/ /-!/ Citrus Station Mimeo Report CES 66-11
/ May 10, 1966
Rootstocks For Spreading Decline Areas
Harry W. Ford
University of Florida
Citrus Experiment Station
A search for rootstocks resistant to damage from the burrowing nematode
has been in progress since 1951. Although more than 1400 different kinds of
citrus have been evaluated, only 15 have been found worthy of field testing.
Three of these rootstocks were formally released to the industry in 1964 for
use on a trial basis although adequate growth and yield data were not avail-
able from field rootstock trials. The rootstocks were named 'Estes' rough
lemon, 'Ridge Pineapple' sweet orange, and 'Milam' lemon. It was suggested
that they be used in and near spreading decline areas where trees had been
removed and the soil fumigated.
Since 1955, the recommended procedure for combating spreading decline
has been to remove all trees infested by burrowing nematodes and to fumigate
the soil before replanting. The treatment is effective if all burrowing
nematodes have been eliminated from the area and the area is not reinfested
by contamination or spread from other infested areas; otherwise, replanted
trees on susceptible rootstocks will eventually show symptoms of spreading
decline. The use of a resistant or tolerant rootstock for replanting is one
method of living with the disease if the grove should become reinfested with
'Estes' rough lemon cannot be readily distinguished from ordinary rough
lemon in Florida. 'Estes' has been classified tolerant to the burrowing
nematode because when planted in infested soil, burrowing nematode populations
are maintained at a relatively high level although growth is not reduced more
than 20%. 'Estes' was found as the rootstock of a single healthy looking
'Valencia' tree in a grove in southern Polk County. The tree was surrounded
by trees with symptoms of spreading decline. The tree had been transplanted
into the burrowing nematode infested site in 1941. Thus, feeder roots of
'Estes' could have been infested with burrowing nematodes for 10 .or more
years before the tree was discovered in 1952. In 1958, root distribution
studies indicated that feeder roots were present to a depth of 11 feet. Root
concentrations fluctuated somewhat each year because of partial damage from
burrowing nematodes. New roots appeared to replace those damaged so that the
root system seemed to be developing fast enough to keep ahead of the damage
from burrowing nematodes. In 1965, the root system was found to be 9 feet in
depth. The deepest root penetrations of adjacent trees with visible symptoms
of spreading decline were found to be only 5 feet. The tree has consistently
yielded three to five times more 'Valencia' fruit than surrounding infested
trees,even after 21 years of infestation by burrowing nematodes. Fruit quality
studies and other observations of the 'Valencia' top, over a period of 12 years,
suggest that,except for nematode tolerance, the horticultural response from
'Estes' has been the same as ordinary rough lemon.
'Estes' was first propagated as cuttings from the root sprouts of the
parent tree. In greenhouse tests, the growth of cuttings was reduced about
20% from feeding injury by burrowing nematodes. Thus, it must be assumed
that the parent tree of 'Estes' has been damaged somewhat by burrowing
nematodes, although it has not been possible to accurately assess the total
effect upon vigor. Seedlings of 'Estes' were not available until 1959. At
that time, it was found that seedlings responded similarly to cuttings in
laboratory studies. Further studies also established that the tolerant
condition was in the rootstock and not in the scion 'Valencia' variety. When
controlled inoculation tests were performed, it was found that burrowing
nematodes penetrated feeder roots and reproduced in numbers similar to those
in ordinary rough lemon. There seemed to be a greater regeneration of new
feeder roots on 'Estes' than was found with ordinary rough lemon. However,
the exact mechanism of tolerance has not been clarified. 'Estes' is considered
tolerant to tristeza, exocortis, and zyloporosis viruses.
The fruits of 'Estes' cannot be visually distinguished from standard rough
lemons used for rootstocks in Florida The seeds, 8 to 14 in number, are
small and identical in appearance to ordinary rough lemon. The variety is
polyembryonic with a nucellar rate above 95%.
A considerable number of flowers have been found with defective pistils
on 'Estes' trees being grown to produce seed. The leaves of 'Estes' are
highly susceptible to anthracnose, one of the characteristics commonly used
in the identification of rough lemon.
'Ridge Pineapple' is a sweet orange variety resistant to the burrowing
nematode because growth is not retarded and burrowing nematode populations
gradually diminish and disappear from the root system. 'Ridge Pineapple'
cannot be readily distinguished from the midseason 'Pineapple' sweet orange
variety commonly used throughout Florida.
