Chapter 23

Greenhouse Disease Management

Types of Greenhouse Plant Diseases

Despite the four basic forms of plant disease causal organisms that commonly infect greenhouse crops (fungi, bacteria, viruses, and phytoplasmas), many microorganisms are beneficial. Beneficial microorganisms control pests, build soil structure, and decompose organic material as well as eliminate toxins. Elimination of all microorganisms results in a lack of natural balance causing disruption and may encourage devastating outbreaks of disease-causing organ­ isms.


Fungi are difficult to describe because they are organisms found differing in many forms, behaviors, and life cycles. These organisms are characterized by a chitinous cell wall and filamentous growth called hyphae, which forms mycelium. Some fungi grow as single cells. Sexual and asexual reproduction is by way of spores. They are heterotrophic, lacking photosyniliesis, which means that they exist either as a saprophyte or as a parasite. Saprophytes share a role in the decay of plant and animal remains into simpler forms that can be later absorbed by future generations of plants and animals. Fungi are grouped into four categories.


Bacteria are single-celled microorganisms. Unlike cells of plants, bacteria cells do not contain a nucleus. Bacteria can be found in every habitat on earth soil, hot springs, radioactive waste, seawater, and deep within the earth crust. Bacterial diseases found that infect greenhouse plants are difficult to control other than through prevention, sanitation, and removal of infect plants. Disinfectants can be used to sanitize greenhouse tools, benches, pots to provide some protection. Bacterial species that cause greenhouse plant diseases include bacterial wilt of carnation (Pseudomonas caryophylli) bacterial blight of geranium (Xanthomonas pelargonii), soft rot (Erwinia chrysanthemi), and crown gall on rose, chrysanthemum, and geranium (Agrobacterium tumefaciens).


Viruses can be defined as noncellular organisms. These organisms consist nucleic acids, RNA and DNA surrounded by protein, which obligately reproduce inside host cells using the host's metabolic machinery and ribosomes. This reproductive process forms products called virions, which protect the virus and are transferred to other cells. Viruses were once considered to be nonliving in that they do not respire, they are not irritable, and they neither move nor grow; however, they do most certainly reproduce, and they regularly adapt to new hosts. They are too small to be seen under a light microscope, they can be observed using an electron microscope.

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