DISEASE

NAME
Phytophthora Rot
Phytophthora species

Pathogen Name
A number of Phytophthora species cause these diseases but the most important species in Australia is the cinnamon fungus, Phytophthora cinnamomi.This species is exotic to Australia and probably originated from south east Asia; it has probably been present in Australia for close to 200 years. Phytophthoraare fungal-like organisms that are related to some protozoa and algae; they are microscopic and cannot be observed by the naked eye.

Description
There are many types of Phytophthora Rot but generally the disease affects the plant by causing a soft rot of the affected plant part. The most common form of the disease is a rotting of the roots that occurs below ground with no visible symptoms of the disease above ground until the disease starts to cause leaf drop.

Symptoms
The disease is initiated below ground in the soil, usually on the feeder roots of a plant. The pathogen grows through the roots killing cells and eventually causing an extensive root decay. This causes the infected plant to lose vigour and the leaves to yellow and die. Entire branches starting from the top then die, quickly during hot weather or linger for months. The infection occurs on the root hairs causing small and large roots to rot. The symptoms of the disease are often very difficult to differentiate from drought symptoms, mainly because the affected root system prevents the uptake of moisture from the soil. Once the disease has progressed far enough large branches will die causing the typical dieback symptoms.

     

There are a number of other diseases caused by Phytophthoraspecies

Twig Die Back(Phytophthora ilicis) attacksIlex species causing black leaf spots and black stem cankers.

Phytophthora cactorumis known by several common names depending on which plant is being attacked and they have various symptoms. Plant species are listed below.

Root Rot(Phytophthora richardiae) infects Zantedeschiaspecies causing the leaves to turn yellow, wilt and die. Flowers are deformed if they bloom and the infection can also be seen in the new growth, eventually killing the plant. On inspection the roots show signs of decay.

Stem Rot (Phytophthora cryptogea) infects the roots and stems turning them brown and seeds are also attacked causing decay. It is commonly found on Tagetes erectaand infected plants wilt, collapse and die.

Source and Dispersal

          
Sporangia          Zoospores          Image by Dr Brett Summerell

Phytophthoraspecies produce sporangia that contain specialised zoospores that have flagella that allow them to move through the soil moisture small distances. These spores encyst on the root and then penetrate the root. The pathogen also produces chlamydospores, specialised survival spores that allow it to survive in the soil for extended periods of time. The chlamydospores can be transported in soil, even extremely small amounts, allowing the pathogen to be dispersed very easily throughout an area and from one location to another. The zoospores are easily moved in water flowing through soil and so are easily dispersed down slopes. It is not uncommon for the disease to move in fronts down a slope.

      Chlamydospores, specialised survival spores
Image by Dr Brett Summerell

Favoured Conditions
Phytophthora root rot is favoured by poorly drained soils or in soils that are waterlogged for short periods of time. The disease generally occurs during periods when the temperature is above 16şC although it has been observed in snow gum country in the Barrington Tops National Park and in southern Tasmania. It also prefers soils that have little organic material.
     
Affected Plants

     Xanthorrhoeaspecies
Image by Dr Brett Summerell

The host range that is attacked by Phytophthora cinnamomiis enormous and is still not well understood but includes many Australian native plants, Rhododendrons, Acer and Prunusspecies, conifers, cabbage tree and strawberries. Some Australian plant families that are quite susceptible include species in the Proteaceae, Epacridaceae and Xanthorrhoeaspecies.

Alphitonia excelsa

Aesculus californicais infected by Sudden Oak disease (Phytophthora ramorum). This disease is primarily a leaf pathogen that thrives in coastal redwood forests or woodlands with cool moist environments. It infects trees through the natural openings in the bark and forms cankers that clog the xylem and phloem tubes. There are two categories, trunk and foliar infections. The trunk infections are commonly fatal and allow other organisms to attack the tree such as bark beetles. Foliar infections vary in symptoms from twig die back to leaf spots and a host rarely die. Control methods include removal of infected trees off-site and burnt and quarantine of infected areas.

Alphitonia excelsacan be infected by Phytophthora cinnamomicausing loss of foliage, death of the upper branches and cankers on the trunk.

Cactussuch as Cereusspecies may be infected with Slimy Collar Rot (Phytophthora cactorum)which forms a soft black area at the base of the plant that is water soaked.

Chamaecyparisspecies may be infected with the Root Rot(Phytophthora lateralis) that attacks roots, trunk, stems and leaves.

Cornusspecies are infected by Crown Canker(Phytophthora cactorum) and in this case the tree is partially infected initially with one side producing smaller leaves that turn reddish in late summer. The leaves may also shrivel and die prematurely, during dry periods and small and large branches die. After a couple of seasons the tree becomes completely infected with poor top growth and an inconspicuous canker develops at the base of the trunk. Eventually the tree dies.

Ericaspecies are infected by (Phytophthora cinnamomi). The symptoms include foliage turning greyish towards the top, and then the plant wilts then dies with evidence of infection at the base.


