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Disease Management

Diseases

1. Damping off (Pythium, Fusarium, Rhizoctonia and Phytophthora spp.)

Symptoms

  • Seed may rot before emergence.
  • Seedlings will show wilt and ultimately collapse.
  • Infected cotyledons will be soft and show rotting and brown discoloration.

Management

  • Seed treatment with any bio-control agent like Trichoderma or Pseudomonas.
  • Crop rotation.
  • Application of FYM.
  • Provision of drainage.
  • Application of Bordeaux mixture @ 1 per cent.

2. Phytophthora root rot (Phytophthora sojae)
Incidence: Phytophthora root rot is a potential problem in heavy clayey soils.


Symptoms

  • It causes damping off in the early stages of crop growth.
  • Water-soaked lesions will appear on the seedlings.
  • The seedlings fail to emerge or get killed after emergence.
  • On the stem a purple or brown discoloration may extend from the roots to the lower nodes of the infected plant.
  • Wilting of the plants and leaves remain attached to the plant even after death.
  • On pulling the infected plant may easily will come out.

Epidemiology

  • Cool and wet weather.
  • Heavy clayey and poorly drained soils favour the incidence of this disease.

Management

  • Crop rotation with non-host crops like maize and rice.
  • Application of well-decomposed manure.
  • Balanced application of nutrients.
  • Field sanitation.
  • Avoid water-logging in the field.

Soybean (Glycine max) (L.) Merr.
3. Rhizoctonia root rot (Rhizoctonia solani)
It is most prevalent on seedlings and young plants, causing root and stem rot, particularly during prolonged wet
periods.


Symptoms

  • Causes pre- and post-emergence damping off.
  • It produces the characteristic reddish brown discoloration on the stem near the soil line.
  • The disintegration of the stem causes the plant to topple down.
  • It also causes wilting and yellowing of the leaves.
  • The disease appears as a patch in the field.

Disease cycle

  • The pathogen is a soil inhabitant.
  • Primary source of infection is the resting mycelium or sclerotia surviving in the soil.

Epidemiology

  • Rhizoctonia root rot is most damaging when cool, wet conditions in the spring are followed by hot (25-29°C), dry conditions.

Management

  • Crop rotation with maize and rice.
  • Seed and soil treatment with biological control agents like Trichoderma viride or Trichoderma harzianum @ 4 gm
  • per kg of seeds and 2.5 kg mixed with 50 kg of sand or FYM, respectively.
  • Application of well-rotted FYM.
  • Provision of good soil drainage.
  • Planting in cool and wet conditions should be avoided.

4. Rust (Phakospora pachyrhizi)


Symptoms

  • Appearance of dark brown or reddish brown pustules on the ventral
  • side of the leaves and also on the petioles, pods and stems.
  • The infected plant will show yellow and mottled appearance.
  • Under severe infection, the leaves will become yellow and fall off.

Epidemiology
It occurs during July to September.

  • Long periods of leaf wetness.
  • Temperatures between 15 and 30°C and high relative humidity favour the disease incidence.

Management

  • Adjustment of sowing time.
  • Application of wettable sulphur @ 0.3 per cent.
  • Growing resistant varieties like PK-73-84, PK-310, IC-89495, IC-89498, etc.

5. Bacterial blight (Pseudomonas syringae pv. glycinea)
Handbook of Organic Crop Production in Sikkim
It occurs during August to September.


Symptoms

  • The symptoms on the leaves will appear as red or black lesions with yellow halo.
  • The infected leaves also show ragged appearance.
  • Infected seed will have water-soaked discoloration at the starting point of the hilum.

Disease cycle

  • The pathogen will survive on the infected seeds and crop residue.
  • The pathogen is spread by rain splash, wind and injury caused by insects and hailstorm.
  • Management
  • Crop rotation with non-host crops like maize and rice.
  • Removal of crop residue.
  • The plants should not be disturbed when the field is wet due to rain or dew.

6. Frog-eye leaf spot (Cercospora sojina)


Symptoms

  • A small circular or sometimes irregular spots with a light brown or light grey centre on the leaves is seen.
  • Initially the spots are dark and show water-soaked appearance.
  • Later the leaves coalesce and occupy larger areas.
  • The severely infected areas may show ragged appearance with shot
  • hole like symptoms.
  • The lesions may also appear on the stem and pods.

Disease cycle
The pathogen overwinters in infected crop residue.
Epidemiology
It occurs during July to August.
The disease is favoured by warm and humid climate.


Management

  • Crop rotation with non-hosts, such as maize or wheat.
  • Use of non-infected seed.
  • Use of resistant variety.

7. Soybean mosaic


It occurs during July to September.


Symptoms

  • Yellow to green mottling and mosaic pattern on the leaves.
  • Puckering, distortion and curling of leaves along the margin.
  • Stunting of plants.

Soybean (Glycine max) (L.) Merr.


Disease cycle

  • This virus is sap transmissible and spread by many species of aphid vectors.
  • The virus is also seed-borne.

Management

  • Plant disease-free seeds.
  • Grow disease-resistant varieties.
  • Control aphids with neem oil 3 per cent, NSKE 5 per cent or petroleum oil-based spray @ 0.7 per cent.
  • Rogue out infected plants.
  • Control of weeds

8. Powdery mildew (Microsphera diffusa)


Symptoms

  • Appearance of powdery coating on the leaves and stem branches,
  • pod and petioles.
  • Severely infected leaf will become yellow and drop off from the plant.


Disease cycle

  • The clestothesia and ascospores surviving in the soil acts as a primary source of infection.
  • The secondary infection spreads through the air-borne conidia formed on the infected plants.

Epidemiology

  • Cool and dry weather favors the disease development.

Management

  • Crop rotation with non-host crops like maize and rice.
  • Removal of the crop residue.
  • Application of wettable sulphur @ 0.2 per cent.
  • Application of sulphur dust.

9. Downey mildew (Peronospora manshurica)


Symptoms

  • Initially the symptoms appear as pale green or light yellow spot on the upper surface of the leaves.
  • Later the spots with pale or dark yellow will occupy the larger areas.
  • During the period of heavy dew or rain the grey or purple fuzzy growth will be visible on the lower side of the leaves.


Disease cycle

  • Oospores surviving on the crop residue and seed act as a primary source of inoculum.
  • The secondary spread is through the conidia formed on the infected plant.

Epidemiology

  • Low temperature and cool weather favor the disease development.

Management

  • Removal of infected crop debris.
  • Crop rotation with non-host crops like maize and rice.
  • Treat the seeds with Trichoderma viride @ 4 gm/kg of seeds.