(a) Discuss briefly the rosette disease of groundnut under the following headings: (i) causal organism: (ii) symptoms (iii) method of transmission; (iv) dam...

(a) Rosette disease of groundnut.

(i) Causal organism: a virus, the groundnut rosette virus.

(ii) Symptoms: stunted growth of the plant; yellowing (chlorosis) and mottling of the leaves; the leaves become small, curled and crowded together, giving a bushy, rosette-like appearance; there is general dwarfing and few or no pods are formed.

(iii) Method of transmission: it is transmitted from infected to healthy plants by an insect vector, the groundnut aphid (Aphis craccivora), which sucks the sap.

(iv) Damage done: it stunts the plants and reduces leaf area and photosynthesis, causing poor pod and seed formation; this leads to a heavy reduction in yield, and severe early infection can cause almost total crop loss.

(v) Preventive and control measures: plant resistant/tolerant varieties; plant early and at the recommended close spacing to reduce aphid spread; control the aphid vector with appropriate insecticides; rogue out and destroy infected plants; and practise good field hygiene and crop rotation.

(b) Plant population calculation.

Spacing per stand = 30 cm by 30 cm = 0.3 m by 0.3 m.

Area occupied by one stand = \(0.3 \times 0.3 = 0.09 \text{ m}^2\).

Total area of land = \(60 \times 30 = 1800 \text{ m}^2\).

Plant population = \(\dfrac{\text{total area}}{\text{area per stand}} = \dfrac{1800}{0.09} = 20{,}000\) plants.