Proceedings
of The World Avocado Congress III, 1995 259 - 264
Ben Faber and Nick Sakovich
University
of California Cooperative Extension
669 County
Square Dr.
Ventura,
CA 9300
U.
S. A.
Abstract
Field and cold storage tests using four foliar
anti-frost materials were evaluated on Hass avocado and Eureka lemon. Frost
Guard®, water check, Kocide®, Frost Shield and Anti-Stress 2000® were applied at
least one week prior to the cold event Without enough cold temperature for the
field trials, the following two years the trials were moved into cold storage.
With mild damage occurring in 1993, there were some treatment tendencies, but
no significant differences. In 1994, even though trees were severely damaged,
those that survived recovered equally well regardless of treatment.
1. Introduction
Many chemicals and some
bacteria have been tested to induce frost tolerance of citrus, avocado and
other horticultural crops. In some trials, protection of a few degrees has been
achieved, but more commonly, the results have been variable or not commercially
successful.
As the costs of conventional
protection, such as wind machines, irrigation and orchard heaters have
increased, a commercially acceptable chemical or biological method would be
beneficial. The treatment must be non-toxic, easily applied and long-lasting.
In the past, materials acting
as growth inhibitors, penetrating agents, antitranspirants, nutritional sprays,
bactericides, and hormones have been used with some success in crops, such as
pear, apple and peach. However, the variability in subtropical tree response to
many of these materials has limited commercial acceptance.
For some years, it has been known that bacteria
residing on cold-sensitive plants initiate ice formation. The ice formed in or
on plants spreads rapidly, causing mechanical injury. Copper-containing sprays
that are applied 10 days before cold weather, kill off these bacteria and allow
enough time for their decomposition. By controlling the ice -nucleating
bacteria, tests have shown temperatures may drop 2-4 ºF below critical levels
without plants showing damage.
For the past four winters, we have been working with
four materials that have shown promise in freeze trials on several crops in
other states. To evaluate their relative efficacy, we have been applying
Kocide, a copper spray which controls ice-nucleating bacteria; Anti- Stress
2000, a plastic film used as a antitranspirant; Frost Shield, another
antitranspirant; and Frost Guard, a systemic/contact nutritional spray. Two
field and two cold chamber trials have been monitored.
2. Materials and methods
2.1 Field Trials
On 19 December, 1991, 8A Lisbon lemon trees in the
ground for six months were sprayed with the materials. Check trees were sprayed
with water alone. Fifteen single-tree replicates were randomized throughout the
plot. Frost Guard was applied to a light wetting, as directed; the other
products were sprayed to runoff. Four days later, on 23 December, a frost hit
with temperatures below 0ºC for four hours and two hours at -2º.
In order to ensure a site
where colder temperatures would be obtained, the original site was expanded to
two other orchards. During the first week of December 1992, the same
experimental layout was done at each of the three sites.
2.2 Cold Storage Trials
On 27 January, 1993, Lisbon
lemon and Hass avocado in liners were treated With the materials. On February
10, twenty trees of each treatment were loaded into picking bins and the root
balls protected with wood chips. At dawn, the bins of trees were placed in a
ready refrigerated trailer at -3ºC. Then every two hours, five trees of each
treatment were removed, so that we had trees that had been at -3º for 2, 4, 6
and 8 hours. Trees were rated on rated on a Damage Severity Index (DSI) visual
rating on 15 February.
The following 3 February, we went through the same
procedure as in 1993. This time we put the trees into the commercial cold
storage room. We had a little better temperature control here and we lowered
the temperature so that it varied between -4 and -3º. After I hour we began
removing some trees because it was evident there was a cold effect. We removed
trees after 1, 2, 4 and 8 hours in the cooler. The trees were rated for DSI on
10 March.
3. Results and Conclusions
For the field trial of 1991,
the trees were evaluated for damage two weeks after the cold period (data not
shown). Only the trees sprayed with Anti-Stress 2000 showed any significant
protection over the water-only treatment; other products showed no aid in cold
tolerance. Damage was minimal however, and two months after the 23 December
cold spell, there was no difference in the appearance of the trees. Why the
copper product showed no effect may be due to the fact that there was such a
short period between application and the onset of cold temperatures.
In 1992, again there was only
a short time below 0ºC at two of the sites and no damage was noted on any
trees.
In the cold storage trial of 1993, the trees were evaluated for damage a week after the cold treatment. The lemon and avocado at all lengths of exposure to the cold showed more severe damage than in the field trials, however even at 8 hours of -3º, only the new leaves had been burned (figures 1 and 2). Contrary to the field trial of 1991, the Anti-Stress 2000 did not seem to confer any frost protection on lemon, while the copper on both crops and Frost
Shield
on avocado did seem to offer some protection. Trees showed no effects of the
cold treatment two months later.
In 1994, all scion were
killed on all avocado and lemon at 4 and 8 hours of cold and the whole tree in
some cases. There was much more significant leaf loss during this trial than
all previous ones, however, there were no effects of the various treatments on
the I and 2 hour periods of cold (figures 3 and 4). We continued to follow the
recovery of the trees. By May 3, citrus and avocado recovery was uniform. By
June 7 there was no difference amongst the trees that had survived.
This is the tale of frost studies - variability. In
the field, you're dealing with the weather - it gets cold or it doesn't. But
even in the controlled environment, there is the tree variability - its age and
stage of growth, crop load, and how acclimated the tree. We know clearly that
tree health is an important factor in resistance to frost. This is shown by the
classic case of two neighbors, one with trees destroyed by frost and the other
untouched. What was the difference? Better irrigation practices, no root rot,
denser canopies, no thrips, anything that improves the tree's strength is going
to make it more tolerant to a freeze.
Bob Burns, Ventura Farm Advisor, made studies of
numerous chemicals during the 60's and 70's. His conclusion was that although
his tests "failed to provide the hoped for cold protection, there is
reason to believe that a chemical will be found." This may be true, but
what we do know is that there is no substitute for proper management and
relying on a chemical protection is the icing on the cake.