Cold injury and boron deficiency produce similar symptoms in tobacco seedlings grown in float greenhouses. In the past, what has often been mistaken for boron deficiency is actually cold injury and does not require any corrective action.
Cold injury is quite common when producing tobacco transplants in the greenhouse. Both burley and flue-cured seedlings are susceptible. Upward leaf cupping and puckering are the main symptoms (see right side of Figure 1). The affected trays may be uniformly distributed through the greenhouse or, more often, the injury may occur in just a few cold spots.
Varieties differ in their sensitivity to cold. Differences in the degree of injury are often seen when varieties of different sensitivity are planted next to one another in the greenhouse (Figure 1). While the example given here shows the difference in two burley varieties, flue-cured varieties also differ in their sensitivity to cold. Despite the name, cold injury is thought to result more from large differences between day and night temperatures and less from a very cold night. Greenhouse temperature can increase rapidly on sunny mornings if manual vents are not opened promptly. It is not uncommon for temperatures in an un-vented greenhouse to increase 40 to 50 degrees within a few hours. Diligent temperature management is the key to reducing cold injury, but even with the best management mild cases may occur.
While cold injury sometimes slows growth, it does not result in permanent harm to the plants. No treatment other than managing greenhouse temperatures is needed to restore good growth. Cold injury is not associated with early flowering in the field.
Boron deficiency is far less common than cold injury. It can occur when fertilizer without boron is used along with a water source that is low in boron. Boron deficiency almost never occurs if a fertilizer containing at least 0.01 percent boron is used at recommended rates. Producers are strongly encouraged to check greenhouse fertilizer labels to verify boron concentration, as slight differences are often observed among formulations.
Boron is needed for proper development of growing points. In a mild deficiency, the growing points at the tips of the leaves are affected (Figure 2). This causes the leaf to be puckered and misshapen, and often it will cup downward. As the deficiency becomes more severe, the growing point at the top of the stem (bud) is also affected. Corrective application of boron at this point can restore normal growth. In extremely severe cases, the bud will turn gray and die (Figure 3), making corrective measures ineffective.
Visual diagnosis of mild boron deficiency can be difficult. The symptoms are easily confused with cold injury. Leaf analysis is the preferred method of diagnosis. The entire above-ground part of the plants should be collected for analysis. Since the plants are small, it may be necessary to collect 50 or more seedlings to get enough tissue for analysis.
Boron can be very toxic to tobacco seedlings, and only a tiny amount is needed for good growth. Slight overapplication causes temporary leaf deformation (Figure 4), while higher rates can result in severe and permanent seedling deformation (Figure 5).
Use of Sol-u-bor or borax to supply boron is not advisable except in cases of confirmed deficiency. The danger of causing boron toxicity from overapplication of foliar boron is far greater than any potential growth benefit. A fertilizer with more than 0.01 percent boron should be used unless irrigation water contains more than 2 parts per million (ppm) boron. It is especially important to use fertilizer with boron if irrigation water has less than 0.5 ppm boron. All water-soluble fertilizers contain a statement of boron concentration on the label. Be aware that different brands or formulations of the same fertilizer can contain different amounts of boron. This is particularly true with 20-10-20.
An extensive study was conducted with both burley and flue-cured tobacco in the Phytotron at North Carolina State University. Controlled temperatures and several boron rates were used to determine if cold injury and boron deficiency are related. Cold temperatures slightly delayed boron uptake, even when boron was present in the float solution in adequate amounts. However, by transplanting, boron levels in seedlings were similar. Boron level in the float water did not affect the appearance of cold injury; therefore, growers should not attempt to cure cold injury with foliar boron.
It was very difficult to produce boron deficiency under research conditions. Painstaking steps were required to eliminate all boron from greenhouse source water and fertilizer. The pictures in Figure 2 and Figure 3 are the result of several attempts to remove all boron from the production system. These conditions should be very rare in a normal greenhouse. This research leads to the conclusion that what is often seen in most greenhouses is cold injury, and it is not related to boron.
As you produce tobacco transplants, keep in mind the following points in managing cold injury and boron.
- Pay attention to greenhouse temperatures to minimize cold injury. Maintain day temperatures below 90°F and night temperatures above 55°F.
- Remember that a small amount of cold injury is normal and that warmer temperatures will correct visual deformities.
- Use fertilizer with at least 0.01 percent boron unless the source water has more than 2 ppm boron.
- Do not use supplemental boron applications unless a deficiency is confirmed by tissue analysis.
For Additional Information
More information on the float system can be obtained from the following NC State Extension publication:
The NC Department of Agriculture & Consumer Services Agronomic Division provides a solution advisory service to assist in the management of float systems.
The research upon which this publication is based was funded by the NC Tobacco Research Commission. The authors are grateful for this support.
This publication is a revision of an earlier version. The authors would like to thank J. Rideout & L. Overstreet for their earlier contributions.
Publication date: Feb. 22, 2019
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