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

BACTERIAL AND VIRAL DISEASES

A) BACTERIAL DISEASES

1. CROWN GALL
Symptoms:

• The disease is initiated with formation of galls on the stems and roots.
• Galls first appear as small protuberances on the plant surface and may eventually enlarge to 10 cm or more in size and are commonly found on crown portion (Plate-5a).
• These are soft at first, but later may become hard and woody.
• Plant growth slows down and in succeeding year many plants die.
• Sometimes aerial galls are produced on the stem, leaf petioles and cut ends of stem where flowers had been removed. Plate 5 b&c

Pathogen:

• Bacterium Agrobacterium tumefaciens (Smith & Townsend) Conn. is responsible for this disease.
• The bacterium is aerobic, gram negative, rod shaped and non-sporulating measuring 0.4-0.8 x 1.0 -3.0 µm.
• They are motile and have 1 to 5 lateral flagella, but non-motile variants are also in existence.
• The colonies are convex, circular, glistening and translucent.

Disease cycle and Epidemiology:

• The bacterium persists saprophytically in the soil for long periods in the absence of the host plants.
• Latent infections can also be a means of dissemination.
• It can multiply and spread rapidly at an optimum temperature of 25-30^oC.
• The bacterium enters the plant through wounds produced after pruning or during cultivation or insect damage.
• Disease spread through infected plants, movement of contaminated soil, water, tools etc.
• Overhead sprinklers and insects are major causes of extensive aerial gall formation.

Management:

• Integration of practices like chemical treatment, selection of clean stock, sanitation, biocontrol methods judiciously can reduce disease.
• Heating of soil to a temperature of 60^oC for 30 minutes in glasshouse by following steaming or aerated steam is useful.
• Tools used for pruning, cuttings or cultivation should be thoroughly washed and disinfected at frequent intervals and care should be taken to minimize wounding during cultivation.
• Gallex or Bacticin (0.1%), copper oxychloride (0.3%), copper sulphate (0.2%) can be applied on wounded areas.
• Biocontrol agent Agrobacterium radiobacter (K-84), a non pathogenic strain can be applied as seed or pre-plant dip for minimizing the disease spread.

B) VIRAL DISEASES

6. ROSE MOSAIC
Symptoms:

• Light green to yellow mottle patterns appear on the leaves (Plate-6).
• The leaf blade around the chlorotic area is sometimes more or less distorted.
• The chlorosis seen in unorganized line pattern or blotches.
• Blossom appears dwarf and often more pale coloured than normal.
• Stunting growth, smaller leaves, bending at the tips, blind shoots on which no terminal blossoms are produced may also considered as a part of rose mosaic syndrome.

Pathogen:

• Rose Mosaic Virus.
• The virus is isometrical partical.

Disease cycle and epidemiology:

• The virus is transmitted mechanically by budding or grafting and nematode.

Management:

• Discarding of infected plants, thermotherapy and indexing on selective indicator plants to eliminate virus from infected material used for propagation are effective strategies for the management of this disease.
• Inactivate the virus at 38^oC by growing plants in chambers for 4 hrs.
• Disinfect tools.

Other minor diseases:

i. Bacterial spot and blight: Pseudomonas syringae
ii. Rose streak: Rose Streak Virus
iii. Rose ring: Rose Ring Virus
iv. Tobacco streak: Tobacco Streak Virus
v. Rose mottled: Rose Mottled Virus
vi. Rose yellow mosaic: Rose Yellow Mosaic Virus
vii. Rose rosette: Rose Rosette Virus
viii. Strawberry latent ringspot: Strawberry Latent Ringspot Virus
ix. Rose spring dwarf: Rose Spring Dwarf Virus
x. Rose colour break: Rose Colour Break Virus