Temperate Vegetables (1+1)

DISEASES

Fungal Diseases
Downy mildew: (Peronospora destructor)

• The first sign of the disease is the formation of elongated patches on leaves varying in size.
• These areas become covered with white to purplish fruiting bodies.
• Often, leaves fold over the affected area and the leaf tips wither away.
• Bulbs do not attain full growth and are often soft and immature.
• Cool temperature and presence of water is required for disease initiation.

Control measures

• Before sowing, treat the bulbs with mancozeb (0.25 %) followed by spray with the same fungicide at the same concentration.
• Regular sprays of metalaxyl + mancozeb (0.25%) checks this disease.

Onion smut : (Urocystis cepulae)

• This disease is more prevalent in temperate regions.
• The fungus survives in the soil.
• Dark brown streaks are seen on the leaves and stem.
• The bending and twisting of earlier infected leaves also occur.
• The linear black lesions most commonly appear near the base of the bulb and grow up to the fourth scale deep.

Control measures:

• Seed treatment with thiram or captan (3g/kg seed) checks seed borne infection.
• Also treat the soil with formaldehyde (1:7) before sowing.

Purple blotch : (Alternaria porri)

• It is very destructive under favourable conditions.
• The first spot usually appears on the oldest leaves.
• In the beginning they are small, elongated, sunken and whitish, generally with purple centre.
• These blotches later on enlarge, coalesce and are covered with the black fruiting bodies.

Control measures

• Seed treatments with captan or thiram (2.5g/kg seed), proper crop rotation, adequate drainage in the fields and fungicidal application of mancozeb (0.25%) have been found very effective.

Black mould: (Aspergillus niger)

• The infection starts from the top or any other injured portion.
• The affected tissue becomes water soaked and at first a white mould develops between the scales.
• This follows development of black spores on the stalk which can be seen with naked eyes and the mass remains on exterior of the scales and can easily be rubbed off.

Control measures

• Avoid bulb injury during various operations and sort out any bulb showing wound, green and thick neck, doubles etc.
• Allow only perfect bulbs for storage.
• Also clean the stores properly and check for sufficient ventilation, and temperature should be below 15^oC.

Stemphylium leaf spot: (Stemphylium vesicarium)

• Purple coloured long spots appear on stems and leaves.

Control measures

• Spray of metalaxyl + mancozeb (0.25%) as soon as the symptoms appear.
• Repeat after 15-20 days if needed

Bacterial diseases
Soft rot: (Erwinia carotovora pv. carotovora)

• The disease is characterized by water soaked soft rot on inner scales of bulbs.
• Sometimes bulb appears healthy from outside but if cut open, 1-2 scales are found infected.

Control measures

• The disease can be managed by applying streptocycline (200 ppm), and the use of resistant varieties.

Viral disease
Onion yellow dwarf virus

• Short yellow streaks appear at the base of the first leaf emerging through the neck of the bulb.
• The leaves fall over and present an abnormal appearance.
• Flower stalks of the infected plant show yellow streaks extending upwards from the base.
• Later the streaks coalesce.
• The stalk becomes yellow throughout, and get twisted and curled in a characteristic manner.
• The leaves turn yellow, become crinkled and drop underdeveloped, although they are usually well shaped.

Control measures

• Indexing of virus free material, production of virus free stock of bulbs in areas where disease is absent and roguing out infected and volunteer onion plants will control the disease.

Pests
Onion thrips: (Thrips tabaci)

• Affected leaves show silvery white blotches which later become brownish.
• Spring summer crop is affected much by thrips.
• A long spell of dry weather is favourable for its rapid multiplication.

Control measures

• Collect the debris, affected leaves, weeds and destroy them.
• Follow crop rotation.
• Grow resistant varieties like Nasik Red and Spanish White.
• Apply malathion (0.05%) or phorate 10 G (1 kg /ha).

Borer: (Helicoverpa armigera)

• Larvae attack leaves, flower and flower stalks.

Control measures

• Collect and destroy affected flower and flower stalk and apply deltamethrin (0.0025 %).
  

Onion maggots: (Hylemyia antiqua)

• It attacks the tender portion of the bulb.
• They remain hiding in the base of the plants or in the cracks of the soil, where they also lay eggs.
• Affected plants become yellow to brown.

Control measures

• Soil application of phorate10G (25 kg/ha) and spray of malathion (0.05 %) can be done.
• Follow crop rotation.

Mites

• These are very small pests and remain mostly on the under surface of the leaves.
• Affected plants become pale yellow.
• Crop infested with mites gives sickly appearance.

Control measures

• Expose infected bulbs to sun for about two days and dust the crop with malathion dust (20-25 kg/ha).

Bulb or stem nematode: (Ditylenchus dipsaci)

• It is one of the important seed borne nematodes.
• Seedlings become pale white, twisted and stunted in growth.
• Leaves become thick and swell, resulting in splitting of the epidermis.
• Bulbs also carry nematodes.
• Tips of the leaves become necrotic.
• Cracks appear in the bulb.

Control measures

• Provide good drainage, obtain and use healthy seeds, follow long crop rotation and sow resistant varieties.

PHYSIOLOGICAL DISORDERS
Bolting

• It means emergence of seed stalk prior to time of bulb formation and adversely affects the formation and development of bulbs.

Possible Reasons

• Transplanting of aged seedlings
• Early sowing of seeds in the nursery beds, which result in the formation of small sets.
• Late transplanting of seedlings
• Restricted or poor vegetative growth also leads to bolting.
• Sharp fluctuations in temperatures at bulb initiation stage.
• Low temperature (10-12^oC) for prolonged period.
• Poor supply of nitrogen in nursery and field.

Correction

• Time of planting should be adjusted in such a way that the crop may expose to moderate temperature at bulbing.
• Sowing of nursery at proper time
• Transplant healthy and 6-7 weeks old seedlings.
• Supply recommended dose of nitrogen.

Sprouting

• An important disorder in storage of onion and garlic and results in huge losses.
• It is associated with excessive moisture at maturity and supply of nitrogen.

Correction:

• Adjust time of planting in such a way that harvesting can be done in dry period.
• Stop irrigation as soon as bulbs reach maturity.
• Spray iron sulphate or borax @ 500-1000 ppm 2-3 weeks prior to harvesting.