Write short notes on the following plant diseases. (a) blackpod of cocoa; (b) leaf blight of cassava.

Answer Details

(a) Black pod of cocoa

• Causal organism: a fungus, Phytophthora palmivora (also Phytophthora megakarya).
• Symptoms: brown to black spots appear on the pod, spread rapidly to cover the whole pod, which turns black, rots, hardens and mummifies; the beans inside are destroyed.
• Spread: favoured by high humidity and rainfall; spread by rain splash, wind, insects and contact.
• Control: regular removal and destruction of infected pods (sanitation); frequent spraying with copper-based fungicides (e.g. Bordeaux mixture); prune to reduce humidity; ensure good drainage and adequate spacing; plant resistant varieties.

(b) Leaf blight of cassava

• Causal organism: a bacterium, Xanthomonas campestris pv. manihotis (cassava bacterial blight); (fungal leaf spots such as Cercospora also cause blighting).
• Symptoms: angular brown/water-soaked spots on the leaves, blighting and wilting of leaves, defoliation, die-back of shoots and gum exudation on stems in the bacterial type; severe attack reduces tuber yield.
• Spread: spread through infected cuttings, rain splash, wind and farm tools.
• Control: use clean, disease-free (resistant) cuttings; practise crop rotation and field sanitation; remove and burn infected plants; avoid overhead injury and disinfect tools; apply appropriate chemicals where necessary.