FRUIT CROPS
Banana (Musa species)
1. Panama disease of banana
Fusarium oxysporum f.sp. cubense
‘Panama disease’, ‘Fusarium wilt’ or ’vascular wilt’ of banana is one of the most serious and destructive diseases of the crop. The disease was first reported from Australia in 1874. Now, it is widely distributed all over the world, including America, Africa, Sri Lanka, Burma, Thailand, Malaysia, Indonesia, Hawai, Fiji, the Philippines, India, Australia and New Zealand. In India, it is prevalent in the states of Assam, Bihar, Karnataka, Kerala, Tamil Nadu, Telungu Desam, Maharashtra and West Bengal.
Symptoms. Usually the symptoms are manifested, when the banana plants are about 5 months old. However, under highly favorable conditions for the pathogen, even 2 - 3 months old plants show wilt symptoms. Initial visible symptoms appear as pale yellow streaks, lengthwise in the petioles of the oldest, lowermost leaves. Two types of symptoms follow this. In the first type, the old leaves turn yellow progressively and finally they break from the petiole and hang down. In the second type, the leaves break from the petiole and hang down, without becoming chlorotic. Gradually, all the leaves collapse and hang down, except the crown, which alone stands upright. The newly formed leaves show yellow blotches and wrinkling of the lamina. Often, the leaf sheaths covering the pseudostem show longitudinal splitting above the ground level. But, sometimes this symptom may not appear. The affected plants may die completely in about 4-6 weeks after the appearance of yellow streaks on the petioles.
The vascular strands show discoloration varying from yellow to dark- brown, which is the characteristic internal symptom of the disease. The discoloration usually appears first in the outer leaf sheaths and extends to the vascular strands of the pseudostem. Vascular discoloration is more pronounced in the corms, but is not common in the roots. When the rhizome of an infected plant is cut transversely, the discoloration of the vascular bundles is distinctly seen as numerous, pin-point like dots all over the cut surface. The roots of affected plants become black and rotten. The suckers growing from the diseased corms are systemically affected and they wilt and die very soon (Fig. 71).
The causal organism. The mycelium of the fungus is hyaline, septate, much branched and mostly intracellular. Few hyphae may also be present in the intercellular spaces. The hyphae are largely confined to the vascular bundles, often filling the cavity of the vessels. The fungus produces three types of spores viz., macroconidia, microconidia and chlamydospores. The sporodochia bearing conidiophores and conidia emerge through the stomata on the petioles and leaves. The conidiophores are vertically branched. Macroconidia are borne at the apical ends of the main and lateral branches and are formed in succession one after another. They are thin-walled, hyaline, pedicellate, sickle-shaped, with both ends somewhat pointed, 2 - 5 septate, mostly 3-septate and measure 22.0 - 36.0 x 4.0 - 5.0µ in size. Microconidia are produced from the branches of conidiophores or from the tips of hyphae in very large numbers. They are single-celled or two-celled, thin-walled, hyaline, ovate or elongated and measure 5.0 - 7.0 x 2.5 - 3.0µ in size. Terminal or intercalary chlamydospores are formed from the hyphae, as well as from the conidial cells. They are oval or spherical, thick-walled and usually occur in pairs. They measure 7.0 - 13.0 x 7.0 - 8.0µ in size (Fig. 71).
The fungus is a facultative parasite. It can invade the host mainly through wounds in the corms or roots. After entry, the fungus develops extensively in the vascular tissue of the corm and then proceeds systemically to the leaf sheaths and pseudostem through the vascular system. The fungus colonizes the vascular bundles and produces masses of mycelium, which bear conidia and chlamydospores, filling the cavity of the vessels. The plugging of vascular elements by the fungus obstructs the translocation of water and nutrients to the aboveground parts of the plant leading to wilting and death of affected plants.
Mode of survival, spread and epidemiology. The fungus is soil-borne in nature. It can survive saprophytically in the diseased corms and other plant parts for prolonged periods and cause fresh infection. The chlamydospores can also remain in a viable state in the soil and diseased plant parts for a long time. The disease can be easily spread through spores present in the infected banana trash and soil, which may be carried in surface flooded water. Suckers used for planting from diseased areas to other places spread the disease easily. Contact of the roots of healthy plants with the roots of diseased plants carrying the spores also leads to fresh infection. The entry of the pathogen is mostly through wounds in the roots or corms, especially deep wounds up to the xylem vessels, caused incidentally during cultural operations or by soil-inhabiting pests, particularly nematodes.
Light textured, sandy loam soils, with acid reaction and low soil moisture of 25 % favor the occurrence and development of the disease. Ratooning of infected banana plants increases the disease incidence to a great extent and leads to continuous accumulation of soil inoculum.
