Phytophthora cinnamomi

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Phytophthora cinnamomi symptoms on chestnut in Chile (top) and on Quercus ilex in Spain (bottom) (click on image to enlarge it)
Author(s): T. Jung
Source: Persoonia 2018, vol. 40, p. 186 + 188

Phytophthora cinnamomi Rands 1922 - (cinnamon fungus)

This oomycete is found mainly in warmer regions and infects a large variety of plants, predominantly fruit and forest trees. It is soil-borne and spreads easily in wet soil. The zoospores are mobile in water and infect the root tips. Root rot leads to wilting, chlorosis, stem necrosis, die-back of branches and often plant death. Trees commonly infected include avocado, eucalypt, oak, chestnut and Banksia.

In Western Australia, many native forest trees and other plants are susceptible and the introduction of P. cinnamomi has caused considerable damage to the ecology of that region. About half of the "Threatened Ecological Communities" within the South-West Botanical Province of Western Australia are infested by P. cinnamomi. Starting in the mid 1940's large numbers of jarrah trees (Eucalyptus marginata) growing in Western Australia were killed by a disease called "jarrah dieback" which was later found to be caused by P. cinnamomi.

Vernacular names
• Deutsch: Wurzelfäule des Avocado
• English: cinnamon fungus
chestnut ink disease
avocado root rot
jarrah dieback
• Español: pudrición de la raíz del avocado
• Français: pourriture de racine d'avocado

The pathogen disperses through water or by human transport of infected plants and contaminated soil. Chlamydospores and oospores may persist in the soil or in plant debris for up to 6 years. Trunk injection of phosphoric acid, phosphite or phosphonate has been found to be effective for control.

The sporangia are pear-shaped or egg-shaped and produce 10-30 zoospores. Globose chlamydospores are also common and have a diameter of 30-50 µm. A1 and A2 mating types have been described, however, homothallic isolates are also known. Oogonia are spherical, about 20-60 µm in diameter and form a thick-walled oospore.

For a review see Hardham & Blackman (2018).