In part one of this series on plant diseases, I described the process of diagnosing a plant problem that might have been caused by a biotic agent (pathogen) or some abiotic factor, such as an environmental stress of some sort. That is the first step in solving the problem: proper diagnosis. If the conclusion is that your plants are infected by some pathogen, it is important to determine what that organism actually is in order to take the proper corrective action.

In this article I describe the different types of pathogens that might be infecting your plants and provide some information about their biology. Knowing the biology of a pathogen allows a grower to choose the best remedy and time the remedy’s application to be the most effective. Pathogens that often lead to basic plant diseases are fungi, bacteria, viruses or nematodes.

What are the Different Types of Fungal Plant Pathogens?

There are many different fungi that can cause plant diseases. Some live in the soil and cause root diseases, while others live above ground and infect the stems, leaves or flowers of the plant. All of these fungi produce spores or other structures (propagules) that can grow and infect plant tissues—both the roots and the leaves.

Many also produce propagules that are resistant to environmental stresses and can live for extended times in the soil and in or on roots and leaves, waiting for the conditions to be right to grow again and infect plants. Such propagules generally have thick walls with dark pigmentation to protect them from physical and biological elements.

When spores of pathogenic fungi contact roots or leaves of a susceptible host plant, they germinate and grow on the plant tissue surface and penetrate the cells. Then they spread within the plant tissues, generally producing chemicals that kill the cells. When enough cells have been killed, the roots become rotten and can no longer function or support the plant. When leaf tissue is infected with a fungal pathogen, dead spots may appear on the leaves.

Eventually, fungal pathogens will produce many new spores, some of which can spread to other healthy roots or leaves, causing new infections. Many fungi will then produce survival spores or structures. The rotted roots will be brown compared to healthy roots that are white, and plants will be stunted due to lack of good root function.

What are the Different Types of Bacterial Plant Pathogens?

Most bacteria are tiny, single-celled organisms (one to three micrometers) compared to the larger fungal spores (at least five micrometers). Bacterial cells have a cell wall, but their genetic material is not contained in a nucleus as with fungi and most other microbes. Some produce spores with a thicker wall, while others do not.

Spore-­forming bacteria can withstand harsher environmental conditions, whereas the rest are quite sensitive and need other means of protection. When cultured, bacteria look slimy, and there can be some cultural variation in the bacterial colony morphology. Identification of specific bacterial pathogens requires laboratory procedures to detect minute genetic differences.

To prove that a bacterium isolated from a diseased plant is the pathogen, healthy plants have to be inoculated and disease symptoms must appear. Many bacteria can be isolated from diseased plants, so identification of the pathogen is critical. However, bacteria cause foliar symptoms on certain plants that are diagnostic.

Most bacterial diseases are on the foliage or stems—few infect roots. One exception is crown gall, a distinctive bacterial disease that occurs on both roots and stems of some plants. The gall structures are usually fairly diagnostic.

Bacterial pathogens spread in splashed water from rain or irrigation. They are able to rapidly multiply in water, doubling their numbers in a matter of minutes. Many bacterial cells can be produced on a leaf or fruit lesion, and splash to more tissue rapidly. There are few or no treatments that can limit the spread of the disease. Culturally, keeping the foliage and fruit dry, that is, without standing water, is the best bet.

What are the Different Types of Viral Plant Pathogens?

Viruses are infectious and intracellular entities, but they are not cells. There is some debate about whether or not viruses are living or non-­living, but they cannot reproduce on their own and require components found in host plant cells. However, viruses do multiply within host cells and can cause disease. Virus particles in plant cells are much too small to be seen, even with a light microscope.

They have various shapes, but all are a thousand times smaller than even a bacterial cell. Some are spherical, some rod­-shaped. They are composed of nucleic acids covered by a coat of protein.

Virus particles can be transmitted from plant to plant by mechanical means (rubbing leaves of infected plants and then touching healthy leaves) or by insects such as aphids that feed on infected leaves and then move to healthy leaves and deposit virus particles that initiate new infections.

Viruses can move systemically throughout the plant and cause changes in the plant’s machinery that result in the development of symptoms such as stunting of the plant or mosaic patterns on the leaves, and as a result the productivity of the plant is likely to be diminished. There is no cure for a virus­-infected plant. A few viruses are carried in the seed, but that is not true for most crop plants.

What are the Different Types of Nematode Plant Pathogens?

Nematodes are tiny roundworms that largely infect plant roots. There are only a few that infect foliage. The most serious infect roots, causing lesions or knots on the roots. Nematodes multiply within the infected tissue and their young or their eggs are released into the soil where they live until they become adults to infect more roots.

Plant parasitic nematodes must infect plant roots to multiply, so they are considered obligate parasites. Root lesion nematodes cause root rot, so their effect on the plant would be to cause stunted growth.

Summing It All Up

To recap, plants can be infected by different types of pathogens, ranging from fungi, bacteria, viruses and nematodes. It is important to diagnose the cause of a plant’s disease in order to consider options for managing it.

Some pathogens are soil-borne and infect roots, while some infect only the aerial parts of plants. The extent of damage caused by the disease can vary, depending on the pathogen and how and when the plant became infected.

In the case of root diseases, whether caused by fungi or nematodes, the pathogen is most likely resides in the soil and begins the infection process by invading the roots, either rotting them or moving from the root into the vascular or water and nutrient-moving cells, sometimes blocking them and causing wilting.

Foliar pathogens, on the other hand, initiate disease by spores or cells of the pathogen splashing onto the leaves or stems, thereafter initiating infection.

Once leaves are infected, pathogens multiply and can spread to other leaves or plants. Viral diseases are most likely to occur when infected plants are touched or pruned, followed by touching or pruning of a healthy plant.

Virus particles are rubbed into the healthy leaf cells, or they are transferred to the healthy leave by insect feeding, such as by aphids. Remember, a virus­-infected plant cannot be cured.

In part three of this series on the plant diseases, I go over options for managing or controlling plant diseases, whether chemical, cultural or biological. For many of the diseases caused by different pathogens, one of the best remedies is to discard infected plants and focus on avoiding the disease next season or on the next planting, either indoors or in the greenhouse.