Plant Diseases: Oak Tree Disease (NJ), blue green algae, green alga

Expert: Jim Hyland - 3/30/2009

Question
QUESTION: My oak tree has a greenish white looking scale. A limb fell and it had strings of brown rubbery lesions (?) on it. When the sun cme out these strings of whatever began to melt. I have never seen anything like this even though most of my trees have this type scale. Any idea as to what it could be?

ANSWER: These are lichens and not a problem.
Lichens are organisms consisting of a fungus and a green or blue-green alga growing together in a mutually beneficial, symbiotic, relationship. The entire structure, called a thallus, is so different structurally from either of its partners that microscopic examination is necessary to distinguish the fungus and the alga.

The fungus obtains water and minerals from the air and the material it is growing on. The alga provides carbohydrates and vitamins. Some blue-green algae fix nitrogen that is used by both the alga and the fungus. Nitrogen is also obtained from bird excrement, organic debris, or plant leachate.

Lichens may be flat, leafy, or branched and hairlike. All three forms occur on tree bark as well as on rocks, soil, and other substrates. Colors may range from white to gray, red, green, yellow, and black.

Although lichens grow on tree bark, they are not parasitic (disease-causing organisms), and do not harm trees. The fact that lichens grow rapidly when exposed to full sunlight may explain their profusion on dead trees. The one conclusion that may be drawn with certainty from lichens on trees is that the air nearby is relatively pure. Most lichens will not grow in a smoky or polluted atmosphere.

Here is a web link to more information on lichens
http://www.backyardnature.net/lichens.htm

---------- FOLLOW-UP ----------

QUESTION: Thank you for your prompt answer. I now understand the lichen fungus but what was the brown rubbery substance that was attached to the lichen. The brown substance was about 5 inches long. I have never seen this attached to the lichen fungus.

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Answer
A lichen is composed of not one, but two organisms: a fungus and an alga or cyanobacterium. The two organisms live in a symbiotic relationship in which the alga provides both partners with energy through photosynthesis and the fungus provides shelter and protection for the alga.

Approximately 20,000 species exist worldwide, about 3,600 of which are found in North America. Lichens cover eight percent of the Earth's terrestrial surface and they can grow just about anywhere: on soil, rocks, trees, even on human-made surfaces. They exist in some of the most extreme, inhospitable environments on the planet, including mountaintops, deserts, and polar regions. While lichens often grow on trees and shrubs as epiphytes, they do not extract nutrients from the surfaces on which they grow, but instead absorb nutrients from the atmosphere. Lichens vary widely in size, color, and shape. They also change color during the rain as they soak up water and produce food energy. This is one of the remarkable qualities of lichens, perhaps even the key to their survival in harsh climates: they can dry out completely, becoming very fragile and brittle, but will quickly rehydrate when moisture becomes available in their environment.
The expanded lichen is usually the fungi partner that is absorbing the moisture and producing its "flower" to produce spore to generate.
Here is a web link to more information on lichens with some pictures. http://www.backyardnature.net/lichens.htm

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