Important diseases of soybean, symptoms of damage and their management aspects are as follows:
1. Alternaria Leaf Spot (Alternaria tenuissima):
Symptoms:
Seed become small and shriveled. Dark, irregular, spreading sunken areas occur on the seed. Appearance of brown, necrotic spots with concentric rings on foliage, which form large necrotic areas. Infected leaves later in the season dry out and drop prematurely.
Management:
Use healthy/certified seeds. Destroy crop residues from fields. Treat the seeds with thiram + carbendazium (2:1) @ 3g/kg seed. Spraying of mancozeb at 2.5g/l or carbendazim 1 g/l of water.
2. Anthracnose/Pod Blight (Colletotrichum truncatum):
Symptoms:
Infected seeds become shriveled, mouldy and brown. Symptoms on cotyledons appear as dark brown sunken cankers. In early stage, irregular brown lesions appear on leaves, stems and pods. In advanced stages, the infected tissues are covered with black fruiting bodies of fungus. Under high humidity, symptoms on leaves are veinal necrosis, leaf rolling, and cankers on petioles premature defoliation.
Management:
Use healthy or certified seeds. Rotate soybean with cereals. Do not cultivate when the foliage is wet. Completely remove plant residue by clean ploughing the field soon after harvest. Destroy last years infected stubble. Maintain well drained field. Seed treatment with thiram or carbendazim 3 g/kg and spray mancozeb @ 2.5g/l as spray or carbendazim 1g/l of water.
3. Bacterial Blight (Pseudomonas syringae pv. Glycinea):
Symptoms:
Seeds may develop raised or sunken lesions and become shriveled and discolored. Small, angular, translucent, water-soaked, yellow to light brown spots appear on leaves. Young leaves are most infected and are destroyed, stunted and chlorotic. Angular lesions enlarge and merge to produce large, irregular dead areas. Early defoliation of lower leaves may occur. Large, black lesions develop on stems and petioles.
Management:
Deep summer ploughing. Use healthy/certified seeds. Destroy infected crop debris. Seed treatment with streptocyclin @ 250 ppm (2.5 g/10 kg seeds). Application of any copper fungicides @ 2 g/l along with streptocyclin at the rate of 250 ppm (2.5 g/10 lit water).
4. Cercospora Leaf Blight, Leaf Spot and Purple Seed Stain (Cercospora kikuchii):
Symptom:
Infected leaves appear leathery, dark, reddish purple. Severe infection cause rapid chlorosis and necrosis of leaf tissues, resulting in defoliation. Lesions on petioles and stems are slightly sunken, reddish purple; severe cause defoliation. Later, blighting of young, upper leaves over large areas, even entire fields occur.
Management:
Use healthy/certified seeds. Previous crop debris should be removed. Seed treatment with thiram + carbendazium (2:1) @ 3g/kg seed. Apply mancozeb or copper oxychloride at 2.5g/l or carbendazim 1 g/l of water.
5. Charcoal Rot or Stem Blight or Dry Root Rot (Macrophomina phaseolina):
Symptom:
This disease occurs when the plants are under moisture stress or under nematode attack or through soil compaction or may be through nutrient deficiencies. It is a most common basal stem and root disease of the soybean plant. Lower leaves become chlorotic and wilting and drying is apparent. The diseased tissues generally develop grayish discolouration.
The sclerotia look like black powdery mass hence the disease is known as charcoal rot. Blacking and cracking of roots is the most common symptom. The fungus survives in soil and crop debris in dry conditions. Dry conditions, relatively low soil moisture and nutrient and temperature ranging from 25° C to 35° C are favourable for the disease.
Management:
Follow deep ploughing in summer. Ensure balanced fertilization of the crop. Rotate soybean with cereals. Maintain well drained field. Destroy last years infected stubbles. Seed treatment with T. viride @4g/kg or P. fluorescens @ 10g/kg of seed or carbendazim or thiram 2g/kg of seed. Spot drenching with carbendazim 1g/lit or P. fluorescens/T. viride 2.5 kg/ha with 50 kg FYM.
6. Collar Rot/Sclerotial Blight (Sclerotium rolfsii):
Symptom:
Infection usually occurs at or just below the soil surface. Sudden yellowing or wilting of plants is the first symptom. Light brown lesions, which quickly darken, enlarge until the hypocotyl or stem is girdled. Leaves turn brown, dry and often cling to dead stem. Numerous tan to brown, spherical sclerotia form on infected plant material.
Management:
Deep ploughing in summer. Adopt crop rotation with maize or sorghum. Destroy infected stubbles. Seed treatment with T. viride @ 4g/kg or P. fluorescens @ 10g/kg of seed or carbendazim or thiram 2g/kg of seed. Spot drenching with carbendazim 1g/lit or P. fluorescens/T. viride 2.5 kg/ha with 50 kg FYM.
7. Frog Eye Leaf Spot (Cercospora sojina):
Symptom:
Light to dark gray or brown areas varying from specks to large blotches appears on seed. The disease primarily affects foliage, but, stems, pods and seeds may also be infected. Leaf lesions are circular or angular, at first brown then light brown to ash grey with dark margins. The leaf spot may coalesce to form larger spots. When lesions are numerous the leaves wither and drop prematurely. Lesions on pods are circular to elongate, light sunken and reddish brown.
Management:
Use healthy and certified seeds. Rotate soybean with cereals. Completely remove plant residue by clean ploughing the field soon after harvest. Seed treatment with thiram + carbendazim (2:1) @ 3g/kg seed. Spray mancozeb @ 2.5g/lit or carbendazim 1g/lit.
8. Rhizoctonia Aerial Blight/Web Blight (Rhizoctonia solani):
Symptom:
Infected seeds have irregularly shaped tan or light brown sunken lesions. Infected leaves appear as water soaked at first instance. They soon take on a greenish brown to reddish brown appearance. The infected portion later turns tan brown or black in colour.
Under high rainfall or high humid conditions, a web like mycelial growth of fungus forms on the leaves. Dark brown sclerotia are formed on leaves and petioles. The pathogen survives as sclerotia in soil. Humid and cool weather conditions (24-32°C) are favourable for spread of diseases.
Management:
Avoid dense planting. Completely cover plant residues by clean ploughing the field soon after harvest. Destroy infected stubble. Seed treatment with thiram + carbendazium (2:1) @ 3g/kg seed. Apply mancozeb 2.5g/l or carbendazim 1 g/lit.
9. Yellow Mosaic – Mung Bean Yellow Mosaic Virus (MYMV):
Symptom:
It is a viral disease and transmitted by white fly (Bemisia tabaci). The disease is dangerous and widely spreaded. Characteristic symptom is conspicuous systemic bright yellow mottling of leaves. The yellow area are scattered or occur in indefinite bands along the major veins. Rusty necrotic spots appear in the yellow areas as the leaves mature.
Management:
Grow resistant varieties to yellow mosaic. Grow seven rows of sorghum as border crop. Treat seeds with Imidacloprid 70 WS @ 5ml/kg to control vector. Rogue out MYMV infected plants early in the season to eliminate the source of inoculum. Give one foliar spray of systemic insecticide (Dimethoate @ 750 ml/ha) on 30 days after sowing.