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Leaf Indicators of Tree Health

As I got closer, I saw that something else was going on. The leaves were yellow with brown blotches spreading from the edges and tips. This is called leaf necrosis, which means the leaves are dying and if nothing is done, the tree could die too.

Leaf necrosis is a classic sign of drought stress. That particular summer was very hot and dry and some types of maple trees are shallow rooted so they experience drought stress before other trees. They are like the canary in the coal mine, and their symptoms tell us to get busy watering if we want all of our beautiful trees to survive and thrive.

The leaves on a sugar maple with drought stress show brown blotches (necrosis) that starts from the edges and tips of the leaves and moves inwards.

Tree with fire blight

Drought Stress


When brown blotches work their way in from the edges and tips of your tree’s leaves, it’s very likely to be a sign of drought stress. But what’s causing the problem? If it’s a dry summer, it may just mean that you need to water your tree. But these symptoms can also result from too much salt in the soil.

Either way, once you see those blotches appearing, it’s time to take action! Set up a soaker hose and give your tree a long, slow water once a week or so (let the soil dry out before watering again) to restore tree health. Water also helps wash excess salt out of the soil.

If you have an apple or pear tree, don’t confuse drought stress with a quick-spreading disease called fire blight. With this nasty bacterial disease, the leaves don’t get blotchy, they look burnt and entire branches die back starting from the tender branch tips.

Carefully prune off infected branches back to the trunk and put them in garbage bags in the garbage, not the compost. Sterilize your pruners before using them again on any other tree.

When a tree has fire blight, the young leaves and tender young branches look scorched. The tips of young affected branches often curve like a hook.

Interveinal chlorosis on a hickory leaf

Nutritional Deficiencies


Later that summer, I took a walk in a nearby arboretum and saw an unusual-looking hickory tree. Instead of being green, its leaves were striped with yellow. It was beautiful, but sadly this hickory tree was experiencing a serious problem.

Interveinal chlorosis is when your leaf goes yellow but only in between leaf veins, which remain green. This is usually a sign that the tree doesn’t have enough manganese or iron, both essential nutrients when it comes to tree health and photosynthesis.

There are a number of reasons why this may be happening. Those nutrients may be lacking in the soil or it could be a pH problem. PH, which measures acidity and alkalinity levels, influences how easy it is for a plant to take up nutrients from the soil. If the pH is off, it’s like going to an all-you-can-eat buffet with your hands tied behind your back. The food is there, but you just can’t get at it.

Interveinal chlorosis on a hickory leaf is evident when the veins stay green while the areas in between the veins turn yellow.

Soil testing kit

Alternatively, this problem could be due to too much phosphorus in the soil, which may be the result of the over-application of fertilizers. While phosphorus is an important nutrient for healthy plant growth, too much is toxic.

So, how do you know what’s responsible for the problem? Get a soil test. It will tell you the levels of iron, manganese, phosphorus and more. You can then help your tree by amending the soil according to the lab’s instructions.

Sometimes you’ll see a tree’s leaves turn completely yellow during the growing season. This is called chlorosis and, in contrast with interveinal chlorosis, this could be a sign of nitrogen deficiency. (Note that soil tests are rarely good indicators of nitrogen in the soil since levels change constantly.) Consider amending your tree’s soil with quality compost in the spring or summer to see if that helps.

Soil testing kit.

Signs of pear trellis rust

Disease


In Ben Nobleman Park in Toronto, Ontario, where I planted a community orchard in 2009, I learned my lesson about disease on leaves early. In the first year, I saw a few orange spots on the leaves of one of our young pear trees. I ignored them.

The next year there were more spots and this time, they were on all three trees. By the end of that summer, the trees were covered with orange spots and looked like three miserable kids covered with chicken pox. Only then did I do my research and discover that our pear trees were infected with a fungal disease called pear trellis rust.

But that’s just one spotty disease that trees can experience. Over the years, I’ve seen all sorts of trees with spotted leaves. They may be big or small, brown, green or red, but they are never a good sign. Often they are signs of fungal diseases.

The important thing is to research the problem early. Some fungal diseases can be nipped in the bud with applications of an organic fungicide such as garden sulphur.

By the time we acted, it was too late. Our young pear trees were too weak and stressed and would never fully recover. We had to dig them out and plant apple trees instead.

Signs of pear trellis rust, a fungal disease.

Evidence of leaf miners

Pests


Some pests are really crafty. Japanese beetles, for example, turn our trees’ leaves into beautiful lace patterns as they nibble away. Others create horrible larvae tents (think of tent caterpillars) that feed on young leaves.

Some problems are more subtle. If a leaf looks unhealthy, pluck it off and hold it up to the sun so you can see through it. You may see signs of a leaf miner that’s been tunnelling through it leaving frass (insect larvae excrement) in brown blotches here and there.

What do you do with pests? Research and then choose your option. You can smoosh them as you see them. You can burn them (an option for tent caterpillars). Or if there are just a few, you can leave them. After all, those pests are trying to survive – just like the rest of us.

To find evidence of leaf miners, hold the leaf up to the light and you may see the tunnelling leaf miner and its frass.

A tree with healthy leaves

Text by Susan Poizner

Photos by OrchardPeople.com

Susan Poizner is the award-winning author of the fruit tree care book “Growing Urban Orchards” and the creator of fruit tree care training website www.orchardpeople.com.

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