Phytophthora blight has also been called soft foot rot, stem canker, soft fruit rot, and root rot. The pathogen, Phytophthora palmivora (E. J. Butler) E. J. Butler, was named in 1919. It was once classified as a fungus, but now it is regarded as a pseudofungus in the stramenopiles. Several strains of P. palmivora have been described due to considerable morphological and pathogenic variation in the species. The host Papaya is just one of many plants affected by this.

The host Papaya is just one of many plants affected by this pathogen worldwide. Other hosts include breadfruit (Artocarpus altilis), palms including the coconut (Cocos nucifera), Catteleya orchids, English ivy (Hedera helix), and cocoa (Cacao sp.). The disease occurs on papaya in the Philippines, Sri Lanka, Santo Domingo, India, Indonesia, Malaysia, Hawai‘i, Mauritius, Mexico, Australia, Brazil, Spain, Taiwan, and perhaps elsewhere. Symptoms on papaya Young fruits. Water-soaked lesions exude milky latex. Fruits may eventually mummify and fall. Mature fruits. Fruit rot initially appears as small, circular, water-soaked lesions about 3/16–3/8 inch (5–10 mm) in diameter. Large lesions, often forming first where the rangia. Fruits can rot, turn soft, and fall prematurely. Stems and foliage. The top portion of the fruit-bearing region of the stem is susceptible to infection during rainy periods. This can cause stem cankers to appear. The infected plant may become more susceptible to wind damage. Older portions of stems are susceptible when wet after extended rainfall, or after injury. As lesions enlarge, infected areas of the stems way weaken, causing stem damage or breaking. Foliage on affected stems may collapse. Roots. Lateral roots of young plants (less than 3 months old) are most susceptible in poorly drained soils. Roots may become dark and rotten, causing stunting of plant growth and yellow, collapsed leaves. Severely infected plants may die. Plants with a heavy load of fruit may topple. Papaya plants with rotten roots are susceptible to drought stress. Morphology and life cycle of P. palmivora The most important developmental factor in P. palmivora is its ability to produce zoosporangia on diseased plant tissue when free water is present. The organism produces hyphae, zoosporangia containing zoospores, chlamydospores, and oospores. Two separate mating types (called A1 and A2) are required for the production of oospores. Oospores do not play a significant role in the disease cycle because the chance of both mating types occurring together naturally is low. Zoospores are motile and infective after their release from the zoosporangia. Chlamydospores are the principal long-term survival structures in soils. Chlamydospores may germinate in water to produce sporangia and release zoospores, which may be transported by wind-blown or splashing rain to susceptible plant tissues. Chlamydospores formed in fallen fruit can survive in soils and infect roots of papaya seedlings in subsequent plantings. The minimum temperature for growth of P. palmivora in culture is 52°F (11°C). The optimum temperature is 81.5–86°F (27.5–30°C), and the maximum growth temperature is near 95°F (35°C). Propagules of this pathogen are dispersed principally by wind-blown rain, splashing rain, slugs, ants, knives, clippers, rodents, soil, or plant growth media.

 Pythium aphanidermatum Symptoms:

 Water soaked spot in the stem at the ground level which enlarge and griddle the stem.

 The diseased area turns brown or black and rot.

 Terminal leaves turn yellow droop off.

 The entire plant topples over and dies.

 Favourable conditions :

 The pathogen inhabits the soil. It is capable of growing and surviving on plant resides, but attacks living plants in the presence of a favourable host.

 Waterlogging is very conducive to disease development.

 Management:

 The disease can be avoided if plants are grown in well-drained soil.

 Affected plants should be carefully dug up and destroyed by burning.

 Replanting should not be done in the same pit where disease has once appeared.

 When trees are weeded, care should be taken so that no injury is caused to the base of the stem.

 Seed treatment with Thiram or Captan 4 g/kg of seed.

 Drenching with Copper oxychloride 2.5g/lit of water or Bordeaux mixture 1% or Metalaxyl 1g/lit of water.

• Control incipient rots (less than 24 hours old) of harvested fruit by dipping fruits in hot water held at 120°F (48°C) for 20 minutes.

• Avoid damage or injury to papaya stems during cultivation.

• Control African snails; they can vector the pathogen.

  • Contributed by : 
    Dr. S K Tyagi
    Scientist (Horticulture)
    KVK, Khargone (MP)

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