'Ridge Pineapple' was first discovered when seeds were screened routinely,
together with many citrus varieties, species, and relatives, in large burrowing
nematode infested soil tanks. The seedlings showed excellent growth and a
satisfactory root system compared to other susceptible kinds of citrus. The
parent tree of 'Ridge Pineapple' was located in the citrus variety collection
of the United States Department of Agriculture. It was one of several selections
of the 'Pineapple' variety that had been growing in the collection. All of
the other selections were found to be susceptible to attack and damage by the
Greenhouse growth tests of 'Ridge Pineapple' seedlings indicated that it
grew equally well in nematode infested and steam-sterilized soil. It required
about 9 months before nematodes were eliminated from the soil or roots of
'Ridge Pineapple.' The mechanism of resistance has not been studied.
It can only be assumed, at the present time, that the varieties on 'Ridge
Pineapple' will respond similarly to trees on regular sweet seedling rootstock.
'Ridge Pineapple' is tolerant to the tristeza virus. Until more data are
available, 'Ridge Pineapple' should be considered as having the same degree of
susceptibility to foot rot as other sweet orange varieties.
'Milam' lemon was the third rootstock officially released in 1965. It is
a citrus hybrid of unknown parentage resistant to the burrowing nematode. Over
a period of 9 months, 'Milam' has consistently eliminated burrowing nematode
populations from containers of infested soil. The feeder roots of 'Milam'
cuttings in an 8-year-old rootstock trial were still free of burrowing nematodes
'Milam' was found as the rootstock of a sweet orange tree in a burrowing
nematode infested grove in 1954. The 15-year-old tree, near Davenport, Florida,
was 16 feet in height and showed no symptoms of spreading decline. At that
time, the yield was six boxes of 'Parson Brown' fruit with about 10 Brix more
solids than comparable 'Parson Brown' on rough lemon rootstocks. Over an
18-month sampling period and before the entire tree was moved to Lake Alfred,
no roots of 'Milam' were found infested with burrowing nematodes. All of the
adjacent surrounding trees on rough lemon rootstock were heavily infested
with burrowing nematodes. The root system extended to a depth of 10 feet and
was quite dense to a depth of 8 feet.
During the past 10 years, grapefruit, 'Valencia,' and 'Orlando' tangelo
scions have been tested on 'Milam.' Brix has been consistently higher, by
about .50, than compared varieties on rough lemon,
Laboratory studies have established that 'Milam' reduces burrowing
nematode populations by preventing the development of nematode eggs in the
root cortex, This process of reducing populations requires about 9 months,
provided no additional nematodes are introduced from other adjacent trees.
'Milam' appears to have little resistance to root penetration or feeding by
the adult burrowing nematode.
'Milam' is not resistant to the citrus nematode, Tylenchulus semipenetrans
Cobb. This factor is very important because citrus nematodes are frequently
present in burrowing nematode-infested groves. Citrus nematodes can be killed
by the same fumigants used for burrowing nematodes.
'Milam' appears to be tolerant to the tristeza virus. Tolerance to
exocortis virus has not been established. It is, therefore, essential that
registered virus-free budwood be used for all scions on this rootstock. Scion
varieties, such as 'Marsh,' 'Ruby Red' grapefruit; 'Hamlin,' 'Parson Brown,'
'Pineapple,' and 'Valencia' sweet orange; and 'Orlando' tangelo, seemed to
grow satisfactorily on the rootstock. All six varieties were bearing fruit
in 1962 when they were severely damaged by the freeze of that year. The root-
stock is reasonably tolerant to flooding.
The leaves of 'Milam' resemble rough lemon except for smooth, slightly
depressed veins on the under surface and long petioles with broader wings.
'Milam' does not have the characteristic poignant odor from crushed leaves
that has been so familiar with rough lemon. The flowers are white except that
on girdled branches they may flush reddish purple.
The fruit is usually mature between September and December. Its shape
changes from rough and bumpy while immature to a smooth pear shape when mature.
The mature fruit is lemon-yellow in color and has a rather thick rind. Seeds
may vary from 0 to 12. They are small and have a rather pronounced beak. The
variety is polyembroyonic. The exact nucellar rate has not been established,
but it must be close to 95% or higher as indicated by the difficulty of
obtaining crosses with trifoliata pollen.
'Milam' grown as an unbudded tree for seed production is extremely vigorous.
