- Immature fruits (green colour) irrespective of their development stages are susceptible.
- Water soaked light brown discoloured spots appear which increase readily showing concentric dark brown rings slightly resembling the markings as a buckeye (Plate-2a).
- The lesions rapidly enlarge and with in 3-4 days, whole of the fruit surface turns dark brown and feels soft to touch.
- In warm and humid weather, white flocculent superficial growth of the fungus consisting of sporangia and sporangiophpores also develops on the diseased fruits.
- Later, these fruits may drop off from the plant (Plate-2b).
Disease cycle and epidemiology
- The disease is caused by Phytophthora nicotianae Breda de Hann. var. parasitica (Dastur) Waterhouse.
- The mycelium of the pathogen is hyaline and coenocytic with branching typically at right angles. The sporangiophores arise from hyphal threads and produce sporangia.
- The sporangia are broadly ovoid to globose in shape having one hemispherical papilla at the tip. Chlamydospores are smooth, globose, and slightly yellowish with thick brown walls, produced abundantly in culture and germinate by producing zoospores or germ tubes.
- Antheridia are amphigynous, spherical or oval and oogonia are rough, thick walled and yellowish brown in colour.
- Oospores are aplerotic, 18 to 20 µm in diameter with 2 µm thick wall.
- The fungus overwinters in the soil in the form of oospores or chlamydospores and can remain active in soil for at least one year without the support of a susceptible host (Fig.1.).
- With the onset of monsoon rains, in the presence of high soil moisture and moderate temperatures (20-25°C), the chlamydospores and oospores start germinating by producing mycelium and sporangia.
- The sporangia in turn produce biflagellate zoospores, which are splashed by rain to the fruits.
- The symptoms develop on fruits after 3-4 day of infection.
- Infected fruit become mummified and fall down on the ground.
- The sporangia produced on infected fruits, liberate zoospores which are again splashed by rain and cause secondary infection.
- Maximum fruit infection under field conditions occurs at a temperature range of 20-25°C, RH > 80 per cent and high rainfall conditions.
- Higher doses of N resulted in higher fruit rot while higher levels of P resulted in more yield of healthy fruits and less fruit rot.
- Based on weekly temperature and cumulative rainfall, short term forecasting of the disease can be done. June 20 is considered as the zero date.
- The disease is not expected to occur at temperatures at or below 20°C, though at temperatures of 22.5°C or above even a slight rainfall (10 mm) will result in disease appearance, which is expected to appear after 4 days of infection.
- Stake the plants erect and remove foliage and fruit up to a height of 15-20 cm to avoid moist and stagnant air conditions.
- Collect and destroy the affected fruits regularly.
- Apply pine needle/grass mulch on the field floor to create a barrier between the host and soil borne inoculum.
- With the onset of monsoon rains, spray the crop with metalaxyl + mancozeb (0.25%) followed by sprays of either mancozeb (0.25%) or copper oxychloride (0.3%) or Bordeaux mixture (4:4:50) and repeat at 7-10 days interval.
Last modified: Monday, 12 March 2012, 6:03 AM