Bunchy top disease of banana has been reported occurring in Australia, Asia, Pacific Islands, and Africa. In India, the disease is prevalent in most of the banana growing States but it occurs severely in Kerala wherefrom it was first reported in 1940.
It is considered that this disease was first introduced in Kerala through planting stalks from Sri Lanka (then Ceylon) and it occupied an area of about 3000 square miles by 1950. Bunchy top disease is economically important as it causes severe losses by destroying the banana plants and reducing effective use of the land.
Symptoms and Causal Agent of Bunchy Top Disease:
Symptoms become evident on leaves. The leaves of infected plants show irregular dark green streaks appearing along small veins (secondary veins) and measuring usually 1.0-2.5 cm x about 0.75 mm.
In badly infected plants, however, the leaves are bunched together at the top of the plant forming a dense rosette, hence the name “bunchy top”. The bunched-top plants remain markedly stunted, do not usually grow taller than two to three feet, and produce no fruit.
Causal Agent:
The disease has been considered to be caused by a virus (Banana Bunchy Top Virus or Banana Virus I), but, more recently, a strong opinion has emerged among the plant pathologists that this disease is caused by phytoplasma.
Bunchy Top Disease Cycle:
The pathogen perennates between the growing seasons in already infected plants, mainly in their suckers. All the suckers produced by infected plant carry the pathogen and serve as the main source of primary infection when they are planted in the next growing season. The secondary infections during the growing season are caused by aphid vectors.
The disease is not sap transmissible and the pathogen is transmitted by aphid, namely, Pentalonia nigronervosa. The aphids require a minimum feeding period of 17 hours to become viruliferous, retain the pathogen in them for a period of 13 days (semi-persistent manner), and cause infection by feeding on the host for a period of one and half hour or more.
Management of Bunchy Top Disease:
(i) Selection and transportations of suckers and other vegetative parts from areas where the disease occurs to disease free areas for plantation should be prohibited specifically by quarantine law. This would check disease incidence in disease-free areas.
(ii) Diseased plants should be marked in the early stage, be up-rooted completely along with their suckers, and burnt.
(iii) Creation of a banana-free wide zone around an infected area may help preventing transmission of pathogen through aphid vectors.
(iv) Use of resistant varieties may be helpful but, unfortunately, no resistant variety against this disease has yet been evolved.
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