Diseases of Vegetable, Ornamental and Spice Crops (2+1)

FUSARIUM WILT

Symptoms:

• 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.

Pathogen:

• 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.

Disease cycle and Epidemiology:

• 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.

Management

• 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.