Diseases of Fruit Plantation Medicinal and Aromatic Crops

Powdery mildew of papaya

Causal organism :Oidium indicum, Oidium caricae, Leveillula taurica
Symptoms

• The disease appears on the foliage and pods. Infection is first apparent on the leaves as small slightly darkened areas, which later become white powdery spots.
• These spots enlarge and cover the entire leaf area. Severely infected leaves may become chlorotic and distorted before falling. Affected fruits are small in size and malformed.
• In more severe cases dieback symptoms develop.
Causal Organism

• Oidium indicum belongs to the Phylum: Ascomycota, and Order: Erisiphales.
• The pathogen is an obligate parasite.
• Mycelium is hyaline, septate, ectophytic and creeping.
• The asexual spore is conidia which are borne in long chains at 2-4 hyaline, elliptical and are barrel shaped.
• The sexual spores are the ascospores which are globose, and gregarious. Each ascocarp contains one ascus.

Epidemiology

• The development of powdery mildew in papaya is promoted by high humidity (80-85%) a temperature of 24-26°C and sunshine duration for 9.1 hours.
• Susceptible varieties like to infection.
Mode of survival

• Primary source of inoculums - Dormant mycelia
• Secondary source of inoculums- Air borne conidia.
• Spread: Air borne, conidia
Disease cycle

• Ascospores produced in cleistothecium are the resting spores in the debris.
• The primary infection may be initiated either by ascospores released from cleistothecia or conidia carried by wind from collateral hosts to papaya.
• The spores after falling on healthy leaves germinate, produce germ tubes, produce haustoria inside the leaf tissues, develop a fungal vegetative body and produce conidia. The conidia are blown away by wind and cause secondary infections. The fungus is strictly an obligate parasite.
Management
• Clean cultivation is the important criterion in reducing infection .
• Increase potassium application and inversely reduce application of nitrogen in the soil so as to improve resistance mechanism in plants.
• As soon as disease symptoms are observed, dusting with Sulphur (30 g/10 liters of water) has been found effective.
• Spray Triademefon (0.1%) during September which may be repeated at 15 days interval.
• Spraying Calixin 75 EC (5 ml/10 liters of water) at 15 days interval also helps to controlling disease.