Some common tree diseases

By Al Lowe
Contributor

I have been asked many times about both of these fungus diseases, so here goes.
Black Knot
This affects plum and cherry trees, both wild and domestic. The organism which causes it is a fungus. The spores are so small that humans can not see them.
When a spore lands on an appropriate place, usually a small branch or twig of a cherry or plum, it starts to grow down into the bark and wood of the the twig. Now this does a funny thing. The fungus causes the cells of the twig to grow in an entirely different manner, much like a cancer in animals.
At first, the new growth is soft and velvety, so you probably would not even notice it. But after a while, it turns black and hard, from which we get the term “black knot”. It continues to get larger as long as the branch is alive, and may easily get up to a foot or more long. After a while the branch is killed.
The only easy means of controlling this disease is by cutting the affected parts out and burning them. This should be done in the fall, when you can easily spot the parts which are diseased. Certainly it should be done in the very early spring, before the fungus begins to grow again.
This disease can spread quickly, and affects flowering plums and cherries as well as fruit trees.
Apple Scab
If your knowledge of apples is confined to the grocery store, you many not even be aware of this disease or maybe have never seen it.
This is another fungus which reproduces by means of spores. It affects both the leaves and fruit of apple and sometimes pear trees.
At first it shows up as a small velvety green spot. As time goes by it gets bigger, turns grey and becomes hard. The (usually) round spot will crack open and split. In bad infestations, a single apple may have many of these blemishes.
Actually, the presence of these “scabs” does not make any difference at all to the taste of the fruit. You can eat them without any ill effects. It just does not look that great.
Apples, of course, are graded carefully for sale and that’s why you do not see any apples with scabs in the supermarket.
For growers of apples, this is only one of many, many diseases which affect their crops. Apples have to be sprayed very carefully many times during the year to keep a lot of fungus diseases and insect pests under control.
Nowadays, apples for market are not allowed to have any disease marks or insect damage. They are also kept at just the right temperature, so we can eat them all year long.
Just be glad, next time you eat an apple, that you did not live a hundred years ago or so.