Biological control of Colletotrichum gloeosporioides.
Colletotrichum gloeosporioides is the causal agent of anthracnose disease of mangoes.
Infection occurs when humidity is high and rain-dispersed spores germinate and form an
appressorium on immature mangoes. The infection then becomes quiescent until the fruit
is harvested. On ripe fruit infection is visible as black, sunken lesions on the surface. At
the pre-harvest stage, the disease is controlled with the application of a range of
fungicides, and at the post-harvest stage by hot benomyl treatment. The extensive use
of benomyl, both pre- and post-harvest, has resulted in the occurrence of isolates of C.
gloeosporioides resistant to this fungicide. To devise an alternative strategy of disease
control, the potential for biological control of anthracnose has been investigated.
Potential microbial antagonists of C. gloeosporioides were isolated from blossom, leaves
and fruit of mango, and screened using a series of assay techniques. In total 650
microorganisms, including bacteria, yeasts and filamentous fungi, were isolated and
tested for their inhibition of growth of C. gloeosporioides on malt extract agar. Of these
650 isolates, 121 inhibited the fungus and were further tested on their ability to inhibit
spore germination in vitro. Of these, 45 isolates, all bacteria and yeasts, were inoculated
onto mangoes, which were artificially inoculated with C. gloeosporioides, and assessed
for their potential to reduce the development of anthracnose lesions. A further selection
was made, and 7 isolates were chosen to be used in a semi-commercial trial in the
Philippines. This final screening procedure yielded two potential candidates for field
trials, isolate 204 (identified as Bacillus cereus) and isolate 558 (identified as
A field trial involving pre-harvest application of the biological control agent, was
conducted using isolate 558. This isolate was chosen for this purpose since in in vitro
experiments it significantly reduced germination of C. gloeosporioides spores. In the field
trial 558 was applied in combination with nutrients and compared to treatments which
had received no treatment or which had received conventional fungicide (benomyl)
application. On spraying, high numbers of 558 were recorded on the leaf surface, but no
reduction in post-harvest development of disease was observed. Failure of disease
control was attributed to rapid death of the bacterium on the phylloplane.
Inpost-harvest trials, isolates 204 and 558 were both tested in combination with different
application methods, including the addition of sticker, peptone, fruit wax or a sucrose
polyester. Application of 204 did not reduce disease development. Application of 558,
however, did significantly reduce anthracnose development compared to the control
fruit. No additional benefit was achieved by incorporating the bacteria in peptone, fruit
wax or sucrose polyester.
The mode of action of isolate 558 was investigated in detail. There was no evidence for
parasitism taking place, or the production of volatile compounds, in the suppression of
disease development. No antibiotic compounds were detected, but isolate 558 did
produce a siderophore. A sharp increase in pH was also observed in culture media in
which 558 was grown. Disease control may result from a combination of these two