Learning From Tree Leaves
Leaves and Tree Health
Autumn is on the way and here in Ontario we’re just starting to see those beautiful leaf colours in vibrant shades of red, yellow and orange. Leaves notify us that the winter is approaching as they draw the nutrients out of their leaves and into their roots for winter storage.. But leaves tell us a lot more than that all through the growing season. They also tell us about the health of our trees.
Last year, for instance, I noticed that the leaves on a beautiful sugar maple in our local park turned yellow. If it happened in October I wouldn’t have worried. The problem was that all the leaves on that tree changed colour in July! It was a hot summer and winter was months away.
So I went closer and saw that something else was going on. The leaves were yellow with brown blotches moving in from the edges and tips. This is called leaf necrosis, It means the leaves are dying and if nothing is done the tree could die too.
Leaf necrosis is a classic sign of drought stress. This 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 our beautiful trees to survive and thrive.
Leaves can tell us so much about tree health. In addition to signalling drought stress, they can show us if the tree is experiencing nutritional deficiencies or even if it is grappling with pest and disease problems. Our job as gardeners is to recognize the symptoms and to fulfill our trees’ needs before its too late.
Tree Leaves and Drought Stress
When brown blotches are working their way in from the edges and tips of your tree’s leaves, it’s very likely to be 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 will also help wash excess salt out of the soil.
If you have an apple or pear tree, be careful not to confuse drought stress with a quickly 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.
If you see this, carefully prune off infected branches back to the trunk and put them in garbage bags in the garbage, not in the compost. Then sterilize your pruners with rubbing alcohol before using them again on any other tree.
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Tree Leaves That Indicate Nutritional Deficiencies
Late last summer, I took a walk in a nearby arboretum and saw a very 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. The leaf veins 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, or soil acidity, influences how easy it is for a plant to take up nutrients from the soil. If the pH is wrong, 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.
Alternatively, this problem could be the due to too much phosphorus in the soil! While phosphorus is an important nutrient for healthy plant growth, too much of it is toxic to plants. This may be the result of the over-application of fertilizers.
So, how do you know what is responsible for the problem? Get a soil test. It will tell you the levels of iron, manganese, phosphorus and more. Then you can help your tree by amending the soil according to the lab’s instructions.
Sometimes you’ll see a leaf turns completely yellow during the growing season including the veins. This is called chlorosis and in contrast with interveinal chlorosis this could be a sign of nitrogen deficiency.
Soil tests are rarely good indicators of nitrogen in the soil since nitrogen levels are constantly changing. In this case consider amending your tree’s soil with quality compost in the spring or summer to see if that helps.
Tree Leaves and Disease
In Ben Nobleman Park in Toronto, where I planted a community orchard in 2009, I learned my lesson about disease on leaves early on. 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 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 I discovered 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 unhappily spotted leaves. They may be big or small, brown or green or red, but they are never a good sign. Often they are fungal diseases.
The important thing here is that if you see any unusual spots on the leaves of your tree, go online and research the problem early on. Some fungal diseases can be nipped in the bud with applications of an organic fungicide like garden sulphur.
In Ben Nobleman Park, by the time we acted it was too late. Our young pear trees were too weak and stressed and would never fully recover so we dug them out and planted apple trees instead.
Pest Damage on Tree Leaves
Finally pests. You have got to admit, some of them are really crafty. Some, like Japanese beetles, turn our trees leaves into these beautiful lace patterns as they nibble away. Others create horrible seething tents of larvae (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 a leaf miner tunneling through the leaf leaving frass (insect larvae poop) in brown blotches here and there.
What do you do with pests? Look them up online, 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’s just a few insect problems, you can also leave them. After all, those pests are trying to survive – just like we humans are.
Director, OrchardPeople.com Fruit Tree Care Education Online
Susan Poizner is the author of the award-winning fruit tree care book Growing Urban Orchards and the creator of the award-winning online fruit tree care training portal at www.orchardpeople.com. She’s also the host of The Urban Forestry Radio Show and Podcast.