Citrus Experiment Station
Mimeo Report Series No. MR-59-3
THE HOST STATUS OF LYCHEE WITH REFERENCE TO THE BURROWING NEMATODE
S -. --- Harry W. Ford
f Florida Citrus Experiment Station
Te burrowing nematode, Radopholus similis (Cobb) Thorne, an endoparasite,
is th pr -may-cause of a serious citrus disease known as spreading decline (7).
At the pe' ttime, the roots of more than 400 citrus varieties, species, and
relatives have been found to be susceptible to attack and damage from this
destructive pest (3). Also, more than 150 kinds of ornamentals and other
plants (6) are hosts of the burrowing nematode. Undoubtedly, more hosts will
be found as additional plants are studied. It is now apparent that the kinds
of plants that cannot be invaded by this nematode will be a small fraction of
the flora of Florida.
Most ornamental and other non-citrus plants have been studied by examining
roots obtained from known burrowing nematode-infested sites in citrus plantings
and greenhouses. The roots were incubated by the Young procedure (9) and also
dissected under the microscope to determine if the nematodes were surviving on
the host. Burrowing nematodes were not recovered from certain of these plants
such as lychee (1), crotalaria (8), and others (6). An additional test was
also made on these plants from which no burrowing nematodes were recovered.
The plants were grown in containers at a temperature satisfactory for nematode
development. After several months, up to a year, the roots were examined for
burrowing nematodes. If no nematodes were found, the plant was considered a
nonhost or at least resistant to the burrowing nematode. This method by which
a plant was judged a nonhost of the burrowing nematode did not give specific
information about the penetration of the nematode into roots, or its subsequent
ability to lay eggs.
Crotalaria spectabilis was recommended as a non-host cover crop (8) after
no burrowing nematodes were found on roots of Crotalaria that had been grown in
infested field soil in a temperature tank. Subsequent studies (4) using a
Petri dish technique involving two-day-old seedlings in the laboratory showed
that the burrowing nematode could penetrate the roots of Crotalaria and damage
the invaded roots. A few nematodes lived for at least 30 days and were able to
reach maturity. The appearance and activity of Radopholus similis in the roots
of this plant suggested that it was actually an unsuitable host rather than a
nonhost of the burrowing nematode. Following these results, studies using
similar techniques were started with lychee, Litchi chinensis Sonn.
METHODS AND RESULTS
Experiment I.-- Four air-layered Bengal lychee plants were secured from the
U. S. Plant Introduction Station, Coconut Grove, Florida, and planned in a
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greenhouse bench filled with soil containing a heavy population of burrowing
nematodes. After 15 and 30 days, root samples were taken for study by digging
with a hand trowel and sifting the roots free of soil. Most of the roots
obtained by this method were one millimeter or larger in diameter. Some of
the roots were dissected under the microscope, others were examined using the
Young procedure, and a few were stained with osmic acid in an effort to see
nematodes inside the roots.
No burrowing nematodes were found by any of the procedures that were
employed. It was discovered that many fine rootlets had broken away from the
collected roots at the time the samples were taken from the infested beds.
The points where the fine rootlets had been attached could be clearly seen in
the stained root samples. Attempts were made, without success, to recover
the fine rootlets and root hairs from plants in the greenhouse beds by using
extreme care in sifting. By using a three-power lens, the tiny structures
could be seen breaking from the larger rootlets as the soil was disturbed.
Finally an entire plant was removed to a screened box without materially
disturbing the root-soil environment. The roots were separated from the soil
with a fine water spray. In this manner a few of the small rootlets were
obtained for study. These roots were about 0.1 millimeter in diameter which
is similar in size to a human hair. Six live female burrowing nematodes were
obtained from roots incubated over a period of 12 days using the Young procedure.
A sample of roots was stained with osmic acid but many of the fine rootlets
broke in the process so that the sample was too small to determine if the
nematodes had actually been inside the roots.
Experiment II.-- A procedure was developed in the laboratory to reduce
the breakage of fine rootlets. A germinated Bcewster lychee seed was placed on
a thin layer of soil above plastic screen wire which was laying on an aluminum
square. The aluminum square containing the lychee plant was placed on one-fourth
inch of sand in a six-inch diameter plastic culture dish. The area around the
new root was marked on the plastic screen with a wax pencil. Fifty burrowing
nematodes were placed on each marked root section and the dish filled with soil.
After four to eight weeks the aluminum squares were lifted from the cul-
ture dish and the undisturbed roots carefully washed with a fine water spray.
The extremely fine rootlets and root hairs were preserved by this technique.