Phytophthora inEucalyptus species
Image by B. Sonsie                Image by Dr Brett Summerell

Eucalyptusspecies are infected by Phytophthora cinnamomi causing rapid die back of the tree with blackened trunk loss or upper growth and is a serious problem for certain species such as Eucalyptus diversicolor(Karri).

Euphorbia pulcherrimais susceptible to Root Rot(Phytophthora and Pythiumspecies). Roots become dark and the rot can extend up the stem. The plant is also susceptible to leaf blight resulting from the same fungal disease.

Hederaspecies are infected by Phytophthora Blight(Phytophthora palmivora). This fungus causes leaf spots, foliage blight and stem rot.

Palmssuch as Archontophoenix, Caryota, Chamaedorea, Cocos, Dypsis, Howea, Liculia, Linospadix,Livistona, Phoenix, Ptychosperma, Rhapis, Roystonea, Syagrus,Washingtoniaand Wodyetiaspecies are also susceptible to Phytophthora Blightforming large irregular areas on the fronds that become dark and rotten and limited by the veins

Liliumspecies are infected by Foot Rot(Phytophthora cactorum) whichattacking the stems just below the soil level causing the plant to topple and if infection occurs as the leaves are emerging the base of the infected leaves which collapse remain attached to the bulb.

Morinda citrifoliais infected by Phytophthora Blight, black flag disease (Phytophthoraspecies) which causes the foliage to turn black and limp. The fruit and stems are also infected causing them to turn brown-black and whither.

      Morinda citrifolia

Orchidssuch as Cattleya, Cymbidium, Cypripedium, Dendrobium, Epidendrum, Paphiopedilum, Phalaenopsis and Zygopetalumspecies are infected by Black Rot(Phytophthoraor Pythiumspecies). This causes the leaves, pseudobulbs, rhizomes and roots to form a dark soft rot, normally occurring towards the base of the plant.

Saintpaulia, Dianthus, Gypsophila, Limonium and Anemonespecies are infected by Rootand Crown Rot(Phytophthora nicotianae). This is a fast moving fungus that turns the roots blackish then extends to the crown and petioles causing wet rot of the crown then wilting, eventually killing the plant. When found as Phytophthora Leaf Spotor Blight, angular spots appear with water-soaked margins as in Cordylineand Philodendronspecies.

Sedumspecies can be infected by up to three Stem Rotfungi including (Colletotrichumspecies), (Phytophthora species) and (Pellicularia filamentosa). Commonly occurring in wet soils.

Ulmusand Acerspecies are also infected by (Phytophthora cactorum) and this is known as Bleeding Canker. This casual disease initially causes cankers in the bark that ooze sap and the sapwood forms reddish lesions with greenish margins. The leaves on affected branches turn yellow, wilt then die and mildly affected trees may survive.

Many species are infected by theBleeding Canker including Acer platanoides, Acer rubrum, Acer pseudoplatanus, Acer saccharinum,Betulaspecies, Liquidambar styraciflua, Aesculus x carnea, Tilia, Salixand Quercusspecies.

Non-chemical Control
The most effective control for all Phytophthoradiseases is prevention primarily because it is extremely difficult to control Phytophthoradiseases after they are established in the plant. As Phytophthoraspecies are most easily transported in infested soil quarantine is an essential component of control of the disease and it is for this reason many areas have hygiene protocols to stop the pathogen being introduced into an area. It is recommended that bush walkers take care not to introduce the pathogen on their boots into un-infested areas and for this reason it is suggested that walking boots be cleaned and preferably sterilised (with 70% methylated spirits) prior to starting a walk.

Cultural techniquessuch as cultivating the soil regularly with added animal manure and other organic substrates to ensure there is good drainage will also help to minimise the impact of the disease. The plants can be mulched with straw or other organic material taking care that the base of the trunk is left clear. Avoid over watering the soil and observe hygiene in regards to tools, containers or shoes to reduce spreading the infection.

Potting mixes should be pasteurised for 30min at 60şC to ensure that they are free of the pathogen. It is also very important to grow pots off the ground to prevent the splash of infested mix or water from an infected plant to clean plants.
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Avoid damaging the bark particularly at ground level and seal any wounds that occur. If cactus or tree species are infected it is possible to cut out the infected area when first seenin order to contain it. Correct tree surgery techniques are required for large trees.

Chemical Control
There are a number of fungicides that are registered for use in the control of these diseases. The most effective chemical control are based on the use of chemicals containing potassium phosphonate. This chemical effectively enhances the defence systems of the plant and has been shown to be most effective in controlling Phytophthora diseases. It is essential that the chemical be applied when the plants is exporting nutrients to the roots, so this is best in the warmer months. The chemical can be applied as a stem injection or a foliar spray, and in some situations as a soil drench. There are specialised stem injecting equipment available for stem injections on larger trees.


Image by Dr Brett Summerell

Note
Always read the label for registration details and direction of use prior to application of any chemicals.
Amendments by
Dr Brett Summerell
Director Science and Public Programs
Royal Botanic Gardens Trust, Sydney