Symptoms
Soft rot is one of the most destructive diseases of ginger, affecting both growing crops and stored rhizomes. The initial symptoms appear as water-soaked lesions on the pseudo-stem near the collar region, which gradually spread upward and downward. The affected pseudo-stem becomes soft, discolored (yellowish to brown), and emits a foul odor characteristic of bacterial decay.
In the rhizomes, the infection begins at the collar region or through wounds and quickly spreads throughout the tissue. Infected rhizomes become soft, mushy, and emit a characteristic putrid smell. The internal tissues turn cream to yellowish-brown and eventually disintegrate into a slimy mass. Above-ground symptoms include yellowing of leaves (chlorosis), wilting, and drooping of the pseudo-stem. In severe cases, the entire plant collapses and can be easily pulled out from the soil.
Etiology
Causal Organism: Pythium aphanidermatum, Pythium myriotylum (primary), and bacterial pathogens including Ralstonia solanacearum and Erwinia species (secondary invaders)
Soft rot is primarily caused by oomycete pathogens of the genus Pythium, with Pythium aphanidermatum being the most common pathogen. These soil-borne pathogens thrive in warm, humid conditions with temperatures ranging from 28-35°C. The disease is often exacerbated by bacterial pathogens that invade as secondary colonizers, particularly Ralstonia solanacearum, which causes bacterial wilt, and various Erwinia species that accelerate tissue maceration.
The pathogens are favored by excessive soil moisture, poor drainage, waterlogged conditions, and high humidity. Heavy rainfall, flood irrigation, and clay soils with poor aeration create ideal conditions for disease development. Mechanical injuries to rhizomes during cultivation or harvesting provide entry points for pathogen invasion.
Disease Cycle
The soft rot pathogen survives between crop seasons as oospores in soil, infected plant debris, and contaminated rhizomes used as seed material. Pythium species can persist in soil for several years as dormant oospores, which are thick-walled resistant structures capable of withstanding adverse environmental conditions.
Primary infection occurs when healthy rhizomes come in contact with infested soil or when contaminated planting material is used. Zoospores, which are motile spores produced by the pathogen, swim through soil water films toward ginger rhizomes, attracted by root exudates. Upon reaching the host tissue, zoospores encyst, germinate, and penetrate through natural openings, wounds, or directly through the epidermis.
Once established, the pathogen spreads rapidly through the rhizome tissue, producing enzymes that break down cell walls and cause tissue maceration. Under favorable conditions (high temperature, moisture, and humidity), the disease spreads quickly from plant to plant through contaminated soil water, farm tools, and cultural operations. Secondary spread occurs through infected plant debris, runoff water, and movement of infested soil. The disease can continue to develop during storage if rhizomes are not properly cured and stored under appropriate conditions.
Management
Cultural Practices:
Crop rotation: Rotate ginger with non-host crops such as cereals or legumes for 2-3 years to reduce soil-borne inoculum. Avoid planting ginger in fields with a history of soft rot.
Site selection and drainage: Select well-drained fields with proper slope to prevent waterlogging. Construct raised beds (15-20 cm height) to improve drainage and aeration. Install proper drainage channels to remove excess water during heavy rainfall.
Seed rhizome selection: Use only healthy, disease-free rhizomes as planting material. Source seed rhizomes from disease-free areas or certified sources. Discard any rhizomes showing symptoms of rot or discoloration.
Seed treatment: Treat seed rhizomes with hot water at 50-53°C for 10 minutes to eliminate surface-borne pathogens. Alternatively, soak rhizomes in mancozeb (0.3%) or copper oxychloride (0.3%) solution for 30 minutes before planting. Biocontrol agents such as Trichoderma viride or Pseudomonas fluorescens can be used for seed treatment (10g/kg of rhizome).
Sanitation: Remove and destroy infected plants along with surrounding soil to prevent disease spread. Clean and disinfect farm tools, equipment, and footwear regularly. Avoid working in the field during wet conditions to prevent disease dissemination.
Water management: Avoid over-irrigation and ensure proper moisture management. Use drip irrigation where possible to minimize soil saturation and humidity around the crop. Irrigate during morning hours to allow foliage to dry during the day.
Chemical Control:
Apply soil drenching with metalaxyl or mancozeb (0.25%) at 30, 60, and 90 days after planting as a preventive measure. In case of disease outbreak, drench the soil around affected plants with copper oxychloride (0.3%) or Bordeaux mixture (1%). Apply fosetyl-Al (0.3%) as soil drench and foliar spray at 45-day intervals.
Biological Control:
Apply Trichoderma harzianum or Trichoderma viride (5-10 kg/ha mixed with farmyard manure) to soil before planting and as periodic drenches during the growing season. Use Pseudomonas fluorescens as seed treatment and soil application to suppress pathogen populations.
Post-harvest Management:
Cure harvested rhizomes properly by exposing them to sunlight for 2-3 days to toughen the skin and reduce surface moisture. Store rhizomes in well-ventilated structures with low humidity (below 70%). Inspect stored rhizomes regularly and remove any showing signs of rot immediately to prevent spread.
Integrated Disease Management: The most effective approach combines multiple strategies including use of disease-free planting material, improved drainage, crop rotation, biocontrol agents, and judicious use of chemical fungicides only when necessary. Regular monitoring of the crop and early detection of symptoms is crucial for effective disease management.