It may reach a height of 10 feet after only 3 years in the field. The pronounced
juvenility of the variety will delay fruiting up to 8 or 10 years unless
girdling is practiced to induce flowering. Fruits have been obtained 3 years
after top working 'Milam' on seedling rough lemons where girdling has been done
correctly. 'Milam' has fruited better on trifoliata than on rough lemon.
Girdling is a rather severe treatment and special care must be exercised to
protect the trees during subsequent development. The girdle consists of
removing a 1/8-inch strip of bark completely encircling the limb and covering
the wound with plastic or budding tape. Girdling should be done between October
15 and November 15. It is recommended that no more than one side of a tree be
girdled in any 1 year. The girdled branches may show severe growth depression,
pronounced chlorotic symptoms, leaf drop, and increased cold susceptibility.
The girdled limbs can die if the wounds do not heal within 2 months. Girdling
has been successful and is, therefore, recommended as the best method to obtain
seeds of 'Milam' within a reasonable period of time.
'Carrizo' citrange (navel orange x Poncirus trifoliata)has been classified
as tolerant to the burrowing nematode. It could not be formally released in
1964 because it was already a named variety. Variations in growth have been
found between different sources of 'Carrizo' seeds. In greenhouse studies, the
California source of 'Carrizo' exhibited the best growth and tolerance to
Radopholus similis in comparison with seeds from Florida and Texas. It is not
known whether differences in vigor are maintained in the field. An attempt is
being made to select an improved burrowing nematode tolerant seedling fro' the
California source of 'Carrizo' citrange. The selection has been made, but it
will be at least 5 years before it will be known whether the selection has
improved vigor and burrowing nematode tolerance
'Carrizo' citrange has many desirable horticultural qualities as a rootstock
and has been planted throughout the State for reasons not associated with
burrowing nematodes. It has better than average tolerance to citrus nematode,
foot rot, and "wet feet." The psorosis virus can be transmitted through seeds
of 'Carrizo' making it essential that a disease-free source be used.
'Algerian' Naval sweet orange is resistant to feeder root damage from the
burrowing nematode. 'Algerian' Navel has not been publicized to the industry
because there is no evidence to indicate that it is a distinct improvement over
'Ridge Pineapple' as a sweet orange burrowing nematode resistant rootstock.
Fortunately for rootstock purposes, 'Algerian' is a seedy navel. The parent
tree is growing in the citrus collection of the United States Horticultural
There are other citrus varieties that show tolerance and resistance to
burrowing nematodes. They will only be released to the industry if they prove
to be superior to the rootstocks already released.
Cuttings vs Seedling Rootstocks
All the common rootstocks used in Florida have been grown as seedlings.
Since 1964, a significant number of 'Milam' plants have been propagated by
cuttings. There has been no other way to quickly obtain trees on 'Milam,'
'Estes,' and 'Ridge Pineapple.' Many nurserymen are growing trees for seed
production, but only small quantities of seed will be available before 1968
to 1970. Thus, it may be 1973 before varieties budded on validated seedling
stocks are available in quantity. Cutting rootstocks are the most expedient
substitute. They are produced by rooting leafy shoots or leaf bud cuttings
under intermittent mist. The cuttings root in 4 to 6 weeks if conditions
are satisfactory. The rooted cuttings are usually transplanted into containers
until large enough to be lined out in the field.
Growth characteristics of cutting root systems are different from seedlings.
One or more roots may develop from the callous tissue formed at the base of
the cutting. There is no tap root nor characteristic whorl of roots around
the base similar to that found with a seedling root system. A cutting requires
more care in the nursery and special handling when dug and planted in the grove
site. The cutting may also require more special care during the first year in
the grove such as more frequent watering. The trees may show a tendency to
blow over in strong winds. This is due to the lack of a well-defined root
system during the early years of development.
Trees on cutting rootsotcks will usually become well anchored at the end
of 1 year in the grove. At 3 years of age, it will be almost impossible to
tell them from seedling rootstocks. Numerous well-defined lateral roots will
have developed near the crown so that the root system will resemble a seedling
except for the absence of a tap root.
A cutting rootstock has the advantage of being absolutely identical to
the parent source of the cutting material. This can be a disadvantage if the
clonal stock is not free of viruses. Cuttings will transmit viruses and
anything else that may be in the clonal source. Other problems probably will
not occur if virus-free, disease-free cuttings are used. It should, however,
be emphasized that there are no data available in Florida concerning the
long-term growth habits of cutting rootstocks.
