses adhere to the tubers giving affected tubers a 'smutty' appearance. Cutting the diseased tuber will reveal browning and necrosis of the vascular ring and in adjacent tissues. A creamy fluid exudate usually appears spontaneously on the vascular ring of the cut surface.
Atypical symptoms on potato (necrotic spots on the epidermis), possibly caused after lenticel infection, have been described by Rodrigues-Neto et al. (1984).
Symptoms of brown rot may be readily distinguished with those of ring rot caused by Clavibacter sepedonicus (EPPO/CABI, 1997). R. solanacearum can be distinguished by the bacterial ooze that often emerges from cut stems and from the eyes and stem-end attachment of infected tubers. If cut tissue is placed in water, threads of ooze are exuded. Because such threads are not formed by other pathogens of potato, this test is of presumptive diagnostic value. For ring rot, tubers must be squeezed to press out yellowish dissolved vascular tissue and bacterial slime.
The youngest leaves are the first to be affected and have a flaccid appearance, usually at the warmest time of day. Wilting of the whole plant may follow rapidly if environmental conditions are favourable for the pathogen. Under less favourable conditions, the disease develops slowly, stunting may occur and large numbers of adventitious roots are produced on the stem. The vascular tissues of the stem show a brown discoloration and drops of white or yellowish bacterial ooze may be released if the stem is cut (McCarter, 1991).
One of the distinctive symptoms is partial wilting and premature yellowing of leaves. Leaves on one side of the plant or even a half leaf may show wilting symptoms. This occurs because vascular infection may be restricted to limited sectors of stems and leaf petioles. In severe cases, leaves wilt rapidly without changing colour and stay attached to the stem. As in tomato, the vascular tissues show a brown discoloration when cut. The primary and secondary roots may become brown to black (Echandi, 1991).
On young and fast-growing plants, the youngest leaves turn pale green or yellow and collapse. Within a week all leaves may collapse. Young suckers may be blackened, stunted or twisted. The pseudostems show brown vascular discoloration (Hayward, 1983). Moko disease, caused by R. solanacearum, is easily confused with the disease caused by Fusarium oxysporum f.sp. cubense. A clear distinction is possible when fruits are affected - a brown and dry rot is only seen in Moko disease.
Seedling wilt manifests itself as yellowing of the mature lower leaves, which show scorching and browning of the tissue between the veins. The younger leaves and terminal shoot become flaccid and droop. Affected seedlings show either a gradual loss of leaf turgidity or sudden wilting. Seedling wilt becomes evident in the early hours of the day and gradually becomes more pronounced by midday, especially on sunny days. The wilted seedlings may partially recover during the afternoon and evening when temperatures fall, but wilting becomes more pronounced on successive days. The roots of affected seedlings exhibit a brownish-black discoloration. In advanced stages of disease, the tuberous portion of the root becomes discoloured and spongy. In due course, seedlings with pronounced wilt symptoms become completely desiccated.
In container nurseries, R. solanacearum infects the cotyledons of emerging seedlings causing greyish-brown, water-soaked lesions, which spread to the entire cotyledon and become necrotic. The infection spreads to the adjoining stem and root tissues and the affected seedlings rot and die. Collar rot appears in 1- to 4-month-old bare-root seedlings as greyish-brown, water-soaked lesions at the collar region of seedlings, just above the soil level. The lesions spread longitudinally on the stem, both above and below ground level, becoming sunken and necrotic. The younger leaves become flaccid and droop followed by leaf scorching and pronounced vascular wilt. In bare-root nurseries, wilt usually occurs in small patches affecting individuals or groups of seedlings, which expand as more seedlings succumb to the infection.
Infection of mature foliage begins as greyish-brown to greyish-black, irregular lesions that spread to the entire leaf lamina. Infection spreads to the petioles and stems.
Control of bacterial wilt is very difficult, largely being dependent on good crop management practices. Yuliar et al. (2015) reviewed recent developments in the management of bacterial wilt disease. The use of certified disease-free seeds/tubers/transplants, reliable and early detection of the pathogen, quarantine measures on infected fields and farms, sufficient crop rotation, control of weed hosts, volunteer plants and nematodes where present, avoidance of surface water for irrigation, and education are key factors in control (Janse, 1996; Pradhanang and Elphinstone, 1996b).
