Plant CSI — Do you have what it takes?

Heather Kirk-Ballard

LSU AgCenter Horticulturist

(08/30/19) Have you ever seen a plant in the landscape or garden that you thought was supposed to be green but wasn’t? Did the coloring include yellow, cream or white in different patterns — streaked, blotched or mottled? Or did you see pink or purple foliage on a plant that had otherwise been only green before? Or maybe you have admired a beautiful flower that has more than one color.

Was your first instinct to think that it looks exactly as it should? Or did you think something is wrong? Is it some environmental stress? Is it a lack of nutrients? Is the plant sick from a pathogen? Did it get too much water? Not enough water? Is it genetic? Or are plant breeders responsible? The answer could be yes to any of these. It is quite possible that any one of these could be the reason the plant looks the way it does.

In some cases, you may have a purchased a plant specifically for those aesthetic characteristics. Plants that display foliage of different colors or patterned colors are referred to as variegated in the plant world. Either or both foliage and flowers can be variegated when more than one color is present in some type of pattern. In most cases, only two colors will be present (these are bicolored). Sometimes it will be three colors (tricolored). Or you can even have four or more colors.

Just how does this happen? Ornamentally speaking, there can be three possible reasons for this. First, it can be a genetic trait that is passed down from the parent plant. Second, it could have occurred randomly, with that specific plant cloned and propagated by plant breeders. (We’ll get to the third later.)

Color variation is often just the absence of the green pigment (chlorophyll) in some of the plant cells. This could be a result of a cell mutation or a genetically inherited trait passed down to offspring. A genetic trait is stable, but random mutations are unstable.

The variegation mutations are an anomaly of nature. Plant breeders will see these mutations as something beautiful and then clone the plants to produce more plants with those specific traits. This can also happen in reverse where you observe a plant that once displayed foliage of several colors “revert” back to a full green, single colored foliage. These are both naturally occurring mishaps of nature.

To make this as clear as mud, here’s the third reason: viral infections. When a plant becomes infected with certain viruses, they can cause symptoms that will result in the two-toned coloring (foliar mosaic) that gives it a distinctive pattern. The veins of the plant may yellow, or the leaves may lose green coloring between the veins. The leaf can also become curled, causing a malformation, or the plant could be stunted. These characteristics can be attractive to breeders even though they were caused by a virus.

Some growers have tapped into this tool of using viruses to manipulate the physical appearance of plants. One example is tulip-breaking virus that was once used in the early 17th century by Danish growers to force the beautiful flower variegation. The variegated tulip flowers increased the commercial value of the crop tremendously and proved to be an economic benefit. However, this virus-forcing variegation is not heavily practiced today as plant breeders have been able to duplicate the beautiful bloom characteristic their use.

There are other examples of viruses causing desirable visual traits in plants. Some of the color variation on specific camellia varieties is caused by the camellia yellow mottle virus while others are purely genetic. Other examples of plants displaying interesting aesthetics due to viruses include cannas that display variegated foliage when infected by canna yellow mosaic virus. Other ornamental plant species that have been modified by viruses include salvias, mints, maples, ageratum, angelonia, New Guinea impatiens and crotons.

Now that we have established how these plants come to hold these traits, let’s consider the mottling, curling and multicolored leaves that are not so purposeful.

If not on purpose, what is causing the leaves to turn yellow and green or to curl and become stunted? It could be for a number of reasons: an insect pest, a nutrient deficiency or iron chlorosis. To determine if it is a nutrient deficiency, you may submit a plant tissue sample for analysis at the LSU AgCenter soil testing and plant analysis laboratory.

Sometimes these issues can be caused by over-watering or poor drainage. They can also be caused by heat or cold. You need to know the full story and get the full picture when trying to determine exactly what is going on.

Put on your detective’s cap. Take a step back. Look at the big picture. Keep in mind what kind of plant it is; its age, size, condition and location; its relationship to other plants, weather and soil: and recent activity. We’ll delve deeper into this later. This is plant CSI, and I challenge you to be the principal investigator on the case.

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Leslie Ann camellia displays tri-colored leaves. LSU AgCenter archive photo by Dan Gill.

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Tricolor Asian jasmine is an example of a plant with variegated foliage. Photo by Heather Kirk-Ballard/LSU AgCenter

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Canna mosaic virus can cause variegated foliage. Photo by Heather Kirk-Ballard/LSU AgCenter

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Tulip-breaking virus was used in the early 17th century by Danish growers to force beautiful flower variegation. LSU AgCenter file photo

8/30/2019 8:56:49 PM
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