Phytophthora Root Rot

Phytophthora palmivora causes bud-rot of palms, fruit-rot or kole-roga of coconut and arecanut. These are among the most serious diseases caused by fungi and moulds in South India. It occurs almost every year inMalnad, Mysore, North & South Kanara, Malabar and other areas. Similar diseases of palms are also known to occur in Sri Lanka, Mauritius and Sumatra. The causative organism was first identified as Phytophthora palmivora by Butler in 1917.

During rainy periods, chlamydospores in soil may germinate in water to produce sporangia and release zoospores. The impact of falling rain drops may splash zoospores into air in droplets. The zoospore-containing droplets may be further dispersed by wind and become the inoculum for infecting fruit and occasionally stems of papaya in the fields. The pathogen produces abundant sporangia on the surface of infected fruit that are further dispersed by wind-blown rain and cause outbreaks of Phytophthora fruit rot in the same and nearby orchards. Chlamydospores formed in fallen fruit survive in soil and serve as the main source of inoculum for infection of roots of papaya seedling in subsequent plantings.

Phytophthora root rot of papaya seedlings is most serious during rainy periods. Under waterlogged conditions, P. palmivora may attack roots of papaya older than three-months of age, the time at which they become resistant to the pathogen under normal conditions. Therefore, Phytophthora root rot may occur on papaya at any age in poorly drained areas. Waterlogged conditions appear to weaken the defense mechanism of papaya roots against invasion by the pathogen. Mobility of zoospores of P. palmivora under such conditions also may contribute to the severity of the disease due to their attraction by papaya roots.

Favorable temperature is also a contributing factor to the severity of Phytophthora diseases because of its effect on growth and sporulation of the pathogen. Phytophthora palmivora has an optimum temperature for growth of 30°C, a maximum temperature of 36°C and a minimum temperature of 12°C. The pathogen produces the most sporangia at 25°C but no sporangia are produced at temperatures higher than 35°C or lower than 15°C.