Mycoflora of Roots of Texas Palm (Sabal mexicana)

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HOLLIER AND DOUGHTY:SABALROOTMYCOFLORA
P r i n c i p e s , 3 6 ( I1) 9, 9 2 ,p p .2 5 - 2 7
Mycoflora of Roots of Texas Palm
(Sabalmexicana)in Louisiana
C. A. HorrruRANDS. C. DoucurY
Louisiana CooperatiueExtension Seraice,Baton Rouge, LA 708O3
Root rot has been reported as a problem
of palms worldwide (Carpenter and Elmer
1978, Djerbi 1983, Joseph1978, Joseph
and Jayasankar I 982). Various fungi have
with seriousdamageto the
beenassociated
root system expressed aboveground as
decline,diebackor death ofthe entire plant
(Carpenterand Elmer 1978, Joseph1978,
Josephand Jayasankarl9B2).
Several dozen Texas palms in New
Orleans, Louisiana display symptoms of
decline or dieback and root rots have been
associatedwith these symptoms since the
early 1980's. Attentionwasdrawnto these
palms becausethey either were leaning at
anglesother than vertical which is normal
for the species or had completely fallen
over. Damage caused by root rots is difficult to assessin the field.
Our objectiveswere to determinel)the
frequency of fungal invasion of roots of
Texas palm and 2) the extent to which
invaded tissuesshow diseasesymptoms.
Materials and Methods
Belowground parts of four asymptomatic and five symptomatic Texas palms
were assayed for internally borne fungi.
Roots were exposed,cut with a shovel and
washedto remove loosesoil. The cut plant
roots were kept on ice en route to the
Iaboratory, where they were cleaned by
agitation in a mild detergent solution and
rinsed under running tap water. Root sections.werecut into about I-cm pieces,and
pieces were randomly selected for assay.
Tissuesassayedfrom each plant included
20 pieces of root replicated five times for
a total of 100 piecesper root system.
After tissue pieces were examined for
discolorationor necrosis,they were surface-sterilized in an aqueous solution of
0.5% NaOCI (Clorox) and 57o ethanolfor
2 rnilrt,rinsed with distilled water, blotted
dry on sterile tissue paper and placed on
potato dextroseagar (PDA) amendedwith
30 rng/L each of chlortetracycline-HCl
and streptomycin sulfate.Plateswere incubated at 26" C for four days. Fungi were
then counted directly or subcultured on
PDA for identification.
Data pertaining to fungi isolated from
symptomaticand asymptomaticplantswere
used to calculate relative frequency (RF)
of invision. RF was definedas the number
of root piecesinvaded per 2O-piecesample.
Results
Roots of both symptomatic and asymptomatic plants rarely showeddiseasesymptoms. From symptomatic plants I I7o of
the total number of root pieces assayed
were discolored while those from asymptomatic plants numbered less than 37a
although one or more fungi were isolated
from over 75% of the root piecesregardless of source.
Rhizoctonia solani andFusarium spp.
were isolatedmost often from symptomatic
plantswith meansof I9.4 and 4.6, respectively (Table l). Curuularia spp., Trichodermaspp.and Phoma spp.wereisolated
Iessoften with RF valuesof 2.1, 1.5 and
0.4, respectively.
[Vor. 36
PRINCIPES
26
Table 1. Relatiue frequency (RF) of
isolation of fungi frorn roots of symptomatic and asytnptomatic plants of S.
mexicanain New Orleans.
Symptomatic
(RF
means)#
Fungus
Curoularia sp.
Fusarium sp.
Phoma sp.
Rhizoctonia solani
Trichoderma sp.
2.1 c
4.6 b
0.4 d
19.4 a
I.5 cd
Asymptomatic
(RF
Signifmeans)# icance
1.2 lr
2.2 b
0.7 c
8.5 a
9.5 a
NS
ns
# RF : the number of root pieces invaded per
20-piece sample. Means in columns followed by the
sameletter do not differ according to LSD (P : 0.05).
* Indicates significant differences between means
in a row (P: 0.05, ns: not significant).
From asymptomatic plants Trichoderma spp. and R. solani were isolated
most often with RF valuesof 9.5 and 8.5,
respectively.Fusarium spp., Curuularia
spp. and Phorna spp. were found lessfrequently (Table l).
When RF valuesof symptomaticversus
asymptomatic plants were compared,
symptomatic R. solani-rnfectedroot values were significantly greater than when
comparedto its presenceon asymptomatic
pla.nts.The same was true for Fusariurn
spp., but the oppositewas true for Trichoderma spp. (Table l). The other fungi
isolateddid not differ in frequency regardless of source.
Discussion
The fungi isolatedfrom the root systems
of symptomatic and asymptomatic Texas
palms were the same regardlessof source.
Root discoloration percentage varied
slightly but the percent of root piecesfrom
which one or more fungi were isolatedwas
very high.
In symptomatic plants the RF of Rhizoctonia solani was significantly greater
than for any other isolated fungus. It was
found more than four times that of any
other firngus, R. solani is a known root
rotting organismof severalspeciesof plants
(Agrios I988). Its presencein such quantity may explain the slight increase in the
observed root discoloration of the symptomatic plants and the presenceof aboveground symptoms. Currently pathogenicity studies are underway to determine the
role of R. solani in this process.
Trichoderma spp. was more prevalent
from the roots of asymptomatic plants. It
was evident that as Trichoderrna spp.
increased,the RF of R. solani decreased.
Trichodernta spp. is a known antagonist
to several fungi (Agrios l9BB, Chet and
Baker 1980) and probably explains the
decreasein the presenceof R. solani. The
reduction in the presenceof R. solani from
the roots of asymptomatic plants further
supports the theory that R. solani plays
an important role in the deterioration of
Texas palm root systemsand subsequently
leadsto symptom expression.
Some fungi commonly isolated from
palm roots, species of Monacrosporiurn,
Cylindrocarpon, Pythium and Penicilliunt. were noticeably missingin our study.
These fungi might have been present but
not detected becauseof competition from
other microorganisms,incompatibility with
Texas palm or unsuitability of the culture
medium used for isolation.
Even though we obseryedrelatively little root discolorationor necrosisindicative
of disease,fungi, including reported root
pathogens,were frequently found. Perhaps
the presenceof fungi in synptomlessTexas
palm root tissue is a more common phenomenon than previously believed. For
symptomlessbut invaded roots to become
diseasedmay require one or more stress
factors or the natural weakeningof senescenceto trigger pathogenic activity by these
fungi.
LrreRerune CIreo
AcRIos, G. N. 1988. Plant pathology, Academic
Press, San Diego, 803 pp.
HOLLIERAND DOUGHTY:SABALROOTMYCOFLORA
r992)
Canprxrnn, J. B. eno H. S. Elrr.trn. 1978. Pests
and diseasesof the date palm. Agric. Handbook,
USDA, no. 527, 42 pp.
Csnr, I. .rtto R. Bernn. 1980. Induction of supto Rhizoctonia solaniin soil. Phypressiveness
topathology 7 0: 994-998.
of date palms(Phoenix
DJERBI,M. 1983. Diseases
dactylifera). FAO (I983), 106 pp.
BOOKSTORE
27
JosEPH,T. 1978. Some fungi associatedwith the
root system of coconut in the root (wilt) affected
area. Current Science47: 586 587.
ANDN. P. JeyesenreR. 1982. Evaluating
root degenerationin coconut in relation to root
(wilt) disease.Plant Disease 66: 667 669.
(Continued
from page 24)
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