Seedling rootstocks have certain definite advantages over cuttings. There
is less risk of virus infestations through the rootstocks. Seedling rootstocks
are easier to handle in the nursery and during the early years at the grove
site. Perhaps the most serious question raised is the possibility that seedlings,
not nucellar in origin, would lose the nematode resistant factor. This risk
is small and it should not be higher than 1 to 3%. The important thing to
consider with'Milam,' 'Estes,' or 'Ridge Pineapple' seedling stocks is that the
seed should come from a validated plant that has been propagated vegetatively
from the original clonal material tested by the research workers that developed
the rootstocks. It is anticipated that when seeds are available in reasonable
quantity they will supercede the use of cuttings as the preferred method of
propagating stocks of 'Milam,' 'Estes,' and 'Ridge Pineapple.'
Field Methods of Using the Rootstocks
It should be emphasized that long-term field trials with 'Milam,' 'Estes,'
'Ridge Pineapple,' 'Carrizo,' and 'Algerian' Navel have not been completed.
It is strongly recommended that the grower fumigate burrowing nematode infested
land before using these rootstocks. 'Milam' may be damaged by feeding injury
during the period that burrowing nematode populations are being slowly
eliminated. 'Estes' would continue to maintain burrowing nematodes in the
grove site. Many old groves may harbor a rather high incidence of the
causal agent of foot rot. One of our older rootstock trials in burrowing
nematode-infested soil has become infested with 50% foot rot. This is a
serious loss of trees for reasons other than burrowing nematodes. Citrus
nematodes are also found in burrowing nematode-infested groves. 'Milam,'
'Estes,' 'Ridge Pineapple,' and 'Algerian' Navel contain no resistance to
citrus nematodes. Thus, the grower may experience decline symptoms from
citrus nematodes even though his rootstock is resistant to burrowing nematodes.
Citrus nematodes and foot rot found in one of our rootstock trials forced us
to abandon the trees. Soil fumigation for burrowing nematodes also controls
citrus nematodes,and there is laboratory evidence that fumigation will
minimize foot rot troubles arising from organisms in the old grove soil.
Burrowing nematode tolerant and resistant rootstocks are a form of
insurance and protection against declining trees should the area become
reinfested. This is the big risk in planting susceptible rootstocks in a
pulled and fumigated site.
Studies have been in progress for 4 years to use 'Milam' or 'Ridge
Pineapple' as burrowing nematode biological barriers. Greenhouse controlled
studies have been promising, but there can be no guarantee that the method
will be entirely successful in the field. It is worthy of trial for those
that understand the problem and are prepared to follow through on a definite
long-term maintenance program. The method for trial consists of planting
four or more rows of trees on 'Milam' or 'Ridge Pineapple' around the border
of the pulled and treated area where a margin breakdown may be anticipated.
This becomes the biological barrier zone. It will also be necessary to
maintain a rather narrow chemical root-killing barrier between the 'Milam'
trees and the trees infested with burrowing nematodes. The purpose of the
chemical barrier is to kill the roots of the infested trees so they cannot
grow rapidly into the region where 'Milam' has been planted; otherwise,
burrowing nematodes may be carried right through the biological barrier on
the roots of susceptible hosts. The treatment must be repeated at periodic
intervals. It is anticipated that by killing the roots of adjacent infested
trees 'Milam' or 'Ridge Pineapple' can eliminate any burrowing nematode
populations that accidentally cross over into the root zone of the biological
barrier. The center of the pulled and treated area inside the biological
barrier zone could be planted with 'Milam,' 'Estes,' 'Ridge Pineapple,' or
'Estes' rough lemon should generally be reserved for those pulled and
treated areas in which there appears to be no immediate threat of reinfestation.
The principal advantage for 'Estes' is that it is a rough lemon, the preferred
rootstock on the sandhills. Hopefully, symptoms of spreading decline will not
occur should the trees become infested with burrowing nematodes. The main
danger is that a reinfestation on 'Estes' would be a threat to surrounding
areas as nematodes spread.
Available evidence indicates rather strongly that 'Estes' rough lemon
is horticulturally identical to the many sources of Florida rough lemon. The
original parent tree of 'Estes' involving a 'Valencia' scion has produced
fruit identical in quality to that of other sources of rough lemon rootstock.
There is merit in considering the use of 'Estes' in the ridge area on virgin
land where spreading decline may be a potential problem.
Florida Citrus Experiment Station
Lake Alfred, Florida