Abstract:
Black rot disease of pineapple (caused by Thielaviopsis paradoxa) is a common post harvest
problem in Sri Lanka and other pineapple growing countries. Anthracnose caused by
Colletotrichum gloeosporioides is the most common post harvest disease of rambutan
(Nephelium Icippciceum L.). Both diseases lead to loss of overall quality and shelf life of
ripening and ripened fruits. Presently fungicides are used to control both diseases. Consumer
resistance to the use of fungicides has precipitated the search for alternative approaches to
control the above mentioned diseases.
Isolates of Trichoderma (TR1, TR2 and TP3) obtained from soil samples collected from
rambutan orchards and pineapple plantations from different regions of Sri Lanka had similar
morphological and physiological characteristics. Analysis of genome DNA of the isolates
showed that all isolates were Trichoderma asperellum. This was the first report of the
isolation of T. asperellum from a Sri Lankan habitat. T. asperellum had antagonistic effect
against both Th. paradoxa and C. gloeosporioides. The antagonistic activity was mainly due
to coil formation around the pathogen hyphae by T. asperellum. The pathogen then fails to
grow, when transferred to fresh Potato Dextrose Agar (PDA) suggesting the fungicidal
effect of the antagonist on the pathogen.
Laboratory experiments demonstrated that out of the several media examined Yeast Waste
Residue (YWR) was the most suitable medium for mass propagation of T. asperellum.
The powder formulation of T. asperellum at a concentration of 1% (aqueous) had the ability
to control the growth of Th. paradoxa. This concentration was found to be the Minimum
Inhibition Concentration (MIC) of the powder formulation. The effect of T. asperellum
powder formulation treatment on Th. paradoxa inoculated soil was also examined. T.
asperellum formulation reduced the pathogen concentration in treated soil to levels below
that needed to cause disease.
The liquid formulation of T. asperellum had high antagonistic activity against both
pathogens in-vitro as well as in-vivo. Pineapple (Ananas comosus) inoculated with 105
conidia/mL of Th. paradoxa, followed by an application of the liquid formulation either 10
or 30 min after inoculation, remained free of the black rot disease when stored at 28 °C for 7
days. Fruits treated with the blank formulation (formulation without conidia) and the control
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fruits (untreated and fruits treated with sterile distilled water) developed characteristic
symptoms of black rot disease. Correspondingly, black rot symptoms of the disease were
observed in fruits that were inoculated and held as controls under similar storage conditions,
with no formulation applications. No significant differences (p > 0.05) occurred among
formulation treated, untreated (controls) and blank formulation treated fruits with respect to
pH value, total soluble solids and titratable acidity. Fruits treated with the formulation more
than 30 min after inoculation with the pathogen developed disease symptoms.
Similar results were obtained with the liquid formulation treated rambutan. As with
pineapples the formulation was not effective if applied more than 30 min after inoculation
of pineapples, with the pathogen. Further, no significant differences (p > 0.05) were
observed between pH value, titratable acidity and total soluble solids of treated, untreated
and blank formulation treated fruits. The shelf life of this formulation was retained for upto
six months as the conidia remained viable for up to six months. The data suggests that the
formulation of T. asperellum would be an alternative eco-friendly approach to the control of
black rot disease in pineapple and anthracnose disease of rambutan.