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

BACTERIAL AND VIRAL DISEASES

B) BACTERIAL DISEASES
6. BACTERIAL BLIGHT

It is an important disease of pea in those areas where cool and moist climate is prevalent during the growing season.

Symptoms:

• All aerial parts of the plants are infected by the disease.
• Symptoms of leaflets and stipules appear as small water soaked lesions usually developing into large irregular areas (Table-9).
• Lesions vary in colour, initially being dark green and water soaked, but generally becoming water soaked at the edge and a lighter shade of brown at centre.
• Under conditions of high moisture, chocolate brown linear streaks are observed on the stem and petiole.
• Later on, the whole stem turns chocolate brown and is shriveled leading to the death of the plants.
• Due to infection, immature pods become chocolate brown, thin, twisted and shriveled, lesions are bigger on older pods.
• Seeds developing in the pods are also discoloured and shriveled.

Pathogen:

• The disease is caused by bacterium Pseudomonas syringae pv. syringae van Hall and P. syringae pv. pisi (Sackett) Young et al.
• The bacterium is gram -ve, non-spore forming and motile rods with one or more polar flagella.

Disease cycle and epidemiology:

• The pathogen overwinters in the infected seed.
• The seed carries the bacterium both externally and internally.
• It colonizes the intercellular and intracellular spaces of the seed coat but does not penetrate the embryo or cotyledons.
• Even a very low level of seed infection can cause economic loss, since the disease can spread fast from primary infection foci.
• The pathogen is carried in irrigation water, splashed by rain or blown in wet winds to other plants and field infection usually occurs through stomata and wounds.
• Cool, moist weather favours the disease while warm and dry weather retards it.
• The extent of disease spread depends upon the frequency of rainy periods.
• If soil is very wet at the time of emergence, it also favours blight transmission from the infected seed to resulting plant.

Management:

• Collect and destroy the infected plant debris.
• Follow at least three years crop rotation with non leguminous crops.
• Use disease free seed.
• Give pre-sowing seed dip treatment in Streptocycline (150 ppm) solution for 90 minutes.
• With the initiation of the disease, spray the crop with Streptocycline (100 ppm) and repeat at 7 days interval.

C) VIRAL DISEASES
7. PEA SEED BORNE MOSAIC
Symptoms:

• The symptoms of the disease include chlorotic pattern on the leaves followed by narrowing of leaves coupled with downward rolling and apical malformation.
• This is followed by vein clearing and production of malformed flowers.
• The size of pods is reduced which get distorted depending upon disease intensity.
• Such pods produce infected seeds which are shriveled.

Pathogen:

• The disease is caused by Pea Seed Borne Mosaic Virus which belongs to poty virus (PSbMV) group.
• The virus particles are filamentous and flexuous rods 770 x 12 nm in size and non-enveloped.
• The genome consists of single stranded RNA.

Disease cycle and epidemiology:

• The virus is transmitted mechanically, or by seed or by aphid vectors either semi-persistently or non-persistently.
• The natural vectors are Acyrthosiphon pisum, Aphis craccivora and A. fabae.
• The host range of the virus is wide but there are only three significant hosts like pea, lentil and broad bean with regard to economic importance, dissemination through seed.

Management:

• Use resistant pea cultivars.
• Use reflective mulches to reduce the incidence of this virus.
• Since seed borne viruses create a within field inoculum source, spray the crop with insecticides like Malathion (0.1%) or Metasystox (0.1%).

Other diseases of importance are:

i. Downy mildew : Peronospora pisi Syd.
ii. Root rot : Aphanomyces euteiches Drechs., Fusarium solani (Mart) Sacc. f. sp. pisi (Jones) Snyd. and Hans.
iii. Leaf spot : Cercospora lathyrina Ell & Ever., C. pisa sativae Stev., Septoria pisi West
iv. Anthracnose : Colletotrichum pisi Pat.
v. Seed and seedling blight : Pythium spp.