Plant Pathology: Diseases in Plants

Plant pathology is the scientific study of diseases in plants. It is also known as phytopathology. Plant disease is a dynamic process where living or nonliving entities impair a plant’s normal function over time.

Disease can affect plants in many ways. It can inhibit photosynthesis if it reduces the amount of sunlight available to the leaf, if part of the leaf dies, or if it leads to leaves falling off prematurely. Root diseases can affect the absorption of water and nutrients and therefore lead to various types of damage to the portion of the plant above the soil’s surface. Diseases can affect the vascular system in plants inhibiting the flow of water and nutrients from the roots to other parts of the plants as well as the flow of food from the leaves to parts of the plant where growth is occurring. Disease can destroy the food supplies of perennials needed to overwinter and start growing again in the spring. It can divert food from plant growth areas of pathogen growth and may inhibit plant reproduction.

Symptoms vs. Signs of Plant Disease

A symptom is a visual effect or characteristic of the plant that indicates that a plant is diseased. It is often hard to detect certain symptoms unless there are healthy plants nearby to which you can compare the diseased plants. Certain symptoms may only be visible if magnified using a microscope. Some local

symptoms may only affect a specific area of the plant, while other systemic symptoms tend to affect large portions of the plant, possibly the entire plant. Many symptoms are external and are visible on the outside of the plant. Other symptoms are internal to the plant and require that the plant be cut open to detect them.

Signs, on the other hand, are parts of the agent that causes the disease, which are visibly noticeable near, on, or in a plant infected with a disease. These signs are helpful in identifying the underlying pathogen causing the disease.

Conditions Necessary for Disease in Plants

For disease to occur in plants, three requirements often referred to as the disease triangle must be met in the same place at the same time:

  • The host plant must be susceptible
  • A plant pathogen must be present
  • Environmental conditions must be favorable to the plant pathogen

Some environmental conditions that are often favorable to plant pathogens include exposure to periods of high humidity, saturated soil, wet plant surfaces, and moderate to warm temperatures.

Once all three requirements of the disease triangle have been met, the pathogen will infect the susceptible host starting the disease. The disease will continue until environmental conditions change such that they are no longer favorable to the hose.

Diseases can be managed by eliminating one or more of these disease triangle requirements. This might mean breeding a host plant resistant to disease, killing the pathogen using chemicals, or changing the environmental conditions.

Two Main Types of Plant Diseases

Diseases caused by living organisms, or pathogens, are called biotic diseases or infectious diseases. Pathogens cause disease but are not diseases

themselves. Once a pathogen infects a plant, the plant is diseased (not before). There are many microorganisms that can cause diseases including bacteria, fungi, viruses, nematodes, and parasitic plants. Fungi cause about 70% of biotic plant diseases. Such pathogens can spread from plant to plant and infect all types of plant tissue. If one plant has a biotic disease, chances are good that it will spread to neighboring plants.

Diseases caused by nonliving factors are called abiotic diseases, or noninfectious diseases. These diseases are also called disorders. Environmental conditions such as winter damage, drought stress, salt injury, nutritional deficiencies, and soil compaction can lead to abiotic diseases in plants. These cannot spread from plant to plant. They are, however, common and should always be considered when assessing plant health.