|
Authors:
Richard T. Hanlin and Mei-Lee Wu
Department of Plant Pathology, University of Georgia
Athens, GA 30602
Timothy B. Brenneman
Department of Planta Pathology, Coastal Plain Experiment Station
Tifton, GA 31793
Abstract:
A hypogeous fungus found among roots in a pecan orchard represents
the first report of a truffle from Georgia. It is identified as
Tuber texense Heimsch. An illustrated description of the
Georgia material is presented.
Introduction:
Records of ascomycetes reported from Georgia have been maintained
for over 70 years (Miller, 1941; Hanlin, 1963), and during that
time Elaphomyces has been the only genus of hypogeous ascomycetes
recorded from the state. In September, 1987, during a visit to a
pecan orchard near Albany, Georgia, numerous ascomata were observed
among roots of several large pecan trees [Carya illinoiensis
(Wang.) K. Koch] that had been exposed by soil erosion caused by
a recent rain. Several ascomata were taken to the laboratory and
examined, and were subsequently identified as Tuber texense.
As this is apparently the first report of a tuberaceous species
from Georgia and since it differs slightly from previous descriptions,
an illustrated description of the Georgia material is presented
here.
Materials and Methods:
General observations and measurements of asci and internal tissues
were made on fresh material mounted in water. Material to be sectioned
was cut into 5mm² blocks, fixed in formalin-propionic acid-alcohol,
dehydrated through a tertiary butyl alcohol series, embedded in
paraplast, sectioned at 6 or 10 µm and stained in iron hematoxylin.
Sections to be examined under the scanning electron microscope were
mounted on an 18 mm round cover glass, deparaffined in xylene, then
the cover glass was mounted on a stub and sputter-coated with gold-palladium
in a Hummer sputter coater (Gaudet and Kokko, 1984). Light micrographs
were taken with a Nikon Optiphot on Kodak Technical Pan film 2415;
scanning micrographs were taken on a Philips 505 SEM with Polaroid
Type 55 P/N film. These procedures have been previously described
in greater detail (Hanlin & Tortolero, 1988).
Observations and Discussion:
Tuber texense Heimsch
Ascomata hypogeous, up to 5.5 cm across, oval to nearly globose,
light brown to dark reddish-brown, surface smooth, often lobed,
especially on one side (Fig. 1). Interior consisting of a gleba
with light and dark veins surrounded by a cortex (= medullary excipulum)
and an exterior layer (= ectal excipulum) of pigmented cells (Fig.
2). Cortex 240-280 µm thick, composed of three regions (Fig.
4, 8). Outermost 2-3 rows of cells angular (textura angularis),
with somewhat thickened, pigmented walls (Fig. 5). These surface
cells integrade into a region ca. 125 µm thick of small, compactly
arranged pseudoparenchymatous cells with thin walls that vary in
shape from globose to angular or short hyphal; the latter are oriented
perpendicularly to the margin of the ascoma. Inner half of cortex
(subcortex) composed of small, thin-walled, globose cells among
tightly interwoven hyphae oriented parallel to periphery of ascoma;
this region extends into gleba as sterile veins. Gleba composed
of white, convoluted sterile veins (venae externae) bordered by
brown fertile veins (venae internae) (Fig. 3). Sterile veins variable
in width, branched, composed of hyaline, interwoven hyphae (textura
intricata) (Fig. 10), becoming compressed by developing asci (Fig.
6, 9). Fertile veins brown, cells crowded, often indistinct and
appearing as surrounded by mucus at maturity, enveloping numberous
asci (Fig. 7, 11). Ascigerous areas in immature ascomata often separated
by small veins of hyaline hyphae that form a loose, open network
that is crushed as the asci develop. Asci unitunicate, thick-walled
(Fig. 16), persistent, variable in size, (70)-88-(122) X (34)-48-(58)
µm (including stipe), subglobose to ovoid, short- or long-stipitate
[stipe (8)-22-(58) µm], containing 1-6 (usually 4) spores (Fig.
