Diseases of Vegetable, Ornamental and Spice Crops

BACTERIAL AND VIRAL DISEASES

BACTERIAL DISEASES

7. COMMON BLIGHT
Symptoms:

• The symptoms appear as characteristic water soaked spots on the under side of the leaves that enlarge and coalesce together to form large blighted areas.
• Infected areas often become flaccid and turn brown giving it a sunscald appearance.
• If infection occurs through the vascular tissues, reddish discolouration of veins and midribs is observed.
• In advanced stages, tissues become necrotic and defoliation may also take place.
• Lesions on stems appear as sunken water soaked areas, which gradually enlarge to form reddish streaks extending longitudinally in the stem.
• Infection on pods may occur at any place as small water soaked spots, which are surrounded by distinct zones of narrow reddish brown or brick red band of tissue.
• On seeds, discoloration appears especially near the hilum region.

Pathogen:

• The disease is caused by Xanthomonas axanopodis pv. phaseoli (Smith) Dye.
• The bacterium is a motile, non-capsulated, non-spore forming, gram -ve, rod shaped, measuring 1.9 x 0.8 µm in size with a single polar flagellum.
• The pathogen forms characteristic yellow coloured colonies due to a water insoluble eacotenoid and a mucoid growth on nutrient glucose agar medium.

Disease cycle and epidemiology:

• The bacterium perpetuate in infected seed, crop debris and weed hosts of which contaminated seed has been observed to be the most efficient mode of survival and dispersal over long distances.
• Upon germination, the bacterium from infected seed infects the developing cotyledons and can then enter the vascular system of the plant.
• Alternatively the bacterium can be splashed to the plants from the plant debris.
• Once inside the plant, the bacteria invade and cause breakdown of the middle lamella.
• The bacterium is a warm temperature pathogen and causes greatest damage to the plants at 28-32^o C.
• High humidity and high rainfall increases the disease spread.
• Secondary spread occurs by wind blown rain (rain splashes), contact between leaves, irrigation water and insects like white flies and leaf miners.

Management:

• Collect and burn the infected plant debris.
• Follow at least three years crop rotation.
• Use healthy seed and treat them with hot water at 50oC for ten minutes followed by diping in Streptocycline (100 ppm) solution.
• With the initiation of the disease spray the crop with copper oxychloride + zineb or copper oxychloride + mancozeb and repeat at 10 days interval.

VIRAL DISEASE

8. BEAN COMMON MOSAIC
Symptoms:

• The symptoms of the disease appear as mixture of light yellow and green areas on the leaves (Plate-10).
• This is coupled with production of narrower leaves which roll upward, mottle, pucker and become malformed.
• Early infection leads to yellow coloured dwarf plants and such plants usually fail to set pods or develop undersized pods.
• The pods on severely-infected plants are usually undersized and contain fewer ovules than those produced on normal plants.
• They are occasionally covered with small, dark green spots and often mature very late.
• The seed may be aborted, smaller or malformed.

Pathogen:

• The disease is caused by Bean Common Mosaic Virus (BCMV) which belongs to poty virus group.
• The virus particles are filamentous flexuous rods measuring 700-800 nm in length and 12-15 nm in diameter.
• The genome consists of unipartite single stranded DNA.

Disease cycle and epidemiology:

• The virus is seed borne in a range of legumes.
• The virus is also sap and graft transmissible.
• The virus can be transmitted in the non-persistent manner by several aphid species which do not normally colonize beans but transmit the virus as winged migrants, especially Acyrthiosiphon pisum, Aphis fabae and Myzus persicae.

Management:

• Use healthy and certified seed helps in checking the spread of the disease.
• Early planting when the incidence of aphid vectors is low gives better yields.
• Intercropping of maize with bean also reduces the incidence of this disease.