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Phytophthora Root Diseases:
Fungi: Phytophthora species; mainly P. cambivora, P. cinnamomi, P. citricola

rootdiseases

 Damage Type

Killing of roots and stem base.

Symptoms and Diagnosis

A tree dies suddenly or slowly, or shows symptoms indicative of a root disorder. Also, all or only the lower branches in one particular area begin to die back.

 

phythophthora damage

Confirmation

Characteristic, though not always present, are tongues of dead bark extending a few inches to several feet up the main stem from the dead roots (accounting for the one-sided branch killing). Roots and probably the stem base are dead, but sound in terms of structural failure.

Additional Indicators

Fungal fruit bodies, mycelium or other fungal structures are absent; resin or gum oozes from the lower stem; the site is liable to lie wet, is irrigated heavily or is heavily mulched with farmyard manure or compost; other susceptible species on the site are affected.

Caution

Easily confused with waterlogging but dead, waterlogged roots are blue-black and like the soil, malodorous. The killed bark is sometimes invaded by Honey Fungus.

Status

Common and widespread in the South, but becomes less common Northwards.

Significance

A frequent cause of debility, dieback and death of amenity trees but often overlooked or misdiagnosed.

Host Trees

Recorded on a very wide range of species. Notably susceptible are Castanea (Sweet Chestnut), Lawson Cypress (Chamaecyparis lawsoniana), Eucalyptus (gunnii). Also a prominent disease of Aesculus (Horse Chestnut), Azalea and Rhododendron, Beech, Apple in orchards, Tilia (Lime) and Prunus (Flowering Cherries). Severe damage to Abies species has been recorded in nurseries and plantations in the formative early developmental stages, though cases in older trees are rare.

Resistant Trees

Any commonly planted species not mentioned above might be regarded as relatively resistant. Rarely recorded on Leylandii, or Quercus (petraea, or robur).

bacterial phtophthora

Infection & Development

Spores moving in soil water, partly passively, partly self-propelled, infect roots and basal stem bark. Mycelium then grows through and kills the Cambium and phloem (water carrying cells). Trees die through the loss of functioning roots or when the stem is girdled at the root collar. The fungus persists in the soil and thus is readily transported on plant roots, machinery, tools and footwear. It is also harboured and carried in water (streams, irrigation, ponds, tanks). Trees can recover but remain open to further attack. Several species of Phythophthora cause identical diseases. Most have host preferences, though some have a very wide host range. Their temperature requirements differ though all require free water for spore production; spread and germination so the disease is most common on sites that are libel to lie wet. It is also encouraged by the presence of large quantities of decaying organic matter such as lawn mowing, farmyard manure and compost.

Control Therapy

None available.

Prevention

  1. The disease is often brought in on planting stock, so use plants from Phythophthora-free nurseries.
  2. Irrigate in moderation; do not allow water to pool around susceptible species.
  3. Replant with resistant species. Fungicides based on fosetyl aluminium and metalaxyl are useful in combating the disease in orchards and may have a very real and successful application within the amenity domain.

Remarks

These diseases are among the world’s most destructive, affecting an enormous range of crops and natural ecosystems. The first cases in this country to be confirmed were on woodland Castanea and Fagus in the 1930’s but the diseases have been recognised as important on amenity trees here only since c. 1970. Root and basal stem-bark killing in trees damaged by de-icing salt may in fact be due to Phythophthora.