14-15). Ascospores unicellular, oval to occasionally subglobose,
golden-brown at maturity, (22)-28-(36) X (16)-19-(24) µm, containing
oil droplets, densely covered with spines that vary from minute
to long and distinct (Fig. 17-18), up to 2 µm in length. The
bases of the spines interconnect to form a reticulate pattern (Fig.
18).
Collected among roots of Carya illinoiensis (Wang.) K.
Koch, Dougherty County, Georgia, September 18, 1987, T.B. Brenneman
and P.F. Bertrand. Specimens deposited in GAM (#12742).
The material collected contained asci in all stages of maturity,
permitting observation of ascus development. Asci arise from croziers
formed from ascogenous hyphae (Fig. 12). The ascus mother cell expands
to form a clavate ascus (Fig. 13), which then develops into a mature
ascus with ascopores. Developing asci often have a clump-like structure
at the base that results from the fusion of the tip and basal cells
of the crozier; this apparently forms the basis for the erroneous
reports in the literature that tuberaceous asci possess clamp connections
(Alexopoulos and Mims, 1979).
The structure of our material agrees well with that described for
T. melanosporum Vitt. (Parguey-Leduc et al., 1987), except
for the large pyramidal scales that cover the surface of T.
melanosporum.
Gilkey (1939) recognized a single species of Tuber in
North America with spinose ascospores, T. candidum Harkness,
to which she later added T. harknessii Gilkey (1954). A
third species, T. texense, was described from Texas by
Heimsch (1958); this species differs from T. candidum and
T. harknessii in the formation of a reticulum on the spore
surface that is associated with the spines. On the basis of these
characteristics, our material is considered to be T. texense.
In addition to Texas and Georgia, this species has also been found
near Gainesville, Florida (James Kimbrough, personal communication).
Like our material, T. texense was found among roots at
the base of a pecan tree. The occurrence of numerous ascomata in
close association with roots of pecan suggests the possibility of
a mycorrhizal association, but no direct evidence was found to support
this. Tuber melansporum has been demonstrated to form mycorrhizae
with Corylus avellana L., Quercus spp., and other
hardwood species (Delmas, 1976).
Acknowledgements:
The assistance of J.O. Owens and P.F. Bertrand is gratefully acknowledged.
The manuscript was reviewed by J.W. Kimbrough and R.G. Roberts.
Literature Cited:
Alexopoulos, C.J., and C.W. Mims. 1979. Introductory Mycology,
3rd ed. John Wiley & Sons, New York, 632 p.
Delmas, J. 1976. La truffe et sa culture. Inst. Natl. Recher. Agron.
Etude No. 60: 1-54.
Gaudet, D.A., and E.G. Kokko. 1984. Application of scanning electron
microscopy to parraffin-embedded plant tissues to study invasive
processes of plant-pathogenic fungi. Phytopathology 74: 1078-1080.
Gilkey, H.M. 1939. Tuberales of North America. Oregon St. Monogr.
Stud. Bot. No. 1: 1-63.
Gilkey, H.M. 1954. Tuberales. No. Amer. Flora, Ser. II (Pt. 1):
1-36.
Hanlin, R.T. 1963. A revision of the Ascomycetes of Georgia. Univ.
Georgia Agric. Expt. Sta. Mimeo Ser. N.S. 175: 1-68.
Hanlin, R.T., and O. Tortolero. 1988. Morphology of Sclerotium
coffeicola, a tropical foliar pathogen. Can. J. Bot. 66 (in
press).
Heimsch, C. 1958. The first recorded truffle from Texas. Mycologia
50: 657-660.
Miller, J.H. 1941. The Ascomycetes of Georgia. Pit. Dis. Reptr.
Suppl. 131: 31-93.
Parguey-Leduc, A., C. Montant, and M. Kulifaj. 1987. Morphologie
et structure de l'ascocarpe adulte du Tuber melansporum
Vitt. (Truffle noire du Perigord, Discomycetes). Cryptogamie, Mycol.
8: 173-202.
|
