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

BACTERIAL AND VIRAL DISEASES

A) BACTERIAL DISEASES

1. BROWN ROT
Symptoms: Plate description

• Both above and underground plant parts are affected.
• There is a sudden wilting and death of the infected plants or collapse of one or more branches is observed.
• Wilting appears during July in the hills about two weeks after the onset of monsoon rains (Plate 1a).
• Many wilted plants also show stem rot at soil level.
• Greyish brown discolouration appears through the stem of affected tubers (Plate 1b).
• Cross-section of such tubers reveals a distinct brown discolouration in the vascular ring.
• A slight pressure on the cut tuber causes oozing of typical grayish white bacterial slime out of the vascular ring.
• Vascular browning is a characteristic symptom of the disease.
• In North-eastern hills, lenticels infection of potato tubers in the form of water-soaked lesions is quite common.

Pathogen:

• The bacterium associated with this disease is Ralstonia solanacearum (Smith) Yabuuchi et al.
• It is a rod shaped, Gram negative bacterium measuring 0.5-0.6 x 0.8-1.2 µm and having 1-4 polar flagella.
• Three races of the pathogen have been identified and potato is affected by race 1 and 3.

Disease cycle and epidemiology:

• R. solanacearum is both soil and seed borne and can survive in infected crop debris, potato tubers, wild host plants and weeds.
• Infested soil and infected seed tubers serve as sources of primary inoculum.
• Bacterial masses released in the soil by brown rot affected plant parts help in the survival of this bacterium.
• This bacterium survives in the soil for extended periods and enters the roots through wounds made by transplanting, cultivation or insects and through natural wounds where secondary roots emerge.
• A high temperature regime (28-30^o C) and relatively high soil moisture are favourable for wilt development.
• The bacteria multiply rapidly inside the water-conducting tissue of the plant, filling it with slime.
• This results in a rapid wilt of the plant, while the leaves stay green.
• If an infected stem is cut crosswise, it will look brown and tiny drops of yellowish ooze may be visible.

Management:

• It is difficult disease to manage as the commercial cultivars are highly susceptible and chemical control is not feasible.
• The reduction of the initial inoculums is of prime importance to manage bacterial wilt.
• Use of disease free seed is important for avoiding introduction of this disease to new fields.
• In disease prone areas, plant whole tuber.
• Follow wheat-maize rotation at least for 2-3 years.
• Application of stable bleaching powder @ 12 kg/ ha has been found to reduce bacterial wilt by 80 per cent when applied in furrows at the time of planting.

2. COMMON SCAB
Symptoms:

• Symptoms of scab are generally seen on tubers and are categorized as shallow and deep pitted.
• In shallow scab, the affected tubers show superficial roughened areas, sometimes raised about but often slightly below the skin of the tubers.
• The lesions of scab consist of corky tissue which is the result of abnormal proliferation of the cells of tuber epiderm due to invasion by the pathogen.
• The lesions may vary in shape and size and the color is brown.
• In deep pitted scab, the lesions are dark brown or almost black and measure 3-4 mm or more in depth surrounded by hard corky tissue.
• They may join together involving entire surface of the tuber.
• Quite often, multiple types of symptoms such as slightly brownish roughening of tuber skin, proliferated lenticels with hard corky deposition, concentric series of wrinkled layers of cork around central black core, raised rough and corky pustules and deep pits surrounded by hard corky tissue are produced.
• Russet scab in the form of superficial encrustation is also becoming quite common in the state.

Pathogen:

• The disease is caused by a bacterium Streptomyces scabies (Thaxter) Waksman and Henrici.
• The mycelium of the pathogen is slender, branched with few or no cross walls.
• The spores are cylindrical or ellipsoid produced on special hyphae that develop cross walls from tip and spores are pinched off.
• The S. scabies is gram positive and aerobic bacteria.

Disease cycle and epidemiology

• Streptomyces scabies can survive in soil for several years under favorable conditions.
• The infected tubers play a major role for its survival from one season to the other.
• The diseased plant debris and infested soil help in survival of the pathogen and also in initiating disease in succeeding crop of potato.
• The pathogen can be distributed through movement of soil by wind, water and cultural operations.
• Various soil factors influence the severity of the disease.
• Streptomyces scabies is active in the pH range of 5.2-8.0 and the severity of disease increases with the increase in pH.
• A temperature range of 20-30⁰ C is most suitable for the growth of the organism.
• Optimum temperature for infection is slightly below 20⁰ C and for lesion development slightly above 20⁰ C .
• The pathogen is active in dry soils and is suppressed by watering.

Management:

• As the disease is both tuber and soil borne, it is difficult to control it.
• However, some management practices have been adopted to check this disease.
• Healthy and blemish free seed tubers should be selected for sowing in order to reduce the primary source of inoculum.
• Green manuring and cultivation of certain legumes before planting potato has been emphasized as important cultural practices in controlling common scab.
• Green manuring probably increases the activity of certain actinomycetes and other bacteria which are antagonistic to S. scabies.
• Soil moisture is important factors in the development of disease, it has been exploited for minimizing the losses.
• Successful control of potato scab can be achieved by frequent irrigations of the field at weekly intervals from tuberisation until maturity.
• The tubers can be disinfected by dipping them for 10 minutes in Emisan (0.25%).

B) VIRAL DISEASES

1. LEAF ROLL VIRUS (POTATO LEAF ROLL VIRUS)
Symptoms

• There is rolling of upper leaves.
• Infected leaves remain upright and turn pale yellow in colour.
• Plants produced from infected tubers show rolling of lower leaves and stunting of plants with upright habit (Plate 1).
• Rolled leaves are leathery, stiff and brittle. Marginal interveinal chlorosis of leaflets is visible on upper leaves.

Disease cycle and epidemiology:

• Potato leaf roll virus is transmitted through infected tubers and spreads through several species of aphids.
• It is more severe in warm climate.

2. MOSAIC (POTATO VIRUS X, POTATO VIRUS Y AND POTATO VIRUS S)
Symptoms:

• PVX induces interveinal mild mosaic symptoms with light and dark green patches, mottling with stunting and often crinkling with more virulent strains.
• There is considerable dwarfing of plants with reduction of leaflet size.
• PVY induces severe or rugose mosaic.
• Veinal necrosis may also occur and plant remain stunted.
• Severe mosaic causes rugosity, bunching or twisting of leaves.
• Symptoms caused by PVS are barely perceptible as mottle or faint vein bending.

Disease cycle and epidemiology:

• Mosaic viruses are transmitted by infected tubers, cutting knife and aphid vectors names .
• Many plants of solanaceae, chenopodiaceae and leguminoseae act as alternate and collateral host hosts of this disease.

Management:

• Always use virus free certified seed from reliable sources.
• Rouge out infected plants as soon as these are first noticed in early crop period.
• Keep the aphid population under check by spraying insecticides like Rogor or Metasystox @ 250 ml/acre or apply 5 kg of Thimet 10G/acre at first earthing up.
• Cut the haulms in 4th week of December when aphid infestation starts (20 aphid /100 leaves) and leave the tubers in soil to mature till the end of February.