Seed potato tubers and seeds of other solanaceous plants should be obtained from crops that have been inspected and found disease-free for the last two growing seasons. Visual inspections should be performed routinely upon export and import. Laboratory checks for low level infection and contamination may be necessary. Plants of Musa spp. should be kept in post-entry quarantine (OEPP/EPPO, 1990b).
Cutting seed potato tubers should be avoided. Crop rotation of 5-7 years without susceptible crops has been recommended. The disease may also be controlled by the application of fertilizers to change soil pH. In the USA, lowering the soil pH to 4-5 in summer and raising it to pH 6 in the autumn eradicated the pathogen (Graham and Lloyd, 1979; Graham et al., 1979).
Tolerant cultivars of potato, aubergine (Dalal et al., 1999; Quezado-Soares et al., 1997), tobacco, groundnut and other crops are available, but the race and strain diversity of the pathogen means that cultivars must be selected with care. Potato cultivars were selected in Colombia with tolerance derived from Solanum phureja and S. demissum (French, 1985; Hartman and Elphistone, 1994). Tobacco-resistant cultivars have been developed (Lopez et al., 1978). In China, wilt-resistant groundnut cultivars appear to be the most important control measure (Tan et al., 1994) although they have a lower yield potential due to reduced nodulation and nitrogen fixation by Rhizobium bacteria (Liao et al., 1992). Because tolerant plants may be infected and contaminated with the pathogen without symptom expression (Grimault and Prior, 1993) movement of these cultivars into disease-free regions may introduce the pathogen.
Grafting of aubergine on resistant Solanum species has been shown to be successful (Mochizuki and Yamakawa, 1979). Grafting of tomato on resistant aubergine rootstock reduced losses by 90% (Lum and Wong, 1976). A new technique of raising brinjal seedlings on coir piths has shown some promise in controlling the disease. Once transplanted, the plants are immune to diseases for about 30-40 days, after which they are not greatly affected by bacterial wilt (ProMED-Plant, 2010). Resistance in tomato and other hosts may be reduced by nematode infection (Yen et al., 1997; Deberdt et al., 1999). Furusawa et al., (2019) demonstrated that simultaneous infection of R. solanacearum and root‐knot nematode Meloidogyne incognita increases the severity of bacterial wilt in tomato, where the galls induced by the nematode are a suitable location for bacterial growth.
Intercropping of potato with maize or Phaseolus vulgaris reduced inoculum density and disease development in some cases (Autrique and Potts, 1987) but the pathogen was found to persist in these alternative hosts (Granada and Sequeira, 1983). In a crop rotation trial it was found that resting land for 3 years reduced wilt from 80.1 to less than 7.5%. Tuber rot was reduced and crop yield enhanced. For potato, a minimum fallow period of 2 years appeared to be adequate to obtain good yields (Mateo et al., 1982).
Wilt severity in tomato was reduced by a rotation system using maize, okra, cowpea or resistant tomato. The onset of bacterial wilt was delayed by 1-3 weeks and wilt severity was reduced by 20-26% (Adhikari and Basnyat, 1998). Tomato in rotation with rice was also effective in reducing R. solanacearum populations in Taiwan (Michel et al., 1996). Rotation of groundnut with rice and with maize, wheat, sorghum and sugarcane was effective in reducing incidence and severity (Hong et al., 1994; Tan et al., 1994). In tobacco, disease incidence was reduced and the yield was increased by cultivar resistance and by 1-year rotation with maize, fescue (Festuca sp.) or soyabean (Melton and Powell, 1991).
