Chapter 12

Plant Disease Management for Organic Crops

Types of Plant Disease Pathogens

A plant disease is any physiological or structural abnormality that is caused by a living organism. Organisms that cause a disease are referred to as “pathogens,” and affected plants are referred to as “hosts.” Disease causing pathogens include fungi, bacteria, viruses, phytoplasmas, or nematodes. An infectious agent is capable of reproducing within or on its host and spreading from one susceptible host to another.

Fungi

Fungi are the most abundant group of plant pathogens. These multicellular organisms are typically microscopic. The “body” of a fungus is composed of filament-like threads called “hyphae.” Masses of hyphae are called “mycelia.” When large enough, these masses can be seen without the aid of a microscope. Powdery mildew is one example of a disease in which fungal mycelia is visible. Fungi reproduce via spores, which can be produced sexually or asexually. Some fungi produce spores within sexual fruiting structures (ascocarps, pustules, mushrooms) or asexual fruiting structures (pycnidia, acervuli). Once a fungal spore makes contact with a plant surface, it germinates, much like a seed, as long as conditions are favorable for the pathogen.

Bacteria

Bacteria are microscopic organisms typically composed of single cells. Due to their small size, a high-magnification microscope is required to observe bacteria. Occasionally, when a large number of cells are present, plants may be observed “oozing” bacteria and other organic byproducts. Bacteria are capable of rapid reproduction through a process known as binary fission. In this process, one cell divides to become two, then two divide to become four cells, and so on. Unlike fungi, bacteria are not able to penetrate plant tissue directly. They must infect via wounds or natural plant openings such as stomata. Free water is required for infection. Once inside plants, bacteria begin to reproduce immediately.

Viruses

Virus is a strand of DNA or RNA, consisting of a nucleic acid wrapped in a thin coat of protein.  Once viruses enter host cells, they “hijack” plants and “instruct” cells to produce more virus particles. As plant cells are converted from their normal function and processes (such as cell division or chlorophyll production), changes in plant growth and development may be observed. Plant viruses do not move in and out of plant tissue as readily as fungal and bacterial pathogens. They require vectors (such as insects or humans) to carry them from one plant to another. After entry into plant cells, reproduction begins. Viruses spread throughout plant hosts, infecting all plant parts (systemic infection). Viruses are dependent upon live hosts for replication, thus disease progresses slowly.

Phytoplasmas

Phytoplasmas are extremely small bacteria-like plant pathogens. While they are similar to bacteria, phytoplasmas differ in their inability to survive without a host, their smaller size, and their lack of cell walls. Phytoplasmas rely on insect vectors, such as leafhoppers, for transmission into hosts. During feeding, leafhoppers acquire phytoplasmas from infected host phloem (nutrient-conducting vascular system) and introduce them into healthy tissue.

Nematodes

Plant parasitic nematodes are microscopic roundworms that primarily infect roots, but a few occur in foliar portions of plants. While there are many species of nematodes, only a few are known to parasitize plants. Nematodes reproduce via eggs that result from either the mating of a male and a female or by the female alone. Symptom development occurs as a result of extracted cellular contents or other plant damage. Nematodes may remain on the exterior of roots during feeding (stubby-root nematode) or penetrate plant tissues completely to feed while inside plants (dagger nematode).

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