Sisal, a plant traditionally known to be highly resistant and rare vegetation in the world, which has hitherto been free from any serious disease atall levels of its lifetime, is gradually losing the immunity and now threatened by strange ailments.
The plant yields a stiff fibre, traditionally used in making twine and ropes, reproducing itself vegetatively as suckers and bulbils.
Bulbils develop from buds below the flowers on the branches of sisal poles, while suckers sprout out at the end of underground stems – radiating from the base of the plants.
It has a life span of between 10 and 15 years, depending on variety and grows a pole in the middle – with a height of 3.5 to 4.5 metres.
Sisal is grown in the tropics and flourishes in all weather conditions. It is drought resistant-unlike any other plant in the globe able to tolerate a variety of soils.
The vegetation has, over decades, never bothered agricultural experts to provide it with pesticides and fungicides at any one time.
But for a considerable time now, researchers in agriculture have had a tough time trying to find out the cause of a strange disease that has been affecting the resistant plant.
The disease – Korogwe Leaf Spot (KLS) -- first surfaced in 1951 in some few sisal estates in Korogwe district in Tanga Region.
KLS has now spread its wings to all sisal plantations in the country, causing considerable yield losses and deterioration in the quality of sisal fibre.
It manifests itself in colouration of sisal leaves which would normally appear in chocolate brown spots – more often in mature sisal.
The serious effects of the ailment include staining of sisal fibre, hence, reduced yield.
Shabani Hamisi, the Mlingano Agricultural Research Institute (ARI) acting director, says his institute has yet to come out with the cause of the ailment despite the ongoing research.
“It is still a puzzle why the disease is spreading so fast. A couple of years ago, the momentum was not as high as it is now,” wonders Hamisi.
Hamisi,a seasoned researcher in sisal growing, likened the disease to malaria which he says had confined itself to lowland areas but were now commonplace on the hillside in many parts of the country.
“We (researchers) ask ourselves ‘Is it because of climate change that the parasite has been aggressive or is it a normal infection?’
According to Hamisi, ARI, the first of its kind in Africa, has already done a survey on the extent and spread of the disease to find out the magnitude of the problem.
“We are trying to see if soil fertility may be the culprit or if it is a normal vegetation infection,” he says.
Presently, says Hamisi, ARI continues with research to see if there is relationship between soil fertility and disease infection.
“The fact is that the disease is so far incurable. What we are doing now, is to mitigate the effects of spread.”
He mentioned such factors as early harvesting, which he said should be between 6-9 months and routine weeding of plantations.
Research findings show that the disease causes losses estimated at about 30 per cent in yield and quality drop, resulting to substandard fibre[ug] which accounts for 70 percent of sisal fibre production in country-leaving the other –hardly 30 per cent for export (ef).KLS mainly affects Hybrid variety -the main commercial high yielding type.
Early symptoms have also been noticed in “sisalana” – the traditioned rariety introduced in the country by a German agronomist – Dr. Richard Hindorf in 1893, according to research findings.
According to research, KLS has also been observed from nurseries and suckers.
Fredrick Malisa, a prominent small-scale farmer at Mwelya/Usambara sisal estate, who owns about 45 hectares, says prerequisites toward elimination of KLS is for farmers to carry out routine cleanliness of farms as well as timely harvesting.
“KLS is likened to cancer, if weeding is not done timely, plantations will be outgrown with shrubs, hence multiplication of the disease,” said Malika who is also chairman of the Usambara Sisal Small Scale Association.
“If the leaves are not harvested in time, the disease goes deeper and completely destroys the quality of fibre,” lectures the retired government official who ventured into sisal farming in 1999 and one of the pioneers of the sisal growers scheme.
He adds: “KLS is not a deadly disease. The only problem is that it affects the grading of sisal fibre,’ adding that a farmer should actually expect nothing short of under grade fibre from an infected sisal leaf, hence an economic loss to the farmer.
Says another ARI official “Several theories have been advanced over the years as to the possible causes of the problem, but up to now, no concrete theory has been able to pinpoint the real cause of the ailment.”
“Various causes have been picked as being responsible for the disease. Some of them suggest virus, bacteria or simply insects,” he adds, saying none of these factors have, nevertheless, been scientifically verified.