|1. Bacterial wilt
- Bacterial wilt is one of the most devastating disease in solanaceous plants.
- It is more serious in warm temperate, subtropical and tropical regions of the world.
- The disease is also called southern bacterial wilt, solanaceous wilt, southern bacterial blight and by many other common names in countries wherever it occurs.
- Characteristic symptoms of bacterial wilt on most of the hosts are sudden wilting and death of infected plants.
- Petioles of the older leaves of the infected plants droop down without yellowing.
- Leaves show epinasty symptoms accompanied by yellowing and stunting of whole plant.
- There is a typical browning of the vascular tissues of roots and stems (Plate-1.).
- Bacterial polysaccharides mechanically block the vascular system, which checks the translocation of water and minerals resulting in wilting of plants.
- Excessive development of adventitious roots may also take place in tomato.
- Cross sectioned part of the affected root or stem yield whitish milky bacterial ooze.
Disease cycle and epidemiology:
- The disease is caused by bacterium Ralstonia solanacearum (Smith) Yabuuchi et al., 1995).
- The bacterium is gram negative, rod shaped, frequently occurs in pairs, motile with 1 to 4 polar flagella and measures 0.5-0.7 x 1.5 –2.5 µm.
- It is aerobic and catalase and oxidase-positive and forms nitrites from nitrates.
- Five races (race1, 2, 3, 4, 5) and four biovars (biovars I, II, III and IV) are known to occur in different parts of the world.
- In India race 1 and 3 and biovar II, III and IV are prevalent.
- The pathogen is both soil and seed borne in nature and overwinters in infected plant parts, in potato tubers, in wild host plants and solanaceous weeds and can survive at least for 2 years in the absence of any host.
- Injured and decaying infected tissues release bacteria in the soil which enter plants through wounds occurring in roots due to cultural practices, nematodes and insects.
- Relatively high soil moisture and soil temperature favour the disease.
- Control of bacterial wilt in infested soil is very difficult as all the commercial cultivars are susceptible and no chemical control is available.
- Follow long crop rotation with non-solanaceous crops.
- Avoid the movement of water from infected plant to healthy plants.
- Shifting of date of transplanting to avoid period of high temperature, heavy rainfall or both.
- Green manuring or biofumigation with Brassica spp. may reduce the bacterial inoculum in soil.
- Bacterial antagonists such as Pseudomonas fluorescens, P. glumae, P. cepacia and Bacillus spp. have also been known to reduce disease incidence.
- Application of bleaching powder (15 kg/ ha) has also been found effective against this disease.
- The disease can also be controlled effectively if dazomet application is combined with soil solarization.
- Seedling dip in Streptocycline (100 ppm) for 30 minutes is also effective to some extent.
- Use resistant cvs./hybrids for cultivation.
2. BACTERIAL CANKER
- Leaves show wilting symptoms.
- On stem, brown streaks and canker develop.
- Small brown, scabby lesions surrounded by white halo appear on the fruits which resembles to bird’s eye.
Disease cycle and epidemiology:
- Disease is caused by Clavibacter michiganensis sub sp. michiganensis (Smith) Davis et al.
- The bacterium is aerobic, Gram + ve, coryneform rod with cell growing either singly or in pairs.
- Pigmented mutants develop due to possession of a range of carotenoid compounds.
- The organism is characterized by oxidation of carbohydrates, being nonlipolytic, it can only liquify gelatin slowly, the ability to hydolyse starch is weak or absent and amino acids like biotin, nicotinic acid and thiamine are required for growth.
- The bacterium perennates in infected seed, plant debris and weed hosts.
- Optimum temperature for disease development is 28o C.
- The disease is more severe in wet weather.
Other important bacterial disease:
- Collect and destroy all infected plant debris.
- Follow crop rotation and tomato should not be grown in infested field for at least 3 years.
- Use disease free seed and treat them by dipping in the Streptocycline (100 ppm) solution for 1 h.
- Spray the crop with Streptocycline (100 ppm) followed by copper oxychloride (0.30%) or Bourdeaux mixture (4:4:50) and repeat at 7-10 days interval.
- Bacterial spot (Xanthomonas campestris pv. vesicatoria (ex. Doidge) Vauterin et al.)