Foliar Nutrient Program for Freeze Protection

Foliar nutrient programs are suggested to aid in recovery from damage to fruit trees caused by two types of environmental stress: drought and freeze. The roles of individual nutrients are examined for their ability to influence tolerance to drought conditions and to aid in recovery from severe freeze damage. The ability of foliar nutrients to provide freeze protection on a wide variety of crops is discussed in this bulletin.

The use of foliar nutrients as frost protectants was being promoted even before publication of the research by Dr. Steven Lindow at the University of California, Berkeley, which showed that certain naturally occurring bacteria such as Pseudomonas syringae and Erwinia herbicola act as nucleating agents for ice formation. The presence of these ice-nucleation-active bacteria results in the formation of ice crystals in plant tissue at temperatures several degrees higher than in their absence. When water in plant cells freezes, it expands and  ruptures the cell walls, leading to cell necrosis. The bacteria can be killed or prevented from acting as nucleating agents, thus providing several degrees of frost protection. This discovery provided a plausible explanation for the previously observed positive effects of foliar nutrients on freeze tolerance. While the mechanism can be explained, there is no general agreement on which chemicals are most effective in controlling the ice-nucleating bacteria. Several specially formulated products are commercially available, but in controlled tests these have generally not given better results than copper-based fungicides or micronutrient mixtures. Certain urea-based products, antibiotics, anti-transpirants, and surfactants have also been claimed to be effective.

Just as low temperatures are not the only factor involved in freeze damage, it is likely that control of ice-nucleating bacteria is not the only factor involved in improving freeze resistance. Gradually lowering temperatures promote cold hardiness. Trees under long term water stress are less cold-hardy(1). Plant growth regulators play an active role in influencing freeze resistance. High gibberellin levels have been reported to increase susceptibility to environmental stress(2). During chilling, a precursor of auxin has been shown to accumulate in buds, followed by the gradual appearance of auxin(2).

Plant nutrients are capable of influencing these factors. Copper, because of its inherent fungicidal properties, is considered the most effective element for control of ice-nucleating bacteria. Zinc also has mildly fungicidal properties and has been shown to be a co-enzyme for the biosynthesis of tryptophane, a precursor of auxin(3). Potassium, Calcium, and Phosphate play important roles in freeze protection and recovery as described in our previous posts.

The following programs are recommended for frost protection. These programs supply Copper, Zinc, Manganese, Phosphate, Potash, and Calcium.

FOR FREEZE PROTECTION ON TREE CROPS

Timing:  On citrus and avocados, apply shortly before freezing weather which normally can be expected around the second week in December. On deciduous fruit and nut trees, apply post-harvest and in early spring at the time a freeze is anticipated or predicted.

FOR FREEZE PROTECTION ON GRAPES & BERRIES

Timing:  Apply just prior to anticipated freezing temperatures.

FOR FREEZE PROTECTION ON FIELD AND ROW CROPS

Timing:  Apply just prior to anticipated freezing temperatures.

REFERENCES

1. Parsons, L. R. Interactions among Environmental Stresses. HortScience 25(11):1363 (1990).
2. Seeley, S. Hormonal Transduction of Environmental Stresses. HortScience 25(11):1369 (1990).
3. Bennett, J. P. and F. Skoog. Preliminary Experiments on the Relations of Growth-Promoting Substances to the Rest Period in Fruit Trees. Plant Physiol. 13:219-225.