Panama disease or Fusarium wilt of banana is one of the commonest and most devastating diseases of banana worldwide. The disease was first reported from Jamaica, Panama, Surinam, and other places at the end of previous century.
Since then, the disease has been recorded from several banana growing countries of Australia, Asia, Africa, North and South America. In India, the disease is a serious problem, and has been recorded collapsed from Andhra Pradesh, Tamil Nadu, Karnataka, petioles Maharashtra, and West Bengal.
Symptoms of Panama Disease:
The characteristic symptoms of the disease are the sudden wilting of the individual leaves and also of the plant as a whole. The leaves become chlorotic, the petioles collapse, and the leaf lamina hang downward and wither. When severely infected, all the leaves of the plant may wilt within two or three days. The leaves collapse in acropetal succession except the central one that remains green and erect for long time.
The pseudostem of banana plant withers developing conspicuous longitudinal stripes on the outer leaf sheaths that form its outer covering. These stripes result in the splitting of the leaf sheaths. Discoloured vascular strands varying from light yellow to dark brown are the distinguishing internal symptoms. Usually the discolouration manifests first in the outer or oldest leaf sheath and extends upto the pseudostem.
The disease appears in the fields sporadically. At first a few plants are being affected, then it spreads concentrically around the infected plants, and ultimately all the plants of the field get affected. The symptoms most commonly appear on at least 5 month old plants of banana. 2-3 month old plants may also be infected and killed under highly favourable conditions.
Causal Organism of Panama Disease:
The mycelium of the pathogen is intra- as well as intercellular. Intracellular hyphae are typically found in xylem vessels often filling their cavity. Intercellular hyphae may be observed in the cortex of roots and in parenchymatous cells in proximity to the site of infection. The hyphae accumulate in substomatal cavities forming a globose mass of pseudo-parenchymatous tissue called sporodochia, which emerge through the stomata.
Conidiophores are borne on the sporodochia and are vertically branched; the side branches are usually single-celled and measure up to 14 µm. Conidia is produced on the apical end of the main and side branches. They are-both micro- and macro-conidia.
The micro-conidia are single- or two-celled, oval or somewhat elongated, and measure 5-7 x 2.5-3.0 µm. The macro-conidia are sickle-shaped, thin walled, hyaline, 2-5 septate (mostly 3-septate), and measure 22-36 x 4-5 µm. The fungus also produces chlamydospores, which are oval to spherical, usually in pairs, and measure 7-13 x 7-8 µm.
Panama Disease Cycle:
(i) Perennation:
The disease is mainly soil-borne. The pathogen survives in the soil mainly as chlamydospores and invades the soft roots or rhizomes, very often through the injuries. The chlamydospores, which are formed by the hyphal and conidial cells, survive in soil for long period of time and infect the plants. Sometimes the fungus survives saprophytically in the infected rhizomes, and on the approach of favourable conditions the infection takes place.
(ii) Infection:
The pathogen propagule surviving as primary inoculum does not attack living cells of the main root. Primary infection always takes place through injured roots. Deep wounds exposing the xylem help in easy infection. It has been observed that the chlamydospore germination is stimulated in the vicinity of injured root surface, while it is inhibited by intact root surface.
When the germ tube of chlamydospore enters into the root, the hyphae proceed internally along the root to the rhizome where they develop extensively in vascular tissues before passing up the vascular system into the pseudostem and the older leaf petioles.
The secondary infection takes place by means of conidia. The disease spreads mainly by contact of the root system of adjacent healthy plants with conidia released by the diseased plants. Floods also help in the local dispersal of the secondary inoculum.
Predisposing Factors:
It has also been noticed that the disease is more prevalent in that crop which is retained in the field for more than one season. Several factors such as soil conditions including texture, structure, temperature, and fertility level of soil are responsible for disease incidence. Sandy loam and light textured loam soils are more conducive for disease development because the pathogen can penetrate deep into these soils.
High soil moisture is also favourable. In soils where soil moisture is maintained at 20 and 100 percent of water holding capacity, the viability of the pathogen is retained beyond four months. The cultural operations and plant vigour also influence the disease incidence and its spread.
Management of Panama Disease:
(i) Sanitation practices should be observed. It includes immediate removal of diseased plants, with surrounding soil from the field, care in cultivation to avoid injury, use of healthy planting stock, control of nematodes, etc.
(ii) It has also been found that application of sodium nitrate and mercuric salts to the soil inhibits the growth of fungus. But the method is not practicable on a field scale, especially among standing plants. Flood fallowing has been found effective to reclaim infested soil. The land is inundated under 2-5 feet of water for six months.
(iii) Conn injection with Carbendazim (2%) plus Agallol or Aretan (0.1%) soil drench has been claimed effective.
(iv) Bavistin capsule application in the rhizome has been reported to be most effective. It has also been found that dipping of suckers in Bavistin (0.1%) solution prior to planting followed by bimonthly drenching of Bavistin (0.1%) from six months of planting onwards provide good results.
(v) The only practicable management device is the use of resistant varieties. Cavendish varieties of banana are resistant, whereas the Michael varieties are susceptible; ‘Giant Cavendish’ replaced ‘Gros Michael’ variety, for example. Some other resistant varieties are Poovan, Moongil, Peyladen, Rajabale, and Vamankeli. However, there is a good hope of getting resistant clones from the Central Banana Research Station, Adhuthurai (Tamil Nadu) in near future where the existing commercial varieties are being hybridized with Musa balbisiana, M. acuminata, and M. coccinea.
No comments yet.