The Difference Between Bacterial and Fungal Plant Pathogens

(This week's blog brought to you by: Diane Plewa (U of I Plant Clinic Student Worker)

The most common pathogens we see at the U of I Plant Clinic are fungi. However, there are plenty of bacteria that can cause problems for plants, and one of the first things we do is try to determine if we’re looking at a fungal or bacterial disease.

There are two ways for us to determine what kind of plant pathogen (fungi or bacteria) we’re dealing with: looking at the disease symptoms displayed by the plant and using the microscope to check for fungal spores or bacterial oozing. When we look at the plant’s symptoms, there are a few that tend to indicate a bacterial infection. Yellow haloing around lesions on leaves is one sign of bacterial disease (though there are several fungi that can cause similar symptoms). Bacterial lesions tend to be limited by veins in the leaves. We also suspect bacteria when we find sticky exudate or stringy ooze in diseased tissue.

 Bacterial lesions on pumpkin and pepper. Photo credit: U of I Plant Clinic.

Fungal lesions usually aren’t surrounded by yellow halos. They may manifest as round, oval, or irregular necrotic areas. Some lesions will have a characteristic “bulls eye” appearance. Under a dissecting microscope or strong hand-lens, fungal fruiting bodies (which contain spores) may be observed.

Clockwise from upper right: Fungal lesions on peony; Fungal lesions on hosta; Magnified fungal fruiting structures on spruce. Photo credit: D. Plewa

Identification of the actual pathogen is done by the appearance of the spore and its fruiting body for fungi.  Using a compound microscope, we can observe the actual fungal spores.

A microscopic view of a few common fungal spores. Photo credit: D. Plewa

While an individual bacterium is too small for us to see, we can see masses of bacteria known as “bacterial ooze” or “bacterial streaming.”  Many bacteria, as well as viruses, can be identified using an ELISA immunostrip assay.  You are unable to see a virus particle under a compound microscope.

Bacterial streaming observed under the microscope. Photo credit: D. Plewa

An ELISA immunostrip assay indicating the presence of the bacteria Xanthomonas campestris. Photo credit: D. Plewa

Lightning Only Strikes a Tree Once?

This week's blog brought to you by: Mike Kwiatek (U of I Plant Clinic Student Worker)

Any tree can be hit by lightning, but oaks, elm, maple, poplar, ash, spruce, fir, pine and tulip trees are known to be prone to lightning strikes. As they are trees that grow large and tower over the forest canopy damage is not uncommon from the forces of nature. Water and sap in a tree provide a conduit for the lightning as it travels toward the ground. As the lightning travels through the tree, water and sap boil, gases inside the tree expand, and often the wood and bark of the tree can split or commonly explode. When the bark is wet, damage is usually less severe as lightning will travel more along the outside. 

Trees struck by lightning can survive many years after a lightning strike, but this is based on the extent of the damage and the treatment of the tree afterward. Most trees that die from lightning strikes years later die as a result of infection or infestation by insects. When pruning can be done to remedy damage, it is suggested. Lightning strikes can be extremely stressful to trees so it is advised that trees be watered in times of drought and fertilized in fall. Do not paint the wound, this can do more damage than good.

To prevent lightning damage a lightning protection system may be installed. Protection systems consist of a series of copper cables attached to the tree’s highest branches and grounded a safe distance from the tree. If lightning strikes the tree, the current flows down the cables to ground. If interested in a lightning protection system, consult with an arborist although the damage may already be done.

During the summer at times of drought leaf abscission may also occur in response to heat stress but we do not suspect that this is the main issue. This issue should also, however, be resolved by watering in times of drought. Fertilize the tree in early fall.

Which trees are less likely to be hit by lightning? -beech, birch, and horsechestnut

For additional information, see http://joa.isa-arbor.com/request.asp?JournalID=1&ArticleID=1862&Type=2