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Fusarium Wilt And Its Effect On Bananas

Mmmm, bananas! There is nothing better than Grandma’s homemade banana-nut bread, bananas in your cereal or a nice banana split. But, in the 1950s, the bananas exported from Central America were in great danger. It was then that vascular wilt, a disease that attacks through the vascular system and ultimately kills by dehydration, reached epidemic proportions. Vascular wilt, also called Fusarium wilt or Panama disease, is caused by Fusarium oxysporum, a common soil fungus of the family Nectriaceae. F. oxysporum often exists harmlessly on the surface or inside the roots of many plants, but over 80 strains cause vascular wilt in particular crops. The strain that specifically attacks bananas is called Fusarium oxysporum f. sp. cubense. It is especially virulent and interesting in its advancement. (www.cbwinfo.com)

The first symptom of an infected banana plant is a “reddish-brown discoloration of the xylem of roots and then the rhizome.” After F. oxysporum has spread, the banana leaves yellow, wilt and then die. The infected leaves fall in order, from oldest to youngest, until they hang around the plant like a “skirt” of dead leaves. A younger plant will die soon after the symptoms occur, but an older plant may live for many months. (www.oisat.org)

Fusarium oxysporum, an asexual fungus, produces spores that spread Fusarium wilt via the water conducting system. Chlamydospores, a resting spore of F. oxysporum, can live dormant in the soil for about thirty years or until it is stimulated to germinate by a susceptible host. Once it has found an acceptable host, usually an injured root, it enters the root system and travels into the xylem vessels.

Upon entering the xylem, the vascular system becomes infected by hyphae and fungus spores called microconidia. The hyphae and microconidia are carried upwards in the water stream, but are halted, after a day or so, by a gel plug produced by the plant. “The gels have a composition similar to primary cell walls, but lack cellulose.” (Vascular Wilt Disease of Plants, Tjamos) If the initial infection is light, and the plant responds rapidly, then the disease can be confined to a few infected vessels. However, if the initial infection is severe and the plant response is slow, the fungus will germinate past the gel blockage by penetrating the gel with its hyphae and producing spores on the other side. (www.helios.com) The plant will repeatedly respond with a gel barrier, but often the spread is already too severe. In fact, the plant’s gel seems to clog the water system, adding to the wilting symptoms. (See link for reasons for symptoms)

Once the spores have penetrated the entire sap system, the F. oxysporum begins to branch outward. “The fungus grows from the xylem into the surrounding, dead fleshy tissues and produces many resting spores (chlamydospores).” (www.helios.com) Interestingly, Fusarium oxysporum can only branch out from the sap once the plant has amplified dehydration by using gel clogs. After the fungus has spread entirely, the plant dies of dehydration.

Multi-celled macroconidia, another F. oxysporum spore, are produced on surface lesions once the plant has died. Those spores can “germinate directly or produce chlamydospores”, which will rest in the soil and start the cycle all over again. (Ploetz)

If you don’t live on a farm, you may not think that Fusarium wilt is important, but it affects you more than you think. Fusarium oxysporum may be the new biological weapon of choice for the United States’ war on drugs. The US government has created a strain of Fusarium oxysporum to eradicate coca plantations in Colombia. Though spraying was halted, in 2000, when the Colombian government and the UN refused to allow testing in Colombia, research is currently underway. (www.cbwinfo.com)

Links to more information:
Use of Fusarium oxysporum as a biological weapon
Table of Formae species
How is Fusarium oxysporum spread?
A Great History of Fusarium wilt in Bananas