A silent killer

 

Sunshine highlighting a grass tree

Grass trees are one of the most recognised victims of the silent killer Phytophthora cinnamomi, but other plants are also impacted. A recent talk highlighted the problem of how to prevent the spread to other areas of bushland.

Here are some brief notes about Phytophthora cinnamomi (P c.) from a presentation by David Smith, from the Dept. of Economic Development, Jobs, Transport and Resources to the Friends of Canadian Corridor at the Earth Ed Centre, Mt Helen.

Often referred as being a fungus it is now classified in the Order Oomycota and therefore closely related to slime moulds. There are currently at least 124 species of Phytophthora in the world, with the number rising each year. Some species only occur on the foliage of plants while others such as P c. only occur in the soil.

Symptoms resemble chronic drought – leaves yellow and then turn brown; pale to dark brown discoloration under the bark of the stem and roots.

Indirect effects of P c. include impacts on insects and small animals that would normally inhabit the ground cover and understory plants (which may be killed).

Indicator plants for the presence of P c. in an area include Xanthorrhoea and Proteaceae sp. such as Banksia, Isopogon etc.

Sources of infection include – pedestrians, machinery, water, gravel, vehicles, motor bikes, bicycles, animals and nursery stock.

P c. will move up and down within the soil profile in response to changes in soil moisture and temperature. It requires moisture to survive and optimum temperature of 240 – 280C (little growth occurs below 60C or above 340C). Prolonged drought may help to reduce a P c. infection.

Control methods include –

  • Eradicants: (for disinfecting tools, footwear, bicycles etc) such as chlorine, methylated spirits, ™Phytoclean.
  • Protectants: e.g. copper based products, Bravo, Mancozeb, Captan etc. and
  • Systemics: e.g. Phosphonate and Metalaxyl which may eradicate the disease in individual treated plants but are not suitable for broad scale use

Finishing on a bright note, it is sometimes possible to collect seed or cuttings from plants which survive the infection of an area (they appear to have genetic resistance to the disease) and use these plants for revegetation projects.

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