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Garden Help Desk: Spots on leaves could indicate a chilling injury

By Usu Extension - | Jun 6, 2021
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Chilling injury can sometimes look like the leaves have dried out. Plants usually recover and damaged leaves are masked by new, healthy leaves.

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Most of our tender warm-season plants are susceptible to chilling injury, especially on young, tender leaves.

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Basil is one of the most tender plants that we add to our gardens. If the exposure to low temperatures is prolonged, the basil plant may not recover.

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Chilling injury can also leave the leaves on a tender plant looking a little wilted. Keep an eye on the weather forecast during the spring and use a protective covering (buckets, small tubs, row cover, hoops and blankets, or something similar) to prevent chilling injury.

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Brown or tan spots on leaves just a day or two after a very cold night can be a sign of chilling injury.

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An otherwise healthy tree can slowly close off pruning wounds and the edges of bark cracks if waxes, tars, paints and sealers are not applied.

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Shade from other trees affects the growth of nearby trees. When trees are growing close to each other, the removal of one tree can affect the health of the other trees.

Question: Almost all my plants, both flowers and veggies, are getting this disease. What is it and how do I fix it?

Answer: When different species of plants in the landscape begin to show similar symptoms, the cause is usually something other than an insect or disease. Environmental (weather), chemical (exposure to weed killer or to an insecticide that wasn’t safe for plants) or plant care problems (over or under watering, too much or too little fertilizer) are the most likely culprits. For the plants in your photo, the problem is probably environmental.

The damage on these leaves looks like chilling injury. Many plants can suffer damage to their leaves when temperatures are very cold, but not freezing. Tomatoes, peppers and similar plants are native to much milder climates and damage like you’re seeing in your garden can happen when overnight temperatures dip below 45 degrees. Basil is even more tender and can be damaged by springtime temperatures below 50 degrees.

Plants with a little chilling injury usually recover, but the injury can set them back and they will lag behind in growth for a bit, but eventually they should catch up to where you’d expect them to be.

In addition to the dry brown or blackened areas you see on some leaves, you may also notice some areas of purple or red discoloration. Out in your landscape, you may find some leaves that are misshapen or have dark or cracked edges.

There’s nothing you need to do about this now. You may have found a few more affected leaves that just hadn’t shown the damage yet, but in general all the new growth should be looking better. In the future, keep an eye on the springtime weather forecast and be prepared to protect your tender warm-season plants when overnight lows are going to dip below 45 degrees.

Question: I was always careful about my fruit trees getting cracked bark when we lived in northern Utah, but now we live in a warmer place, and this isn’t a fruit tree. What does this crack in our red maple mean? The tree is tucked into the northeast corner of our back yard. We lost part of our shade in our yard when our next-door neighbors cut their trees down a few years ago so we really want to keep the shade we get from our own trees. Should we be doing something?

Answer: This crack is probably caused by sun scald on the bark. You can tell from the way your tree has grown mostly toward the west that the shade from the neighbor’s tree was significant. Once your neighbor’s tree was removed, your own tree was exposed to direct sun. After all those years of growing in the shade, the bark on your tree wasn’t adapted to this new exposure. The rays of the sun overheated the area of bark that was getting the most direct exposure and it was scalded, killing bark cells in that location. Eventually, that dead bark started to peel away. The reddish, thick, rounded-looking edges around the crack are a wound response. The tree has closed off the edges of the wound to (hopefully) keep out pathogens and disease. Eventually, those rounded edges will meet.

You can see that the damaged branch is also angled a bit so that the cracked surface is pointed more directly at the sun. This made that branch even more susceptible to the effects of the summer heat and direct sun.

Cracks like this can happen in the winter wherever overnight temperatures drop below freezing. If the low southern and western sun warms the bark during the day, causing cells to break dormancy, and then drops below freezing for long enough during the night, those cells can freeze and die. The cracking and peeling on the trunk will look like the bark on your tree branch. When this happens because of sun exposure during the winter, it’s called Southwest Winter Injury.

The sun scald damage happened at least a couple of years ago, and in your photos it doesn’t look like there is any new damage, so your tree may be adapting to the situation. Don’t put any wax, paint, tar or “pruning sealer” on the damaged area; that would just interfere with your tree’s ability to finish closing off the wound. You can carefully cut away some of the loose, dead bark if you’re worried about the bark being torn away and causing further damage, but the best thing you can do for your tree now is to continue to give it good care with deep, infrequent watering.


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