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

Bacterial Diseases

6. Bacterial spot

The disease affects bell pepper/ chillies and tomato. Bacterial spot of capsicum was first reported from Florida, USA by Sherbakoff (1918), but in India, disease was observed for the first time at Poona during 1948.

Symptoms:

• On leaves, initially the lesions are water-soaked, circular or irregular that becomes necrotic with brown centers and thin chlorotic borders (Plate-7a).
• These lesions are generally sunken on the upper surface of the leaves and slightly raised on the bottom.
• In favourable environmental conditions, these spots coalesce and give a blighted appearance and such leaves turn yellow and fall down prematurely.
• Lesions on the stem are narrow, elongated and raised.
• Lesions on fruit initially begin as green spots, which enlarge and later become brown in colour (Plate-7b).
• These spots are raised with a cracked, roughened wart like appearance.
• During periods of high humidity, fruit around the lesions may start rotting.

Pathogen:

• The bacterium responsible for this disease is Xanthomonas campestris pv. vesicatoria (ex Doidge) Vauterin et al.
• The bacterium is motile, strictly aerobic, Gram -ve, rod shaped with round ends, non-spore forming and possesses a single pollar flagellum (Monotrichous).
• Bacteria occur singly but occasionally in pairs and rarely in short chains.
• Its colonies on nutrient agar or on specific medium yeast dextrose agar (YDA) are lemon yellow, slightly convex, wet and shining.

Disease cycle and epidemiology:

• The bacterium persists in infected seeds and plant debris.
• Moderate temperatures along with high precipitation and relative humidity favour disease development.
• Infection of the plants can take place at a wide range of temperature i.e. 15 to 35 o C.
• The optimum temperature lies between 22 to 34^o C and maximum disease develops in between July and September.
• The bacterium is disseminated within a field by wind driven rain droplets, clipping of transplants and aerosols.

Management:

• Since the disease development requires high precipitation and high relative humidity, fields should be well drained, free of low-lying areas to minimize water-logged conditions.
• The field should be rotated with non-solanaceous crops and bell pepper/ chillies should not be rotated with tomato.
• Use of disease free seeds also reduce the immediate availability of primary inoculum and ensures disease free transplants.
• Sprinkler or overhead irrigation of the field should be limited to keep the disease under check.
• Treat the seeds by dipping in Streptocycline (100 ppm) for 30 minutes. Spray the crop with Streptocycline (100 ppm) with the initiation of the disease.

7. Bacterial wilt

• It is an important disease of this crop. Symptoms, pathogen, disease cycle and epidemiology and management are same as described in tomato.