- The disease first appears in the field as yellowing of the lower leaves most commonly at the time of flowering stage.
- The affected leaves die prematurely and the symptoms continue to appear on successively younger leaves.
- One or more branches may be affected while the others remain symptom less.
- Browning of the walls of the vascular bundles follows rather promptly, and is best seen by cutting of stem or petiole and examining a cross section.
Disease cycle and Epidemiology:
- The disease is caused by the fungus Fusarium oxysporum Schlechtend f.sp. lycopersici.
- Mycelium is septate and hyaline at first becoming cream coloured with age, however, some isolates produce blue or red pigment.
- Micro conidia are ellipsoidal, straight to curved, one to two celled and hyaline.
- Macro conidia are hyaline, 3-5 septate, falcate having gradually pointed and curved ends and appear on sporodochia.
- Chlamydospores, both rough and smooth walled, characterized by thick walls are terminal or intercalary.
- They are often solitary but occasionally form in pairs or chains.
- Three physiological races viz., race 1, 2 and 3 have been reported world over.
- The pathogen is soil borne in nature and overwinters in the infected plant debris and in the soil as mycelium and spore forms especially as chlamydospores.
- It spreads over small distances by means of water and contaminated farm equipments, and over long distances, primarily in infected transplants or in the soil carried with them.
- Usually, once an area becomes infested with Fusarium, it remains so indefinitely.
- The optimum soil temperature for disease development varies from 25-31 oC.
- Hot dry weather favour wilt development.
- The disease is more severe in acidic soils (5.6 to 6.5 pH) than in alkaline ones.
- Presence of root knot nematodes and monoculture enhance the disease further.
- Low nitrogen and high potassium levels in soil predisposes the plant to this disease.
- Follow careful crop rotation and use of healthy seed.
- Avoid movement of water from infected to healthy plants.
- Combined application of inorganic fertilizers and organic manures is considered effective in reducing disease incidence.
- Application of phosphate fertilizers and nitrate fertilizers help in reducing the wilt incidence whereas ammonium fertilizers increase disease development.
- Treat the seed with carbendazim (0.2%).
- Drench the affected plants with carbendazim/ benomyl (0.1%).
- Pseudomonas isolates from rhizosphere of different plants can also reduce infection.
Last modified: Friday, 2 March 2012, 6:15 AM