Foot Rot of Papaya
Symptoms
Foot rot, also known as collar rot, manifests as water-soaked lesions at the base of the stem near the soil line. The affected area becomes soft, dark brown to black, and gradually extends upward and downward. As the disease progresses, the bark peels off easily, exposing the discolored underlying tissue. The rot may girdle the stem, causing wilting, yellowing of lower leaves, and eventual collapse of the plant. In advanced stages, white mycelial growth may be visible on the rotted portions, and the entire plant may topple over due to weakened stem base.
Etiology
Foot rot is primarily caused by Phytophthora palmivora and Pythium aphanidermatum, both being oomycete pathogens. Phytophthora palmivora is the more common and destructive causal agent. These pathogens thrive in warm, humid conditions with temperatures between 25-30°C. The disease is favored by poor drainage, waterlogged soils, and excessive irrigation. Deep planting of seedlings and injury to the stem base also predispose plants to infection.
Disease Cycle
The pathogens survive in soil as oospores, chlamydospores, or mycelium in plant debris. During favorable conditions, these survival structures germinate and produce sporangia, which release motile zoospores. Zoospores are attracted to root exudates and swim through soil water to reach the host. They penetrate through wounds, natural openings, or directly through the epidermis at the collar region. Once inside, the pathogen colonizes the cortical tissues, causing cellular breakdown and rot. Secondary spread occurs through irrigation water, contaminated tools, and movement of infested soil.
Management
Cultural Control: Plant papaya in well-drained soils with proper land leveling. Avoid deep planting and ensure the collar region remains above soil level. Implement proper irrigation management to prevent waterlogging. Remove and destroy infected plants promptly. Practice crop rotation with non-susceptible crops for 2-3 years. Apply organic mulch to reduce soil splash and maintain soil moisture.
Chemical Control: Drench the soil around the stem base with metalaxyl (0.125%) or fosetyl-Al (0.3%) at 15-day intervals during rainy season. Apply copper oxychloride (0.3%) or Bordeaux mixture (1%) as a protective spray and soil drench. Treat planting pits with Trichoderma-enriched compost before planting.
Biological Control: Use Trichoderma harzianum or Trichoderma viride as soil application or seedling root dip. Apply Pseudomonas fluorescens formulations to enhance plant defense mechanisms.
Leaf Curl of Papaya
Symptoms
Leaf curl disease causes characteristic downward curling and distortion of young leaves. Affected leaves become thickened, brittle, and develop a leathery texture with reduced size. Severe vein clearing and yellowing occur on the lower leaf surface. The leaf margins curl downward and inward, giving a cupped appearance. Plants show stunted growth with shortened internodes and a bunchy top appearance. Severely affected plants produce few or no fruits, and existing fruits remain small and malformed. The disease is most severe during hot, dry weather conditions.
Etiology
Papaya leaf curl disease is caused by Papaya leaf curl virus (PaLCuV), a begomovirus belonging to the family Geminiviridae. The virus contains single-stranded circular DNA genome. Several strains of the virus exist, showing variation in symptom severity. The virus is transmitted by the whitefly vector Bemisia tabaci (tobacco whitefly) in a persistent, circulative manner. The virus can also spread through infected planting material, though this is less common.
Disease Cycle
The primary source of inoculum is infected papaya plants and alternate weed hosts harboring the virus. Adult whiteflies acquire the virus by feeding on infected plants for 15-30 minutes. After an incubation period of 8-24 hours within the insect vector, the whitefly becomes viruliferous and remains so for life. When the infected whitefly feeds on healthy plants, it injects the virus into phloem tissues. The virus multiplies in phloem cells and moves systemically throughout the plant. Symptoms appear 10-15 days after inoculation. Disease spread is rapid during periods of high whitefly population, particularly in hot, dry seasons.
Management
Cultural Control: Use virus-free planting material from certified nurseries. Remove and destroy infected plants immediately to reduce inoculum source. Control alternate weed hosts such as Ageratum, Euphorbia, and Croton around papaya fields. Plant marigold or other barrier crops around the papaya orchard to deter whitefly movement. Maintain proper field sanitation and avoid overlapping crop cycles.
Vector Management: Monitor whitefly populations using yellow sticky traps. Apply neem oil (3%) or neem seed kernel extract (5%) as a repellent. Use chemical insecticides such as imidacloprid (0.3 ml/L) or thiamethoxam (0.2 g/L) for controlling whitefly populations. Spray insecticides during early morning or evening when whiteflies are less active. Target the lower leaf surface where whiteflies congregate.
Resistant Varieties: Plant tolerant varieties such as Pusa Dwarf, Pusa Delicious, or CO-7 that show reduced susceptibility to leaf curl virus. Screen local varieties for tolerance and promote use of resistant germplasm.
Biological Control: Conserve natural enemies of whiteflies including Chrysoperla spp. (lacewings), Encarsia formosa (parasitic wasp), and predatory beetles. Apply Verticillium lecanii or Beauveria bassiana as mycoinsecticides for whitefly control.