The rootlets were taken from the original area of inoculation and dissected
under a stereomicroscope. The remainder of the roots were incubated by the
Ten lychee seedlings were inoculated and held for four, six, and eight
weeks. Two plants were examined after four weeks. On one plant, 14 individual
burrowing nematode lesions were found in the fine rootlets, 0.1 to 0.2 mm. in
diameter, near the original area of inoculation. Extensive damage to the root
cortex in each lesion could be seen. Six live females were found inside the
roots together with numerous eggs. No larvae were found. No lesions or nema-
todes were found in roots larger than 0.2 mm. in diameter.
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The root system of two plants was removed from the plastic dishes after
six weeks. A lesser number of rootlets was found in the region of inoculation.
It is not known whether this was the result of nematode damage. One dead female
and one dead larva was found in one lesion.
After two months the roots of the remaining six plants were removed from
the dishes and the roots dissected. There were considerably fewer rootlets in
the inoculated area than were presentoon roots infested for only one month.
None of the roots from the original inoculated area that were dissected contained
burrowing nematodes. The roots outside the inoculated area were incubated by
the Young procedure for seven days prior to dissecting and no burrowing nematodes
DISCUSSION AND CONCLUSIONS
The presence of extremely small rootlets on lychee has not been reported
in Florida. Lui (5) stated that lychee roots are finely branched with abundant
root hairs. There was no reference to size. These fine rootlets cannot be
adequately sampled in the field by digging with a shovel or auger. A small
sample of fine rootlets was obtained by pressing a screened structure under the
root system, cutting around the structure, and washing gently with water. The
roots were studied in the laboratory and it was evident that most of the fine
rootlets had broken and were lost even by this procedure.
The terminology used to define the relationship of the host plant to the
nematode has been used with different connotations by various workers. Any
condition of nonsusceptibility could be called resistance. Since this covers
a rather wide range, varying degrees of resistance are frequently reported
depending upon the interpretation of the individual. In this paper the writer
prefers to reserve the term immunity for the highest form of resistance where
the nematode does not feed upon and does not penetrate the root tissues of the
plant. A plant is called resistant to the nematode, even though invaded by the
nematode, if plant growth is not retarded and the nematode population gradually
diminishes to a low level or. completely disappears. This is the interpretation
used in the citrus rootstock screening program (2).
The data presented in this paper are in agreement with DuCharme and Suit (1)
who reported that populations of burrowing nematodes can not be maintained on
the Brewster lychee. However, nematodes did penetrate the root system. The
fact that one larva was found indicates that at least one viable egg was produced
and hatched. The presence of lesions on the fine rootlets show that these
structures can be damaged by the activity of the burrowing nematode. Apparently
this damage is superficial to the root system of the lychee since there are
many feeder roots larger than those that can be invaded. Therefore, it is
suggested that the host status of lychee should be considered as one of high
resistance rather than immunity.
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1. DuCharme, E. P. and R. F. Suit. 1955. Immunity of the lychee from the
burrowing nematode. Proc. Fla. State Hoct. Soc. 68: 270-272.
2. Feder, W. A. and H. W. Ford. 1957. Susceptibility of certain citrus
varieties, species, and relatives to the burrowing nematode. Proc. Fla.
State Hort. Soc. 70: 60-63.
3. Feder, W. A., H. W. Ford, J. Feldmesser, F. E Gardner, R. F. Suit,
A. Pieringer, and P. C. Hutchins. 1958. Citrus varieties, species, and
relatives susceptible to attack and damage by the burrowing nematode,
Radopholus similis. Plant Dis. Reptr. 42: 934-937.
4. Ford, H. W. and C. I. Hannon. 1958. The burrowing nematode, Radopholus
similis, in roots of Crotalaria spectabilis. Plant Dis. Reptr. 42: 461-463.
5. Lui, Su-Ying. 1954. Studies of Litchi chinensis, Sonn. Unpublished Thesis,
University of Michigan.
6. State Plant Board of Florida. 1958. (Unpublished). Hosts and suspected
host plants of the burrowing nematode Radopholus similis (Cobb) Thorne.
7. Suit, R. F. and E. P. DuCharme. 1953. The burrowing nematode and other
parasitic nematodes in relation to spreading decline of citrus. Plant
Dis. Reptr. 37: 379-383.
8. Suit,R. F., and E. P. DuCharme, and T. L. Brooks. 1955. Effectiveness of
the pull-and-treat method for controlling the burrowing nematode on citrus.
Proc. Fla. State Hort. Soc. 68: 36-38.
9. Young, T. W. 1954. An incubation method for collecting migratory endopara-
sitic nematodes. Plant Dis. Reptr. 38: 794-795.
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