Why is it a problem? Chilli blight is the most serious problem in chilli cultivation in Bhutan. It causes sudden wilting and death of plants, both in the nursery and in transplanted fields, thereby reducing yields. In severe cases, entire crops can be lost. Chili blight is now wide spread in Bhutan. It can be managed through cultural practices.
Where and when is it a problem? It has been recorded from all major chilli growing districts, but is most severe under high rainfall and where drainage is poor (such as flat, heavy soils).
Any chilli growth stage can be affected and all plant parts are susceptible. Damping-off (rot and dying) of seedlings may occur, though this can also be caused by other diseases. Two main disease phases are often distinguished: the stem phase and the aerial phase. In the stem phase, collar rot and wilting in combination with discolouration of lower part of the stem are the common symptoms. Collor rot and stem discolouration often appear as dark brown to black lesions on the stem or at the soil line. Wilted plants often have disintegrated roots. In the aerial phase, water-soaked leaf and fruit lesions, and blackened sections higher up the stem are the symptoms and are usually associated with heavy rainfall. Quite often, infected fruits appear blanched due to coalescing of the water-soaked lesions.
Confusion with other pests. A wide range of diseases and other factors could potentially result in wilting and death of chilli. Careful inspection for symptoms is needed. Sometimes expert input will be required to confirm the cause.
Lifecycle: It is caused by an oomycete plant pathogen, Phytophthora capsici. The pathogen reproduces through both asexual and sexual means and can cause multiple infection within a season. Reproduction is most rapid in warm and wet or humid weather by producing millions of short-lived spores on the surface of infected plants. These spores can be splashed from the plants to soil and back to plants, or between plants, and they can also be carried by moving water in a field. Each one can also release 20-40 motile spores that can swim short distances through standing water or saturated soil towards plant roots. A second type of spore with much thicker walls is produced inside infected plant tissue. These spores can survive for years in the soil until a susceptible crop is planted. Once the pathogen is present in the soil it cannot be eradicated.
Dispersal: The fungus is spread within and between plants by irrigation water, drainage water, and rain. Moving infected plant material, soil and farm equipment can spread the pathogen to new fields and farms.
When can damage be expected? Diseased plant material in the soil forms a major source for infection. Warm weather with wet or waterlogging conditions help spread the disease. Extensive rainfall, overhead irrigation, standing water in the irrigation furrows, splashing rain and run-off water all form important factors. Continuous cropping of chilli is associated with an increase in blight and heavy yield losses. More blight infection occurs in densely planted fields.
Hosts: The pathogen can infect a diverse range of plant species. Thus, the disease is more commonly known as Phytophthora blight in other countries. In Bhutan it is mainly a disease of chilli but has also been reported from tomato, eggplant (brinjal), cucumber, pumpkin and black pepper.
Losses to this disease can be minimized through cultural control provided it is grown in the right places. Prevention is the best control method.
Monitor transplanted fields at least once a week, and remove affected plants as they are found.
Effect of variety
No resistant chilli varieties available in Bhutan.
- Raise seedlings on elevated beds where no chilli has been grown for three years. Ideally, every grower should raise their own chilli nurseries in disease free plots to ensure that the seedlings are pathogen free.
- Do not buy low-quality seedlings that have suffered from a long transit period and might come from infested fields
- As far as possible chilli should be alternated with other non-host crops such as maize or rice to reduce build-up of the disease inoculum. A minimum crop break of 3 years is needed after the field has been infected with rot disease.
- Plant chillis on elevated beds, about 30 cm high and 1.2 m wide, to provide a well-drained root zone. Plant rows 30 cm apart (3 rows per bed) and 30 cm between the plants in the rows. Avoid plants touching each other as this can result in cross-infection.
- Maintain the beds properly during the whole season.
- Do not flood the fields or beds when irrigating.
- Provide good field drainage to remove standing water and puddles
- Monitor regularly, especially after rainfall, and uproot and destroy by burning the infected plants, as well as left over host plant debris. Do not throw infected plants in the drains or furrows.
- Mulch beds with FYM (farm-yard manure). This ensures proper nutrient as well helps prevent splashing of inoculum from the soil and provides weed control.
Chemical spray is recommended only if there is a history of severe infection in the area, and if the weather conditions are very conducive for disease development. Chemical sprays are only preventative and cannot cure infected plants. That is, chemicals are only used to prevent infection and its spread further in the field.
- Seedlings can be treated by dipping in fungicide solution to prevent early onset of infection. Fungicide solution can be prepared in plastic bowls, then bundles of chilli seedlings immersed for 15 minutes. Fungicide solution can be prepared using Ridomil or Metalaxyl (2 g/litre of water) alone, or a mixture of Ridomil or Metalaxyl and copper oxychloride (2.5 g/litre of water).
- Spray Ridomil (2g/litre) or Metalaxyl (2 g/litre) within the first week after transplantation.
- Follow up with Copper oxychloride (2.5 g/litre) 14 days after Ridomil.
- It is important to note that continuous use of fungicides will lead to development of resistance in the pathogen. Therefore, the aforementioned cultural practices must be observed to manage the disease.
Image acknowledgements: NPPC & AVRDC