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Fungi
Associated with Plant Mortality in Dogwood (Cornus florida)
Seedlings
Frank A.
Mrema
Institute of Agricultural and Environmental Research Seminar Series
Nursery Crop
Research Station
Tennessee State University, McMinnville, TN
March 9, 2005
A persistent problem of crown rot, stem
rot and dieback has resulted in the loss of many dogwood (Cornus
florida) plants in shadehouse, greenhouse and field environments at
the Otis Floyd Research Center, McMinnville. Several fungi were isolated
and identified using morphological and cultural characteristics as well
as DNA sequences. The fungi were tested for their ability to produce
extra-cellular phenol oxidase enzyme associated with the depletion of
polysaccharide and lignin components in wood. Out of 40 fungal isolates,
7 isolates (18%) displayed the ability to produce phenolic enzymes, but
only three fungi showed strong enzyme activity. DNA sequences of the 3
isolates showed a GenBank match to Collectotrichum spp. and
Rhizoctonia spp. one isolate did not match any GenBank reports.
Based on morphological and cultural characteristics, some of the 7
isolates were identified as Botryosphaeria spp. and some were of
the genus Phytophthora. However, the DNA sequence submitted to
the GenBank did not produce a match for either Botryosphaeria spp.
or Phytophthora spp. and some isolates did not get close matches
to any reported fungi. The six fungal isolates that displayed enzyme
activity were tested for pathogenicity on healthy dogwood seedlings, the
isolates included Botryosphaeria spp., Phytophthora,
Collectotrichum spp. and Rhizoctonia spp. Host response was
evaluated on the basis of lesion size around the inoculated site. All
six isolates caused stem lesions of different lengths, but none of the
fungi reproduced the symptoms they were isolated from. Rhizoctonia
is known to cause root and crown rots, Phytophthora spp. cause
root rots, or stem cankers, and foliage blights, Botryosphaeria
is known to cause stem cankers, and Colletotrichum cause die-back
and leaf blights. In this study, each organism was tested individually;
it is possible that plant mortality observed in dogwood seedlings was
caused by a synergistic effect of more than one pathogen.
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