Diseases of Vegetable, Ornamental and Spice Crops

BACTERIAL DISEASES

1. BLACK ROT
Symptoms:

• Initial symptoms of the disease appear as chlorotic lesions along the margins of leaves which progress in the direction of midrib forming “V”-shaped lesions (Plate-5).
• The veins and veinlets in the chlorotic area turn black and with the passage of time, the blackening of veins advances to the stem and from there to other leaves and roots.
• The stem and stalk of infected leaves show blackening of vascular tissues.
• Due to the systemic infection, black spots appear on flower stalk and siliques.
• The heads of cabbage and curds of cauliflower are also invaded and become discoloured. The roots of radish and turnip are also invaded from leaves which show discolouration and internal breakdown.
• The infection of this pathogen may be followed by attacks of soft rot bacteria or Sclerotinia sclerotiorum.

Pathogen:

• The disease is caused by Xanthomonas campestris pv. campestris (Pammel) Dowson.
• It is a small, rod shaped, aerobic, gram negative, non-spore forming bacterium.
• The bacterium has a single polar flagellum (monotrichos) and it is catalase positive, hydrogen sulphide positive, oxidase negative and does not produce nitrate or indole.
• It produces a yellowish extracellular polysaccharide (EPS) called Xanthan on media containing glucose.
• X. c. pv. campestris hydrolyses starch, a characteristic that is used for easy recognition of the bacterium on Schaad’s selective medium.
• Its growth is inhibited or retarded in acidic range of pH.

Disease cycle and epidemiology:

• The pathogen survives in infected seeds, diseased plant debris and on cruciferous weeds (Fig.4.).
• When the infected seed germinates, the seed coat that comes out along with the cotyledons serves as the source of primary inoculum.
• The bacterium enters cotyledons through the marginal stomata and passes into true leaves through hydathodes.
• On entry, the bacteria move to the xylem vessels where they multiply rapidly and move up and down in the plant.
• When the disease inoculum is present in the field, secondary spread is facilitated by splashing of contaminated soil by irrigation water and/ or rains, blowing of fallen leaves and occasionally by insects.
• The optimum temperature for the disease development is about 26.5 to 30^oC, minimum being 5^oC and maximum 36^oC.
• Heavy rains have been reported to be responsible for the fast spread of the pathogen and the disease through splashes.

Management:

• Follow crop rotation with non-cruciferous crops for at least two years.
• Use disease free seeds and treat them either with hot water (52^o C) for 30 minutes followed by same duration dip in Streptocycline solution (100 ppm) or by dipping the seeds in Streptocycline (100 ppm) solution for 30 minutes to eradicate the pathogen.
• In disease prone areas, apply grass or pine needle mulch on the field floor.
• With the initiation of the disease, give fortnightly sprays of combination of Streptocycline (100 ppm) and copper oxyxhloride(0.3%) or mancozeb (0.25%).