PANAMA DISEASE OF BANANA: The Fusarium Wilt

         Banana is major fruit crop cultivated in different countries in the world, contribute major chunk of international fruit trade, after Citrus. Banana are not only known for its year round availability, affordability, nutritive and dietary value, but also as one of the top ten world's staple food.  Commercial cultivation of crop is a economically viable venture.

        Being such an important crop, banana farming  faces plenty of constrains during its cultivation. One important among them is incidences of diseases. Succulent nature of the crop and its humid tropic climatic requirements favors the occurrence and spread  disease causing pathogens, especially fungal counterparts.     Panama disease, otherwise called Fusarium Wilt is one such devastating fungal disease affecting banana cultivation and has been considered   as one of the sixth most  destructive plant disease in the world.

           Fusarium Oxysporium. f.sp. cubence is the causal  organism and  the same was   infamous for  vanishing of then  popular variety of banana like Gros Michel. The cultivar was renowned  for its exportable quality owing to its  thick peel and longer shelf life. Occurrence of  panama wilt completely affected the cultivation and production of banana world wide, leading to huge setback in international banana trade then. It involved longer time and extensive effort to bring back the banana cultivation to normalcy and the same was achieved with the advent of yet another famed cultivar like Cavendish.   After 50 years or so, the pathogen became belligerent to even on Cavendish. But this time with new variant called TR-4.  TR- 4 ( Tropical race-4) of panama wilt pathogen was reported first in Taiwan  in 2019 and spread gradually  to  China, Philippines and Indonesia, Australia and other banana growing countries.

        The disease was reported in Australia as early as 1876.  Wide spread of disease was  reported since 1890 to Caribbean Islands and by 1930 to most banana growing regions.  Worst hit was Caribbean Islands  which were   known for growing export quality banana. In India,   first incidence of  disease was noticed in 1911  and later  found causing  huge damage in states like Tamilnadu, Karnataka, Kerala. 

Fusarium oxyporium f.sp.cubence, the causal organism of fusarium wilt

       The Fusarium oxysporium.f.sp, cubence was first isolated in Cuba in 1910 and its detailed pathogenicity was published on 1919. The pathogen shows specificity to genotypes of banana and related host. Race- 1 affected Gros Michel cultivar, Race- 2 on Bluggoe, Race-3 on helicona, a related banana species  and Race -4 impacted the cultivar like  Cavendish.

        The fusarium oxysporium.f.sp, cubence is a soil born fungi, known to produce  chlamydospores, a thick walled structures which aids the pathogen to overcome adverse conditions. Activation of chlamydospores usually happens when they come in contact with root exudate secreted by host plants. Wounds in host's feeder roots extending to vascular tissues becomes easy target. Later pathogens may spread larger portion of rhizome, then reach pseudo-stem and petioles of older leaves. It eventually blocks the xylem vessels, thus transportation of water and nutrients.

       Spread of wilt causing pathogens happens  in different way. When the roots of healthy plants come in contact with roots of infected, localized spreading happens. Flowing water, flood water also aid in disbursal of disease to distant area. Use of soil inhabited with fungus is also source of inoculum when used in new plantations. Use of infected propagating material is another prominent  source of spread. Though suckers from infected mat may not exhibit any symptoms and looks healthy but carry the source of inoculum. Machinery, equipment's, visitors and workers may become carrier of infection. At times, irrigation water also carries the pathogen to infect new area.

Symtoms of the Disease

          Development of faint yellow streaks on petiole of lower leaves is the initial symptoms that affected plants exhibit in case of incidence of panama disease. Later the affected leaves either  continue to show yellowing or remain green before they collapse. Newly formed leaves remain distorted, blotchy, of irregular shape and size, with pale margin. As disease advances, most of the leaves collapse and dry and started hanging from the plant, leaving few central leaves erect or even entire plant collapse. The pseudo-stem shows longitudinal splitting at peripheral leaf sheath. At time, split on the stem may not be a symptoms. Oldest plant in the mat gets affected first, but younger plants or suckers may remain without showing any symptoms. Fusarium pathogen does not infect fruits bunch, however suckers developed from infected mat may show per-matured and uneven ripening. Affected plants show root discoloration and rotting of roots if we uproot and closely examine the diseased plants.

Management of Fusarium Wilt:

            Unlike other fungal pathogen, chemical means of control has not proved  much effective against panama wilt  fungus.  Thus other methods like mechanical, cultural and sanitary are to be well integrated to mitigate the threat. In this regard following measures may be considered for effective management,

 Selection of infection free prorogation materials:  Suckers or tissue cultured seedling used should be free from fungal inspections. Suckers from infected mat are to be strictly avoided. Though, suckers from infected mat  may not show external symptoms, but may become carrier of infection into new area. Such plant may not produce good yield, may be with defective, unevenly ripened, less merchantable  fruits.

Keeping the plantation free from  weeds: Some kind of  weeds belong to Asteraceae, Poaceae , Comellinaceae  act as alternate host fusarium causing pathogens without showing external symptoms. Thus regular weeding to to keep the field free from weed and other vegetaions is very essential.

Fallowing of infected field: Keeping the infected field uncultivated for some duration, may be a season or two,  has also proved beneficial . Some have recommended to keep the fallowed land inundated with stagnation water.   Rice cultivation in affected field shown success in Thaiwan.   Here care may be taken to avoid flow of  such field to fresh area which may source of infection.

Cultivation of resistant variety:  Cultivation resistant variety like cavendish has been found very economic way of management.  Fusarium wilt fungal is host or cultivar specific and cavendish has remained resistant for long period and contributed sustained banana production.  Other cultivar reported to have resistant are poovan, Rajabale, Vamankeli, etc.

Fungicidal application: It is not much effective in curing of affected plants. But can be taken up as preventive means, by injecting Carbendazim 2 % with 0.1% Agallol into propagating material.

Biological control: According to Meredith, supplying of  antagonistic fungus like Pseudomonas fluorescence near the root zone of  banana plant ensure some kind of biological control.

Mechanical measures: Removal and destruction of infected plants along with entire mat is essential to avoid spread of disease. Damage to roots and rhizomes of banana plants has to be avoided to prevent its entry.