R.A. Fullerton{ XE "Fullerton, R.A." }A and S.G. CasonatoA
A
The New Zealand Institute for Plant and Food Research Limited, Private Mail Bag 92169, Auckland Mail Centre 1142, New Zealand
INTRODUCTION
Mycosphaerella fijiensis, cause of black leaf streak, has a very
slow incubation period compared with many other pathogens.
The fungus infects leaves as they emerge from the plant. The
first symptoms (rusty streaks) appear in the highly susceptible
‘Cavendish’ types in 2–3 weeks, extending to over 35 days for
the more resistant ‘Ducasse’, ‘Pahang and ‘Pisang Mas’. In some
genotypes symptoms may not be expressed until the leaf begins
to senesce (1). These patterns of host response show that the
pathogen has the ability to survive without symptoms in the leaf
for extended periods. There are no records of infection of the
fruit of dessert bananas. A record of infection in plantain (2)
shows that the fungus has the capacity to invade the skin of
fruit. This study aimed to investigate whether the fungus can be
present without symptoms in the skin of ‘Cavendish’ bananas.
MATERIALS AND METHODS
Source of fruit. Green, fully developed fruit were obtained from
diseased ‘Cavendish’ plants in the field in Samoa. Additionally,
‘green‐mature’ and ripe fruit were obtained from the local
market.
Isolation protocol. Pieces of skin tissue approximately 5 mm
square and 0.5–1.0 mm thick were excised aseptically and plated
onto Potato Dextrose Agar (PDA) or V8 agar, both modified with
streptomycin and penicillin (100 µg/ml of each). Plates were
incubated under continuous white/near‐UV light and were
examined microscopically after five days
Overall, a total of 1040 skin pieces were taken from 60 fruit from
13 different sources. Five pure cultures of putative M. fijiensis
were returned to New Zealand under a Biosecurity New Zealand
permit to confirm their identities.
Identification of isolates. The identities of the cultures returned
to New Zealand were confirmed by spore morphology,
polymerase chain reaction (PCR) using species‐specific primers
provided on a confidential basis by the Cooperative Research
Centre (CRC) for Tropical Plant Protection, and by the
sequencing of the internal transcribe spacer region of rDNA.
RESULTS AND DISCUSSION
Of the five fruit isolates returned to New Zealand, two were
positively identified as M. fijiensis, with concurring results from
spore morphology, PCR, and sequencing (100% homology to
sequences of M. fijiensis in GenBank). PCR results are shown in
Figure 1. Both isolates were obtained from different skin
samples from the same fruit. Microscopical examination (in
Samoa) of a cluster of hyphae on skin of a different fruit revealed
dark hyphae strongly resembling the early stages of a stroma of M. fijiensis. The structure was insufficiently developed to obtain
a positive identification based on morphology, and overgrowth
by other fungi prevented its isolation into pure culture. In all
three cases, (two confirmed M. fijiensis, one suspected), the
fungus was associated with minute (~1 mm diameter) red,
necrotic flecks on the surface of the skin. Red flecks were
relatively common on many of the test fruit. Most did not yield
any fungi and others were overgrown by various fungal species
before the slow growing M. fijiensis would have had time to
emerge. It cannot be determined from this study whether there
is a constant association between the red fleck symptom and M. fijiensis.
In many cases the presence of M. fijiensis may have remained
undetected because of overgrowth of the skin pieces by other
faster growing fungi such as Colletotrichum spp., Cordana
musae, Penicillium sp., Cladosporium sp., Curvularia sp.,
Fusarium sp., Rhizopus sp., Trichoderma sp., and Pestalotiopsis
sp.
This study has shown that M. fijiensis can infect and survive
without symptoms in the skin of ‘Cavendish’ banana. While only
a very low recovery rate was achieved in this study, the
incidence in nature may be much higher.
–—– A ––– B – – C – – D –– E –– F – – G –– H – – I – – J
Figure 1. Amplified Mycosphaerella fijiensis products of approximately
1050 bp. Lane A: 100 bp ladder B: 88a; C: 88b; D: Mfb; E: Mfc; F: Myc#1; G: 589 yellow Sigatoka; H: 748 M. fijiensis (positive control); I: blank (negative control); J: 100 bp ladder. Arrow indicates 600 bp on 100 bp ladder.
ACKNOWLEDGEMENTS
We gratefully acknowledge the Samoa Ministry of Agriculture,
Fisheries, Forests and Meteorology for access to the laboratories
of the Nuu Crop Research Centre, and assistance in this study.
We thank Dr Juliane Henderson of CRC for Tropical Plant
Protection for supplying primer sequences.
REFERENCES
1. Cedeño L, Carrero C, Quintero K. 2000. Identification of Mycosphaerella fijiensis as cause of specks on fruits of plantain cv Harton in Venezuela. Fitopatologia‐Venezolana. 1:6–10.
2. Fullerton RA, Olsen TL. 1995. Pathogenic variability in
Mycosphaerella fijiensis Morelet, cause of black Sigatoka disease in
banana and plantain. New Zealand Journal of Crop and
Horticultural Science. 23:39–48.
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