LEAF BLIGHT (LB): Lasiodiplodia theobromae
Coconut seedlings and mature palms are both seriously threatened by the fungal pathogen-caused leaf blight disease. Leaflets that appear charred and dry are the main symptoms of this disease, which mainly affects the outer whorls of adult leaves. Nuts, it should be noted, are particularly vulnerable; they exhibit dark lesions with undulating margins that cause early dropping and yield losses that can reach 25%. Desiccated and malformed nuts are the result of the fungus penetrating the kernel through the mesocarp and causing endosperm decay. The summer is when the incidence of leaf blight peaks, and the spores and fungal structures on the afflicted leaves act as sources of wind-borne transmission. Management strategies include chemical controls such as Bordeaux mixture or Copper oxychloride sprays, along with root feeding with fungicides such as Carbendazim or Hexaconazole/Tridemorph at specific intervals, biological approaches using Bascillus spp. with organic supplements, and cultural methods such as removing and burning of affected leaves to prevent further infection. These coordinated strategies seek to maintain coconut yield while reducing the impact of disease.
Symptom:
A serious fungal disease called leaf blight damages leaves and nuts extensively in coconut palms of all sizes, from seedlings to mature trees. The disease usually affects the outer whorls of adult leaves, where the afflicted leaflets dry out from the tips down, looking burnt or scorched. Nuts are also affected; they develop dark grey to brown lesions that start at the apex of the nut and have wavy borders. The fungus penetrates the kernel via the mesocarp, causing endosperm to decompose and the nuts to become desiccated, shrunken, and malformed. This premature dropping of the nuts results in yield losses that can range from 10% to 25%. The occurrence of leaf blight is seasonal, peaking in the summer when the weather is conducive to the growth and spread of fungi. The main sources of new infection on afflicted leaves are spores and fungal structures, which wind-carries to nearby palms.
Management
To manage leaf blight in coconut palms effectively, a combination of cultural, biological, and chemical methods is essential.
Cultural method
Culturally, it is crucial to promptly remove and burn severely affected leaves to prevent the spread of the disease within the plantation. This practice helps in reducing the availability of fungal inoculum that could otherwise lead to further infections. Additionally, adopting good field hygiene practices such as maintaining proper spacing between palms and ensuring adequate ventilation and drainage in the plantation can minimize favourable conditions for disease development.
Biological method
Application of beneficial microorganisms like Bascillus spp. is recommended. This biocontrol agent, when applied at a rate of 200 grams per palm per year along with 50 kilograms of farmyard manure (FYM) and 5 kilograms of neem cake, enhances soil health and suppresses the fungal pathogens responsible for leaf blight. These microbes help in promoting a balanced soil microbiome and improve the coconut palm's resistance to diseases.
Chemical method
Chemical management involves strategic spraying of fungicidal solutions during critical periods. For effective control, 1.0% Bordeaux mixture or 0.25% Copper oxychloride should be applied twice at 45-day intervals, especially during the summer months when disease incidence is high. These fungicides protect the foliage from fungal infections and reduce disease progression. Additionally, root feeding with Carbendazim at 2 grams or Hexaconazole/Tridemorph at 2 ml mixed with 100 ml of water should be administered three times at three-month intervals. This systemic treatment ensures that the coconut palms receive adequate protection from soil-borne pathogens, contributing to overall disease management and healthier palm growth.
