Fungal, Viral or Bacterial
Disease is defined as “suboptimal plant growth brought about by a continuous irritant, such as a pathogen (an organism capable of causing disease) or by chronic exposure to less than ideal growing conditions.” Plant pathogens are very similar to those that cause disease in humans and animals. Even though most plant diseases, around 85%, are caused by fungal or fungal-like organisms, here are some examples of the diseases caused by the three main pathogenic microbes: fungus, bacteria and virus.
A sign of plant disease is the physical evidence of the pathogen. When you look at powdery mildew on a lilac leaf, you’re actually looking at the parasitic fungal disease organism itself.
A symptom of plant disease is a visible effect of the disease on the plant. Symptoms may include a detectable change in color, shape or function of the plant as it responds to the pathogen and can generally be placed in the following categories:
• Abnormal tissue coloration: Tissue color may change (leaves, stems, roots). Examples include: chlorosis (yellowing), necrosis (browning), purpling, bronzing and reddening. Mosaic or mottled patterns may also appear on some tissues.
• Wilting: Water stress (too much or too little) can cause a plant to wilt. If a pathogen interferes with the uptake of water by the host plant, a part of the plant or the whole plant may die.
• Tissue death: Necrotic (dead) tissue can appear in leaves, stems or root, either as spots or as entire organs. Decay of soft succulent tissue, as in damping off in young seedlings, is common and can result in other symptoms, such as bad odor from rotting tissues.
• Defoliation: As the infectious disease progresses, the plant may lose all its leaves and sometimes drop its fruit.
Abnormal increase in tissue size: Some diseases increase cell numbers or cell size in the plant tissues, twisting and curling the leaves or forming galls on stems or roots.
• Dwarfing: In some cases the pathogenic organism will reduce cell number or size, stunting parts of the host plant or the whole host plant.
• Replacement of host plant tissue by tissue of the infectious organism: This occurs commonly where floral parts or fruits are involved. Examples include ergot on rye and other cereal crops.
Fungi and FLOs are heterotrophic (cannot make their own food), eukaryotic organisms that have a filamentous growth habit and which may or may not produce spores.
Fungal disease signs:
• Leaf rust (pictured left)
• Stem rust
• Sclerotinia (white mold)
• Powdery mildew
Fungal disease symptoms:
• Birds-eye spot on berries (anthracnose)
• Damping off of seedlings (phytophthora)
• Leaf spot (septoria brown spot)
• Chlorosis (yellowing of leaves)
Bacteria are microscopic, single-celled prokaryotic organisms that reproduce asexually by binary fission (one cell splitting into two). Phytoplasmas are a certain type of bacteria that lack a cell wall.
Bacterial disease signs:
• Bacterial ooze
• Water-soaked lesions
• Bacterial streaming in water from a cut stem
Bacterial disease symptoms:
• Leaf spot with yellow halo
• Fruit spot
• Crown gall
• Sheperd’s crook stem ends on woody plants
Viruses are intracellular (inside the cells) pathogenic particles that infect other living organisms. They live off the host’s nutrients.
Viral disease signs:
• None – the viruses themselves can’t be seen
Viral disease symptoms:
• Mosaic leaf pattern (pictured right)
• Crinkled leaves
• Yellowed leaves
• Plant stunting
Remember, monitoring your plants is key. If plant disease is suspected, careful attention to the plants appearance can give a good clue regarding the type of pathogen involved, and you may be able to successfully take steps to limit the disease. If you see something that doesn’t look “normal”, contact a Branch Tree & Landscape Service certified arborist for an evaluation.
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