Cold-Hardening Young 'Valencia' Orange Trees on Swingle

Table 2. Mean freeze* injury to 18-month-old Valencia orange trees on
different rootstocks.
Rootstock
Swingle
Rusk citrange
Carrizo citrange
Sour orange
Trifoliate orange
Rough lemon
Foliage kill*
C
72 ax
86 a
82 a
90 a
95 a
92 a
NC
100
100
100
100
100
100
f
% 0
Stem dieback
c
6a
7a
10 a
3a
6a
39 b
NC
100
100
100
100
100
100
z-6.7 C (20 F) for 4 hr.
yC = cold-hardened for 14 days at 21.1 C (70 F), 12-hr light and 10 C
(50 F) dark followed by 14 days of 15.6 C (60 F) light and 4.4 C (40 F)
dark. NC = not cold-hardened (glasshouse).
xOdds are 19:1 that means are different if followed by different letters.
9- than with 18-month-old trees. In the field, rootstock effects
tend to increase as trees age. <strong>Young</strong> trees are known to be
more cold susceptible than older trees because of size dif
ferences. Size differences among older trees sometimes re
quire adjusted freeze ratings in the field (3). Little is known
about the cold-hardening potential of young, budded citrus
trees. Controlled-environment screening will help provide
information.
<strong>Cold</strong>-hardening in 9-month-old <strong>'Valencia'</strong> trees on
'Swingle' was sufficient to prevent apparent injury at —3.9 C
(25 F) for 3 hr (Table 3). This was ice tolerance, and not
ice avoidance, as supercooling was prevented by precooled
Table 3. Freeze injury to 9-month-old Valencia orange trees on Swingle
rootstock at different temperatures for 1 and 3 hr.
Treatment
C
NC
C
NC
C
NC
Tem
perature
C(F)
-3.9(25)
-5.0(23)
-6.1(21)
Leaf
kill
0
100
4±3
100
12±9
100
1 hr
Stem
dieback
0
11±9
0
57±5
0
92±6
i
Leaf
kill
0
100
8±5
100
44±11
100
3h
Stem
dieback
0
22 ±18
0
89±4
0
100
yC = cold-hardened for 14 days at 21.1 C (70 F), 12-hr light and 10 C
(50 F) dark followed by 14 days of 15.6 C (60 F) light and 4.4 C (40 F)
dark. NC = not cold-hardened (glasshouse).
a.
10
H2O mist sprays at test temperatures. <strong>Cold</strong>-hardened
<strong>'Valencia'</strong> stems showed no injury, even after 3 hr at —6.1 C
(21 F). Both lower temperatures and longer freezes in
creased leaf kill. After 1 hr at -5 C (23 F), leaf kill averaged
4%. This percentage doubled after 3 hr. A similar pattern,
but greater injury, resulted after the —6.1 C (21 F) test.
This test was essentially lethal to nonconditioned <strong>'Valencia'</strong>
on 'Swingle' rootstock. All treatments killed 100% of the
leaves of nonconditioned trees.
In this study, 'Swingle' compared favorably with mod
erately cold-tolerant citrus rootstocks. Observations on much
older trees in field plantings by other researchers indicated
similar results. Gardner and Horanic (3) rated 'Swingle'
close to sour orange in cold-hardiness ratings in a 10-yr-old
<strong>'Valencia'</strong> planting in Weirsdale, Florida. As a rootstock
for grapefruit in Texas, Cooper, et al. (1) rated 'Swingle'
moderately cold-hardy and equal to that of sour orange.
<strong>Young</strong> and Olson (9) reported that most grapefruit trees on
'Swingle' survived -11.1 C (12 F). During the 1951 Texas
freeze, 100% of the grapefruit trees observed on 'Swingle'
survived, whereas only 58% survived on 'Rusk' citrange (2).
As seedlings, 'Swingle' compared favorably with cold-hardy
trifoliate orange during freezes in Texas' (%) and survived
-16.1 C (3 F) in Georgia (7).
<strong>'Valencia'</strong> orange on 'Swingle' rootstock is considered
relatively cold-hardy based on available information.
Literature Cited
1. Cooper, W. C, E. O. Olson, N. Maxwell, and G. Otey. 1956. Review
of studies on adaptability of citrus varieties as rootstocks for grape
fruit in Texas. /. Rio Grande Valley Hort. Soc. 10:6-19.
2. , , , and A. Shull. 1957. Orchard per
formance of young trees of red grapefruit on various rootstocks in
Texas. Proc. Amer. Soc. Hort. Sci. 70:213-222.
3. Gardner, F. E., and G. E. Horanic. 1958. Influence of various root
stocks on the cold resistance of the scion variety. Proc. Fla. State
Hort. Soc. 71:81-86.
4. Hutchison, D. J. 1974. Swingle citrumelo—a promising rootstock
hybrid. Proc. Fla. State Hort. Soc. 87:89-91.
5. Wutscher, H. K., and A. V. Shull. 1972. Performance of 13 citrus
cultivars as rootstocks for grapefruit. /. Amer. Soc. Hort Sci 97*
778-781.
6. Yelenosky, G. 1972. Nocturnal radiation in a citrus planting Proc
Fla. State Hort. Soc. 85:71-74.
7- , R. T. Brown, and C. J. Hearn. 1973. Tolerance of tri
foliate orange selections and hybrids to freezes and flooding Proc
Fla. State Hort. Soc. 86:99-104.
8. <strong>Young</strong>, R. 1963. Freeze injury to young seedlings of citrus cultivars
and related species in the lower Rio Grande Valley. T. Rio Grande
Valley Hort. Soc. 17:37-42.
9- > and E. O. Olson. 1963. Freeze injury to citrus trees on
various rootstocks in the lower Rio Grande Valley of Texas Proc
Amer. Soc. Hort. Sci. 83:337-343.
Proc. Fla. State Hort. Soc. 89: 1976.