So, what is happening when an otherwise green plant suddenly develops another color? Generally, the new color is yellow since that is the underlying color of leaves which is usually overwhelmed by the green of the chlorophyll.

If someone asks why their leaves have turned yellow, the one absolutely certain answer is that it has lost its chlorophyll. This loss of green color is called chlorosis and affected plant tissues are said to be chlorotic. The larger question is, “Why did it turn yellow?”. The answer to that one can get a lot more complicated.

In the autumn of the year when the day lengths shorten, leaves of deciduous plants and the older needles of evergreens turn yellow. At low light levels, the leaves are not capable of maintaining chlorophyll molecules and they fade away. Depending on the species of plant, other pigments such as anthocyanin and carotene may increase depending on weather conditions. That is why some plants have bright orange or red foliage in the fall. Warm, sunny days followed by cool nights will encourage these pigments which explains why the intensity of fall color will vary from year to year.

Fungal leaf spot diseases such as apple scab or anthracnose in maples, oaks and sycamores cause leaves to yellow. Needlecast diseases on pine, spruce and fir cause a similar effect on needles. Viruses cause the loss of chlorophyll in a mottled pattern on leaves and stems.

Sucking insects such as aphids, mealybugs and scale remove nutrient laden sap from the plants restricting their ability to make more chlorophyll. Spider mite feeding produces a mottled pattern of yellowing similar to a virus.

Many environmental factors cause a plant to yellow. The most basic is a lack of adequate light. When plants become shaded by other plants or by branches higher up on the same plant, the chlorophyll fades away and the leaves yellow.

Nutrient deficiencies also cause chlorosis. Nitrogen (N) is a vital component of chlorophyll so inadequate levels of this element make leaves turn yellow. Iron (Fe) deficiencies cause chlorosis between the veins in leaves while the veins themselves maintain their green color. This often occurs when the pH of the soil is too alkaline for acid loving plants such as rhododendrons, azaleas or pin oak trees. Magnesium (Mg) is also part of the chlorophyll molecule which is why Epsom salts (MgSO4) may help in some occurrences of yellow leaves.

Too much or too little water can cause yellowing. Under water stress, plants shut down internally and cannot transport nutrients to the leaves. Chlorophyll production can be affected causing loss of green color.

Low temperatures can affect leaf color. Under cool conditions, chemical reactions are slowed and, again, chlorophyll production may be reduced. In some cases, low temperatures can block the uptake of phosphorus resulting in the plant taking on a pinkish cast.

Herbicides (weed killers) may also cause yellowing. Some of these chemicals, including the widely used Round‑Up i.e. glyphosate, work by inhibiting photosynthesis in the plant. Often, they do this by blocking the production of chlorophyll.

Of course, we gardeners often prize plants that have lost their chlorophyll in parts of their leaves. We call these plants variegated and, in plants such as hosta, they may be the centerpiece of our gardens. A lot of this type of “good” chlorosis is caused by a change i.e. mutation, in the genes of the plant so that some of the cells that should produce chlorophyll are inhibited from doing so. We don’t always know why this happens but when we discover such plants, we tend to value them for their uniqueness.

The good news for most of these situations is that, if the causative factor i.e. lack of nitrogen, drought, disease, etc. is controlled, the green will often come back. Of course, with the return of longer days in the spring, our trees will once again give us the shades of green that we have come to relish.

There are literally hundreds, perhaps thousands, of cultivars of crabapples currently available. Every nursery or plant store will offer many from which to choose. So, how do you make the right choice for your landscape? The following traits should be considered when purchasing one for your yard:
Disease Resistance. Certain species of ornamental landscape plants come with a lot of “baggage” that could cause you problems in the future. One big problem is diseases. From the day you put certain plants in the ground, you can bet with a high degree of certainty that someday, they will get sick. So, the key is to do a little research before hand and AVOID future problems.

Although we make a big deal of calling these ornamental trees, crabapples, the fact is that they are just apple trees with small fruit. Therefore, they have the same range of problems as their Red Delicious or Granny Smith cousins. In the landscape, we are not so concerned with the quality of the fruit as they are in the orchard. However, crabapples disfigured by diseases can also be a big negative in certain trees in the landscape.

Like other apple trees, crabapples are susceptible to the fungal diseases called apple scab and cedar-apple rust. These diseases can disfigure the leaves and, in extreme cases, cause defoliation of the tree by mid to late summer. This is not a pretty sight. Successive years of defoliation can weaken the tree and lead to its decline. Some sort of fungicide treatment is the most common approach to combating this disease both in the landscape and the orchard.

A serious, life-threatening, bacterial disease caused fireblight also infects crabapples. This disease will kill the tips of branches in its early stages and may eventually kill the entire tree. Since it is bacterial, fungicides do not work against it and it is very difficult for the homeowner to treat.

Our goal should always be to minimize the use of pesticides in the home landscape. This is a place for beauty and nature. So, the preferred approach is to avoid these diseases by seeking out cultivars that are resistant to them.

Plant breeders work hard to select crabapple cultivars resistant to the common diseases. Resistance does not mean immunity. Resistant plants may still become infected but generally not as easily as susceptible types. If they do develop the disease, it may not be as severe.

We will cover the ornamental traits that you need to consider when selecting crabapples in the next few paragraphs. However, for the general home garden, disease resistance should trump all. Sure a certain cultivar may have magnificent flowers in May, but, if it loses all its foliage to apple scab, it will spend most of the summer looking pretty ratty.

Fruit. Crabapples develop fruit in a wide range of colors, quantity, sizes and persistence (how long they hang on the tree). These factors are important to consider especially if the tree is located over a driveway or sidewalk where they would cause a mess.

If the tree is to be located away from the house, it may be worthwhile to have a heavy fruiting cultivar that hangs onto its fruit throughout the winter. That will provide a great view from the house and any mess with the fruit will be minimized.

Also, some cultivars are “alternate bearing” which means that they produce a heavy crop one year and a small one the next. For consistent display, choose annual bearing types. Sprays are available which will “thin” the fruit from the tree but this process is dependent on proper timing and the results can vary greatly from year to year. The best control for excessive fruiting is to plant a low producing cultivar if that is what you desire for a particular location.

Flowers. Most crabapples have pinkish colored buds that open into white to pink blooms. A few darker red flowering cultivars are available but many of these are susceptible to apple scab. As with most members of the Rosaceae Family of plants, apple blossoms have five petals. Double flowering cultivars with many more petals are also available. If you have room for several crabapples, you can also choose early, mid and late season blooming cultivars to extend the season.

Foliage. Leaf color on crabapples may range from light to dark green to dark purple. Be sure the contrast between flower, leaf and fruit color is such that each trait is properly displayed.

Water Sprouts and Suckers. Water sprouts are vigorous shoots that grow straight up from branches often appear after a heavy pruning the previous season. Suckers are those annoying stems that come up from the ground at the base of the tree. Both types of growth are undesirable and certain cultivars are more prone to produce them than others.

Check on these factors before purchasing that crabapple to cut the level of maintenance and increase the level of enjoyment for years to come.