Phytophthora palmivora

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 in Malnad, 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.

Biology
Phytophthora palmivora produces abundant sporangia on V-8 agar under continuous fluorescent light. However, light is not required for sporangia production on infected papaya fruit. Sporangia are usually produced in clusters sympodially. Sporangia are papillate and ovoid with the widest part close to the base. They are easily washed off and each detached sporangium contains a short pedicel. The average size of the sporangia is 50 X 33 µm with a length of about 1.6 times longer than it is wide. Sporangia germinate directly in a nutrient medium by producing germ tubes that develop into mycelial masses. In water, however, zoospores are released from germinating sporangia. Zoospores aggregate and form distinct patterns at 16ûC in water.

Chlamydospores produced in infected papaya fruit and pure papaya juice are thick-walled. However, chlamydospores produced in papaya juice at lower concentrations or in other kinds of fruit juice are mostly thin-walled. In the presence of nutrients, chlamydospores germinate by producing germ tubes that continue to grow and form mycelial masses. In water, chlamydospores germinate by producing short germ tubes, each with a sporangium at the tip.

Sexual reproduction in Phytophthora palmivora requires the presence of opposite mating types known as A1 and A2. Both A1 and A2 isolates can produce oospores by selfing when stimulated by sex hormones produced by A2 and A1, respectively. Light is inhibitory to oospore formation but stimulatory to oospore germination. Mature oospores can be induced to germinate by treatment with 0.25% KMnO4 for 20 min and incubation under light during germination.

Although sporangia and zoospores may survive in soil for short periods, chlamydospores are the main survival structure for P. palmivora in nature. Oospores are capable of long-term survival but do not play a significant role in the disease cycle because sexual reproduction in P. palmivora requires the presence of opposite mating types, and the chance for this to occur in nature is very low.

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 30ûC, a maximum temperature of 36ûC and a minimum temperature of 12ûC. The pathogen produces the most sporangia at 25ûC but no sporangia are produced at temperatures higher than 35ûC or lower than 15ûC.

Epidemiology
Rain and wind are the two major factors in the epidemiology of Phytophthora fruit rot of papaya. Rain splash is needed for liberation of sporangia of P. palmivora from the surface of infected fruit into the atmosphere and for projection of the soil inoculum into air. Wind is required for dispersal of the inoculum once it reaches the air. Therefore, wind-blown rain is essential for initiation of the primary infection and the development of epidemics in papaya orchards.

Non-Chemical control
Root rot of papaya seedlings, caused by P. palmivora, in replant fields can be controlled with the virgin soil technique. Virgin soil (soil in which papaya has never been grown in before) is placed in planting holes about 30 cm in diameter and 10 cm deep with a mound about 4 cm high. Roots of papaya plants are protected by the virgin soil during the susceptible stage, and become resistant to the pathogen when they extend to the infested soil. Trees established with the virgin soil method in the replant fields produce fruit as abundantly as those growing in the first planting fields. The virgin soil method has the advantages of being relatively inexpensive, very effective and nonhazardous.

Cultural practice is also important in the management of Phytophthora diseases of papaya. Incidence of Phytophthora root rot of mature trees in waterlogged areas during the rainy periods can be greatly reduced by improving drainage in the orchards. Infected fruit on the trees and those that have fallen to the ground should be removed to reduce the inoculum for aerial infection of fruit and stems, and infection of seedling roots in subsequent plantings.

Chemical control
Phytophthora fruit rot of papaya can be controlled by fungicides. Application of a preventive fungicide such as mancozeb or basic copper sulfate to the fruit column are effective in protecting papaya fruit from infection by P. palmivora.