MICHIGAN FORESTS FOREVER TEACHERS GUIDE

WHAT IS A DISEASE?

Diseases of trees are a little more difficult to visualize than insects.  Most tree disease organisms are fungi.  Most of these fungi can be divided into two major classes, the ascomycetes and basidiomycetes, although there are other taxonomic units for fungi.  The differences are based on fruiting bodies that produce and release spores.  Fungal life cycles have many variations.  Most of a fungus, however, is underground or in a host.  This "hidden" portion consists of filaments called hyphae that form masses called mycelia.   It's this part of the fungus that does the damage, usually by digesting living and/or non-living parts of the tree.  Destroyed tissue interrupts tree function.  If enough damage is done, the tree will die.

Chestnut blight, Dutch elm disease, oak wilt, and powdery mildews are examples of ascomycetes, or sac-fungi.  So are morel mushrooms, truffles, bakers yeast, and Pencillium spp. (penicillin is derived from one species).   Ascomycetes form tiny sac-like bodies that contain either four or eight spores.   Usually, there are many of these structures in a single fruiting body.  For the most part, ascomycetes decompose non-living organic matter.

Most of our common mushrooms, toadstools, and puffballs are members of the basidiomycetes, or club-fungi.  So are many of the rusts that infect trees and food crops.  The spores are released from tiny clubs.  The fruiting bodies are often conspicuous and sometimes very colorful.  Bracket fungi on trees are examples of basidiomycetes. 

Spores are very tiny.  They float on the wind and water, or can be transported by other organisms, especially insects.  Spores cannot germinate and survive on the bark of trees.  A wound is required for entry.   Once in a tree, the degree of spread will depend on many factors, including overall tree vigor, the species of tree, and the species of fungus. 

Non-fungal Diseases

Disease symptoms can be caused by agents other than fungi.   Bacterial and viral disease sometimes afflict trees.  Mineral or chemical deficiencies can stress a tree.  Sun scorch, frost-cracking, winter kill, and toxins (natural and human-made) can all cause damage to bark or leaves.  Sometimes the effects of insects or other animals appear to be diseases.  Witches brooms and mistletoes are parasitic plants. 

Disease Symptoms

Anthracnose:  A very common ailment of leaves, especially during wet/humid weather.  Dead areas appear on leaves that might follow the leaf veins or a random pattern.  Typical on maples, ashes, and oaks.

Bleeding or Sap Drip:  Many times a simple mechanical injury (bark rubbed off by something or a broken branch) will cause bleeding in a tree.  Other times, such drips may be evidence of a disease, such as white pine blister rust or wood borer injury.

Blights:  These are diseases that kill young living tissue, particular leaves, twigs, and shoots.  Fire blight of apples is a good Michigan example.  Sometimes in the Fall, the yellowing of conifer needles is mistaken for a disease, when it it the normal fall "leaf" drop.

Cankers:  These fungal organisms that cause cracks or dead areas on trunks and branches.  Oftentimes, the canker will ooze sap.  Nectria spp., Hypoxylon spp. and Eutypella spp. are common cankers in Michigan.

Damping-off:  This occurs with new seedlings, evidenced by the seedling wilting and falling over.  Certain soil fungi attack the seedling at ground level, killing the vascular tissue. 

Galls:  Galls are usually malformations caused by insects or mites, and can be found on leaves, needles, twigs, and small branches.  A small larva produces chemicals that force the tree to produce these odd-looking growths.  They are not usually harmful to the tree, but are visually unattractive to most people.  Galls are a particular problem in the Christmas tree business.  Other gall-like malformations may be caused by bacteria and can be fatal.

Leaf spots:  Generally circular patches of dead leaf tissue.  Spots might be brown, yellow, or translucent.   The can even become holes.  Fungi are often responsible, but certain insects will cause a similar appearance.  Irregular spots are called "blotches".  Maple tar spot disease is a common residential fungus .

Mildews:  Not very common with trees, a powdery coating, often white, gray, or bluish, is caused by the fruiting bodies of fungi.  You may have seen similar mildews in your bathroom or on damp clothing after a few days.

Mistletoes:  Somewhat similar in appearance to witches brooms, a mistletoe is a parasitic plant that produces seeds.  It's most common on black spruce, but also grows on white spruce and tamarack.

Rots:  These might be the most common of tree diseases.  Fungi invade and consume the dead wood in a tree, causing structural weakness and opens the tree to other invaders, such as woodpeckers, squirrels, ants, et al.  Conks are usually the first sign of rot, but by the time conks occur, the rot is already well-established in the tree.  Stands with 20-30% of the trees bearing conks often have infection rates in over 50-60% of the trees.

Rusts:  Orange or reddish-brown color on leaf tissue or other living tissue is named "rust" because of the color.  Rusts are a group of "basidiomycete" fungi.

Scorch:  Tissues, especially leaves, that brown and curl from the edges are called scorch.  Very hot sunny days may scorch leaves and needles, but so will very cold, windy winter days.  A scorched appearance also be caused by disease agents or certain environmental pollutants, especially salt injury along main travel routes.  Sometimes in the fall, the yellowing of conifer needles is mistaken for a disease, when it it the normal fall "leaf" drop.

Wilt:  Withering of leaves, often with discoloration can be caused by many things, from a lack of water during a dry spell, or from a disease organism preventing the transport of water to tree crowns.   Dutch elm disease and oak wilt are two good examples.

Witches Brooms:    An excessive and dense amount of branching is caused by a rust fungus.  Resembles dwarf mistletoes.

The U.P. Tree Identification website has a pretty good collection of common insects and diseases.      .

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