a brief introduction to plant diseases

A disease, as it relates to plants, is a disturbance from plant pathogens or environmental factors that interfere with plant physiology.  When a disease is present, plants will express symptoms. Symptoms are the detectable expression of a disease, pest or environmental factor.  These symptoms are usually the result of complex physiological disturbances.  They result in changes to plant appearance, yield and/or quality.  Knowledge of plant diseases is essential because plant diseases:

  • Affect food quality in terms of safety to eat
  • Impact landscape value
  • Alter the quantity, quality and diversity of both domesticated and wild crops

The relationship between environmental, pathogen and host is a complicated one.  There are varying components of this relationship are explained by a “disease triangle”.  The original concept of the disease triangle was amended by Mazz who incorporated the influence of humans and time.

photo credit: finegardening.com

Environmental factors include soil and air temperature, soil type, pH-value, nutrient availability, precipitation humidity and moisture levels.  Factors relating to the environment can be influenced by the effects of human cultural practices, such as monoculture agriculture, rotation practices, the introduction of foreign pathogens via trade, the quality of seeds used and the amount of chemical ingredients greatly impacts the growth environment

Hosts are susceptible crops and cultivars.  All plants are considered hosts.  The age of a plant (developmental phase) affects disease development.  Each plant has a different level of susceptibility to different types of disease

Pathogens are viruses, viroids, bacteria, phytoplasma, fungi, pests (insects, nematodes, mammalian, etc.).  Their impact is largely dependent on the amount of inoculum, the genetics of the pathogen, the virulence of the pathogen and how the pathogen reproduces (monocyclic or polycyclic).  The ecology and mode of distribution (air, soil, seed or vector dependent) are also major influencing factors

Each infectious disease requires a series of sequential events for its development:

  1. Dispersal of the pathogen to the host
  2. Penetration and infection of the host
  3. Invasion and colonization of the host
  4. Reproduction of the pathogen
  5. Pathogen survival between growing season and/or the continuous presence of a host

Viruses are not technically living organisms because they cannot reproduce without a host.  They are not fully understood, although efforts are (of course) being made to expand knowledge surrounding their biology.  It is possible for them to exist in surface water, soil, sewage sludge, ancient glaciers clouds, sea water, plants, etc.  In most cases, they are very stable and can infect a wide range of hosts, although some are host specific.  Upon infection, viruses change the mechanical production process of cells which turns the cells into production sources for new viruses.  The cell then bursts, releasing copies of the virus which then infect new cells.

tomato mosaic virus, photo credit: gardenaction.co.uk

Fungi are the largest group of plant pathogens with approximately 8,000 species.  They are characterized by their reproduction via spores and the production of threading hyphae (mycelium).  They can be transferred by wind and water, rhizomorphs and sclerotia.  Fungi infect via natural openings, wounds, natural openings (stomata, lenticels, hydathodes), intact surfaces (enzymatic) and during pollination.  

One of the most important fungi plant pathogens is the Banana Panama Disease that is caused by Fusarium oxyprorum f. Sp. cubense which affects >40% of all cavendish and 21% of all plantain varieties.

banana panama disease, photo credit: media.padil.gov.au/

Bacteria are prokaryotic microscopic organisms that are free-living cells that produce filamentous colonies.  The reproduce via binary fission.  The daughters are identical to the mothers and they require a host or growth medium to survive.  Bacteria infect via natural openings, non-cutinized parts of the plant (root hairs, nectaria) and through cell walls.

soybean bacterial blight, photo credit: agdev.anr.udel.edu/

Nematodes are the most numerous multi-cellular organisms on the planet.  They are, however, poorly understood.  They feed on plants, fungi, bacteria and even other nematodes.  Signs and symptoms of nematode infection are stunting and slow growth, wilting, yield reduction, a lack of response to various treatments (ex. fertilizer) and damage to roots systems e.g. lesions on the roots, galls at root tips or along roots and root tip stunting.  These symptoms are often mistaken for nutrient deficiencies.  As a result, treatment is often attempted when it is too late.  Plant feeding nematodes also cause mechanical injury to cells and tissues, result in cell death, modify cell function, disrupt the uptake of water and nutrients, alter photosynthesis partitioning, create new avenues of ingress and predispose plants to other diseases.  

Nematodes can be transferred via wind or water.  Infections from nematodes are very difficult to address.  The best options are crop rotation, strip cropping and solarization.  Areas that have higher moisture levels are more susceptible to infections.

damage from a root knot nematode, photo credit: infonet-biovision.org/

To reduce disease transmission and engage in effective management techniques, the following guidelines should be followed:

  • Choose resistant cultivars.
  • Engage is thoughtful cultural practices by avoiding activities that stimulate rapid tree growth which causes weak plants by avoiding excessive nitrogen application
  • Prune infected tissue and prune during dormancy in order to reduce the possibility of disease transmission. Pruning should be completed at 30 – 40 cm below the infected area and should never be done when the plant is wet.  After pruning, burn the infected tissue.

Some interesting factoids:

  • The likelihood of infection during the embryo phase is almost 100% while infection risk during the seed phase is only approximately 2%.
  • Aphids are one of the number one transmitter of plant pathogens.  Ironically, they can be both host and vector.  
  • In 1 tsp of soil there are 100 million – 1 trillion bacteria, 6 – 9 ft of fungal strands and several thousand flagellates and amoebae.
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