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