The Identification of Bacteria, Fungi, & Yeast in Soil

Common Infestation Symptoms

• When soil becomes overrun with fungi, bacteria or yeast, symptoms show in plants. Overall health declines, leaves turn yellow or brown, and branches or stems die back. Growers should be especially cautious during periods of extended rainfall or humidity, as these conditions are ideal for promoting fungal and bacterial growth. The effects on plants vary depending on the species and severity of infection. A dose of targeted fungicide is usually required.

Yeast Identification

• Yeast is a type of fungus with approximately 1,500 species, is found in most environments and is common in soil. It aids in breaking down organic matter, making it useful for plants to obtain nutrition. There is no chlorophyll in yeast, and they reproduce through budding. Identifying yeast is generally done with a gram stain, a process requiring preparing a slide and staining a sample with crystal violet and safranin solutions. When correctly done, yeast appears as single or budding cells, or may be present in chains called pseudohyphae. Yeast cells are gram positive, meaning they appear dark blue and are as much as four times larger than bacteria.

Bacteria Identification

• Identifying specific bacteria types may be needed to target a plant infestation. Some, such as sooty molds or certain blights, are easily identified, as they leave telltale signs on the above ground portions of the plant. Others, such as root rot, turn roots into a mushy gray substance, useless for transporting water or nutrients. For others, growing a sample in a Petri dish with an agar solution may aid in identification. Color, shape and growth pattern helps identify the species. Members of the Rhizobiaceae family are among the most common, widespread species of bacteria in the world. They come in several colors, and are generally not harmful to the soil's ecosystem and are highly beneficial to legumes.

Fungi Identification

• Fungi are usually different in structure from bacteria or yeast and are commonly found growing in large threads called hyphae, which may span very small or very large distances. There are three main groups of fungi, and only one is harmful. Saprophytic fungi are decomposers that aid in converting organic material into something that is useful to plants, while mycorrhizal fungi colonize roots but are not parasitic. In these relationships, both the plant and fungi benefit, usually because the fungi converts minerals into useful forms. Pathogenic fungi, including verticillium, pythium and rhizoctonia, are each treated with fungicide, and may cause plant death if left untreated. Identification is done in a similar way to identifying bacteria, taking a sample and allowing it to grow with an agar solution in a Petri dish.