Hot-air treatment of ginger roots for 30 min at 50°C has been successful (Tsang and Shintaku, 1998). Treatment of soils using stable bleaching powder gave disease suppression of 70-89% in greenhouse and field trials (Dhital et al., 1997) and in combination with deep ploughing (Kishore et al., 1996). Data collected over 3 years revealed that pre-treatment of soil with bleaching powder controlled the disease by 68.4% (Verma and Shekhawat, 1991). Effects of soil amendments are soil dependent (Michel and Mew, 1998). Soil fumigants showed either slight or no effects (Murakoshi and Takahashi, 1984). Some compounds based on hydrogen peroxide and peracetic acids (and catalase-inhibitors) show promising results for disinfection of contaminated surface water (Janse et al., 1998; Niepold, 1999).
Along with the integrated disease management strategies (Biswal and Dhal, 2018), several attempts have been made to develop a biological control method against the bacterial wilt pathogen (Rocha and Moura, 2013; Kheirandish and Harighi, 2015; Chen et al., 2019). Biological control of the pathogen has been reported using bacterial strains isolated from soil (Nguyen and Ranamukhaarachchi, 2010). Application of clove oil, chitosan and Paenibacillus strains significantly reduced the incidence and severity of the bacterial wilt disease (Huang and Lakshman, 2010). The application of chitosan as a seed treatment reduced wilt incidence by 48%, and the application of the Paenibacillus polymyxa strain MB02-1007 reduced wilt incidence by 88% (Algam et al., 2010). Furthermore, amending tomato field topsoil with cocopeat, farmyard manure compost and green compost suppressed bacterial wilt severity and pathogen survival in the soil, and increased tomato yield in Ethiopia (Yadessa et al., 2010). Positive results were achieved in laboratory experiments with the antagonistic bacteria Bacillus polymyxa and Pseudomonas fluorescens (Aspiras and Cruz, 1985). Success has been claimed for P. fluorescens in potato in laboratory and field trials. Avirulent mutants of the bacterium have also been used in some studies (Ciampi-Panno et al., 1989; Gallardo and Panno, 1989; Hartman and Elphinstone, 1994). It has been reported that Ralstonia pickettii strain QL-A6 is an effective biocontrol agent for bacterial wilt of tomato (Wei et al., 2013). Several studies have also been conducted to evaluate the use of bacteriophages which are capable of infecting R. solanacearum (Lee and Park, 2016; Elhalag et al., 2018). Effectiveness of essential oils from different plant species and incorporation of plant materials into soil has also been investigated to combat the bacterial wilt pathogen (Pontes et al., 2011; Alves et al., 2014; Tu et al., 2020). However, none of the above-mentioned biological methods has successfully been used on a commercial scale.
Chemical control is ineffective. Antibiotics, streptomycin, ampicillin, tetracycline and penicillin showed hardly any effect (Farag et al., 1982); in fact, streptomycin application increased the incidence of bacterial wilt in Egypt (Farag et al., 1986). The field use of calcium carbonate (CaCO3) reduced disease incidence and it has been suggested that CaCO3 could be used as a potential soil amendment for management of bacterial wilt disease (He et al., 2014).
R. solanacearum is the most serious pathogen of solanaceous plants in tropical regions and can cause serious losses in temperate regions. Accurate data on yield losses and further economic impacts are not available. A review of the older literature can be found in Kelman (1953). A method to determine yield loss/disease severity for brown rot in potato has been described (Elphinstone, 1989). New high-yeilding but susceptible cultivars in place of older tolerant varieties, may cause problems in areas where the disease is endemic (Weingartner and Shumaker, 1984).
Many factors influence disease incidence and yield loss. In a study in India on sesame wilt incidence was significantly correlated with mean temperature, rainfall and relative humidity during the crop growth period (Hazarika and Das, 1999). In a study on the effects of physical soil properties it was found that sandy loam soil with a high sand content and low silt or clay content, with low water-holding capacity, was unfavourable for the pathogen and wilt incidence. Elevated disease levels were expressed in clay soils with high water-holding capacities (Keshwal et al., 2000).
Root colonization by ectomycorrhizal fungi is important in reducing disease levels and increasing tree growth in Eucalyptus spp. In China, disease in nurseries was reduced by 40-72% and in fields by 20-39% when seedlings were inoculated with eight fungal isolates. Height and basal diameter growth of trees in field trials were enhanced by 11.7 to 59.7% (Gong et al., 1999).
