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Horticulture/Phytopathology

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Phytopathology is the study of how disease processes develop in plants. These processes are of particular concern in the fields of agriculture and horticulture. There are several major groups of organisms that cause disease on plant, each of these are outlined below.

Fungal Phytopathogens

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There are over 10,000 fungal pathogens of plants. The fungal plant pathogens themselves fall into to major groups, the biotrophs which feed from living plant tissue and the necrotrophs which kill plant cells and then live off the nutrients released. However, many plant pathogens adopt a hybrid lifestyle, initially acting as biotrophs before converting to a nectrophic lifestyle. These are called hemibiotrophs.

The majority of phytopathogenic fungi belong to the ascomycetes and the basidiomycetes.

The fungi reproduce both sexually and asexually via the production of spores. These spores may be spread long distances by air or water, or they may be soil bourne. Many soil bourne spores, normally zoospores and capable of living saprophytically, caring out the first part of their lifecycle in the soil.

Fungal diseases can be controlled through the use of fungicides in agriculture, however new races of fungi often evolve that are resistant to various fungicides.

The Oomycete Phytopathogens

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The oomycetes are fungal like organisms that until recently used to be mistaken for fungi. They include some of the most destructive plant pathogens including the genus Phytopthora which includes the casual agents of potato blight and sudden oak death, and members of the genus Pythium, which causes damping off of seedlings.

Despite not being closely related to the fungi, the oomyctes have developed very similar infection strategies and so many plant pathologists group them with fungal pathogens. The oomycetes are particularly difficult to control, and many can live in the soil as saprotrophs for long periods.

Protozoa

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There are a few examples of plant diseases caused by protozoa. They are transmitted as zoospores which are very durable, and may be able to survive in a resting state in the soil for many years. They have also been shown to transmit plant viruses.

When the motile zoospores come into contact with a root hair they produce a plasmodium and invade the roots.

Bacterial Phytopathogens

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Most bacteria that are associated with plants are actually saprophytic, and do no harm to the plant itself. However, a small number, around 100 species, are able to cause disease. Bacterial diseases are much more prevalent in sub-tropical and tropical regions of the world.

Most plant pathogenic bacteria are rod shaped (bacilli). In order to be able to colonise the plant they have specific pathogenicity factors. There are 4 main bacterial pathogenicity factors:

1. Cell Wall Degrading Enzymes - used to break down the plant cell wall in order to release the nutrients inside. Used by pathogens such as Erwinia to cause soft rot.

2. Toxins. These can be non-host specific, and damage all plants, or host specific and only cause damage on a host plant.

3. Phytohormones - for example Agrobacterium changes the level of Auxin to cause tumours.

4. Exopolysaccharides - these are produced by bacteria and block xylem vessels, often leading to the death of the plant.

Bacteria control the production of pathogenicity factors via Quorum sensing.

Phytoplasma and Spiroplasma are a genras of bacteria that lack cell walls, and are related to the mycoplasmas which are human pathogens. Together they are referred to as the mollicutes. They also tend to have smaller genomes than true bacteria. They are normally transmitted by sap-sucking insects, being transferred into the plants phloem where it reproduces.

Plant Viruses

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There are many types of plant virus, and some are even asymptomatic. Most plant viruses have small, single stranded RNA genomes. These genomes may only encode 3 or 4 proteins: a replicase, a coat protein, a movement protein to allow cell to cell movement and sometimes a protein that allows transmission by a vector.

Plant viruses must be transmitted from plant to plant by a vector. This is normally an insect, but some fungi, nematodes and protozoa have been shown to be viral vectors.

Plant Parasitic Nematodes

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Nematodes are small, multicellular worm like creatures. Many leave freely in the soil, but there are some species which parasitise plant roots. They are mostly a problem in tropical and subtropical regions of the world, where they may infect crops. Root knot nematodes have quite a large host range, whereas cyst nematodes tend to only be able to infect a few species. Nematodes are able to cause radical changes in root cells in order to facilitate their lifestyle.

Examples of Plant Pathogens

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Significant fungal plant pathogens

  • Ascomycetes
  • Basidiomycetes
    • Rhizoctonia spp.
    • Puccinia spp.; causal agents of severe rusts of virtually all cereal grains and cultivated grasses

Significant oomycete plant pathogens

Significant bacterial plant pathogens

Significant abiotic disorders can be caused by:

Natural
Drought
Frost damage, and breakage by snow and hail
Flooding and poor drainage
Nutrient deficiency
Salt deposition and other soluble mineral excesses (e.g. gypsum)
Wind (windburn, and breakage by hurricanes and tornados)
Lightning and wildfire (also often man-made)
Man-made (arguably not abiotic, but usually regarded as such)
Soil compaction
Pollution of air and/or soil
Salt from winter road salt application
Herbicide over-application
Poor education and training of people working with plants (e.g. lawnmower damage to trees)
Vandalism
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