Monday, November 5, 2007

Diseases - How Plants Defend Themselves Against Diseases

One of the issues with plant diseases during and following a drought year is that there is reduced resistance to plant diseases due to reduced plant defences. The following is a list of the different ways plants defend against disease.

How plants defend themselves against diseases

Constitutive (passive) plant defenses

- bark
- bud scales
- collenchyma/sclerenchyma (protect vascular bundles)
- defense trichomes (hairs)
- position, size, and shape of stomata on lower leaf surface
- the suberized outer layers of bulbs, corms, and tubers
- waxy cuticle
- suberin (similar to waxy cuticle) on primary roots (however, root hairs and root apical meristems are extremely vulnerable to attack)

Chemical (these are secondary metabolites, which are not necessary for growth of the plant)
- cyanogenic glucosides (cyanide bound to sugar molecules)
- phenolic glucosides (phenols bound to sugar molecules)

Induced (active) plant defenses

- abscission layers (infected portions of leaves drop out and block further invasion of the pathogen)
- cork layers (block further invasion of the pathogen)
- gums or resins (create barriers against invading pathogens in wounds or vascular tissue)
- papillae of callose (thickenings that prevent entry of a pathogen into a cell)
- thickening or lignification of cell walls
- tyloses

- pathogenesis-related (PR) proteins (enzymes and other proteins produced as defense compounds)
- phytoalexins (defense compounds toxic specifically to the pathogens of the host plant)

Information from "The Relationship of Plant Stress to Plant Disease" by Ann B. Gould, Ph.D., Specialist in Plant Pathology, Rutgers University in the May 31, 2007 issue of the Plant and Pest Advisory, Landscape, Nursery, and Turf edition.

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