Greatest economic losses have been reported on potato, tobacco and tomato in the south-eastern USA, Indonesia (Sunarjono, 1980), Nepal, Uganda (Busolo-Bulafu et al., 1993), Brazil (Melo et al., 1999), Colombia and South Africa. In the Philippines, there were average losses of 15% in tomato, 10% in aubergine and Capsicum, and 2-5% in tobacco (Zehr, 1969). In the Amazon basin in Peru, banana plantations have been seriously affected with rapid spread of the pathogen in previously unaffected plantations (French and Sequeira, 1968). In India, there are sometimes total losses in tomato crops. Bacterial wilt also appears to be very common in wild and cultivated turmeric (Curcuma spp.) in Thailand and Indonesia (Thammakijjawat et al., 1999). Bacterial wilt is also a problem in ginger (Zingiber officinale); it was present in 80% of 310 fields surveyed in Himachal Pradesh, India (Sharma and Rana, 1999), and severe losses were reported from Thailand (Titatarn, 1985). R. solanacearum has been intercepted regularly from rhizomes exported for cut flower production in Europe. The disease may cause serious indirect losses when quarantine measures entail restriction movement of, or destruction of plant products (Hyde et al., 1992; Tuin et al., 1996).
Multiplication by cutting seed potato seriously increases the risk of high losses. Cut seed potato increased disease incidence by 250% and reduced yield by 40% (Vijayakumar et al., 1985). Extensive losses of potato were reported in Greece (Zachos, 1957). In Israel, losses were heavier in the spring potato crop than the autumn crop, because of the higher growing temperatures in spring (Volcani and Palti, 1960). Tuber rotting averaged 10%, reaching 50%, in stored potatoes in Nepal (Shrestha, 1996). Complete crop losses in small holdings in Nepal resulted from poor cultural practices including using seed from affected crops for subsequent plantings (Gurung and Vaidya, 1997). In Venezuela, in the period 1992-1996, R. solanacearum was found in most localities between 1100 and 3000 m above sea level, but was not found in localities at altitudes greater than 3000 m. Bacterial wilt disease incidence increased from 22% in 1992 to 37% in 1996 with disease incidence varying between 5 and 75%. Biovar 2 was present in greatest frequency and in most of the affected areas for potato (Garcia et al., 1999).
In tomato hybrids, field grown in Taiwan for the fresh market, bacterial wilt incidence was 15-26% on improved tolerant hybrids compared to 55% in other hybrids (Hartman et al., 1991). In India, an investigation of the effect of time of infection showed that disease incidence, measured by plant mortality and plant yield, diminished with age of the plant at the time of inoculation. Maximum losses were recorded during the summer season (Kishun, 1987).
Groundnuts and Other Crops
In Vietnam, infection in groundnut was most severe in dryland cropping systems, especially on sandy soils along riverbanks, and on uplands (Hong et al., 1994). Tolerant varieties are infected by the pathogen but are not affected and can produce high yields (Liao et al., 1998). Bacterial wilt in groundnut (Race 1, biovars 3 and 4) is widespread in China. Annual disease incidence ranges from 4 to 8% on resistant cultivars. Pathogenicity varies between regions, the disease generally being more serious in southern provinces where losses of up to 20% were common (Yeh, 1990; Tan et al., 1994). Disease severity mostly increases if R. solanacearum is found in association with root nematodes. In tobacco, nematode infestation leads to greater susceptibility to bacterial wilt (Chen, 1984). When bacterial wilt of teak was initially recorded during the 1980s in Kerala, India, it appeared to be of little consequence. However, the incidence of the disease has increased over the years, both in nurseries and plantations. R. solanacearum causes mortality of bare-root and root trainer seedlings raised in high rainfall areas. Young plantations raised in waterlogged sites in areas of high rainfall (>3000 mm per annum) are more seriously affected. In these areas, the incidence of disease varied from <1% to ca 20% (Sharma et al., 1985). Synergistic interactions between R. solanacearum and Meloidogyne javanica have been reported (Sitaramaiah and Sinha, 1984; Verma et al., 1997; Pathak et al., 1999).