DISEASE

NAME
Fusarium Wilt (General)
Various Wilt Species

Description
Fusarium wilt is caused by specialised strains of the common soil fungus, Fusarium oxysporum.This fungus is microscopic, and that can live in the soil for many years and primarily attacks plants by entering through the roots. This affects corms, stems and leaves by moving through the water conductive tissue in the plant.
The specialised strains are known as formae speciales (f. sp. or special forms) and each f. sp. has a relatively narrow host range. For example Fusarium oxysporumf. sp. cubensecauses Panama wilt of banana, F. oxysporumf.sp. vasinfectuminfects cotton and F. oxysporum f. sp. lycopersiciattacks tomatoes. Each strain is specific to that narrow host range and will not affect other hosts; this is important as it makes control options involving the planting of other plant species possible.

Symptoms
Generally the plant wilts quickly during hot weather or lacks growth and vigour. Typically the symptoms are often indistinguishable from drought stress. Leaves yellow and wither or stems split open near the base. Individual branches may collapse and die or entire plants perish. One of the distinctive symptoms is a one-sided yellowing and death of a leaf or branch. Corms become rotted with open wounds and affected stems, when split open are yellowish. There are variations in the symptoms depending on the type of plant but generally plants wilt, turn brown and die.



Basal Rot(Fusarium oxysporum) infects Tulipaand Crocusspecies by turning leaves reddish, which wilt and normally die. It also affects the bulbs by forming few roots and rotting the base, turning them dull white.
This disease also infects cactussuch as Cereusspecies and Zygocactus truncatuscausing soft black rot that appears at the tips of the plant then spread rapidly downwards. It is particularly virulent in hot beds, under glass. The infected plants cannot be cured and should be disposed off site.

Fusarium Patch, Bent Fusarium Patch, Blue Grass

Other Fusarium(or related species) diseases
Fusarium Patch(Microdochium nivaleformerly known as Fusarium nivale) is also known as Crown or Root Rot and infects warm or cool season Turf Grasses. The fungus infects the roots or rhizome turning them black, sometimes with a pink ting. During hot periods light green to brown patches appear in the turf as the damaged roots are unable to supply water to the leaves. This diseases is not a significant disease in Australia
Minimise irrigation of infected areas to reduce spread of infection as the spores are distributed in water.

Root Rot(Fusarium verticillioides) is a seed born fungus that infects Strelitzia reginae. It also forms a Leaf Spot in Dracaena species where rounded or irregular yellowish to reddish spots with a pail green border form. These are found on immature leaves and heavy infections may rot the crown.

Source and Dispersal
The fungal spores are dispersed by wind from infected plants or parts of plants. It is also dispersed by splashing water or contaminated stock or the movement of soil. With Fusarium wilt of canary island date palms the transfer of infected plant material on chain saws has been shown to be an important way in which the fungus can be transmitted from one tree to another. It is essential in these cases to sterilise the chain and chain bar between trimming the fronds on each tree.

Fusarium oxysporumproduces three types of species, macroconidia, microconidia and chlamydospores. The latter two spore types play very important roles in the disease. The microconidia move in the vascular system of the plant and collect at the sieve plates in the xylem of plant. Here they germinate and grow and eventually block the sieve plate thus stopping the flow of water in the plant. The chlamydospores are specialised survival spores that persist in roots and in the soil for very long periods of time, thus allowing the fungus to survive periods when there are no susceptible plants present.

Favoured Conditions
The fungus prefers warm humid climates and is commonly seen in coastal regions and may continue living in soil for many years.

Affected Plants
Many species are infected including tomatoes, carnation, cucurbits and freesia. Die Back(Fusarium solani var. martii) causes damage to the branches of Ilexspecies. Fusarium also attacks palm species such as Phoenixcausing wilt.

Aglaonemaspecies are infected by the Fusarium Stem Rot (Fusarium spp.) Symptoms include the base of the plant or of cuttings becoming soft and mushy with purplish to reddish margins around the infected area. Bright red globular fruiting bodies may also appear on the stem of severely affected plants. Control methods include removing and destroying infected plants or applying the appropriate pesticide.

Albizia and Alternantera species are infected by the wilt (Fusarium oxysporum var. periciosum ) that causes the leaves to wilt, shrivel and die. This infection also extends to the branches and eventually may kill the tree.

Callistephusspecies are attacked by the wilt (Fusarium oxysporum f. sp.callistephi) which infects the vascular bundles turning brown, particularly on one side of the stem causing the plant to become stunted, eventually dieing.

Dendranthemaand Thymusspecies are susceptible to the Stem Rot(Fusariumspecies) and (Pellicularia filamentosa)which rots the base of the stem.

Dianthusspecies are infected by Bud Drop(Fusarium poae). This fungus rots flower buds prior to opening and infected flowers open with distorted, unattractive petals. The control of thrips is essential.Dianthusspecies are also infected by Wilt (Fusarium oxysporum) that causes yellowing and wilting of the lower leaves, normally on one side. The infection causes stems to rot with vascular discolouration.

      Fusarium Wilt Image by B. Sonsie     
      Image by Dr Brett Summerell

The only palms susceptible to Fusarium wilt are Phoenixspecies, especially Phoenix canariensisand juvenile Washingtonia filifera.Initially the leaflets turn yellowish then brown, on mature fronds causing them to die. Eventually only tufts of new growth remain the plant. As the infection continues the plant collapses and dies.

Fusarium Patch

Turf grasses are infected by Fusarium Patch(Microdochium nivale). Plants include; Digitaria didactyla(Blue Couch), Cynodonspecies (Couch), Eremochioa ophiuroides(Centipede Grass), Festucaspecies, Paspalum vaginatum(Salt Water Couch), Pennisetum clandestinum(Kikuyu),Poaspecies (Bluegrass), Stenotaphrum secundatum(Buffalo) and Zoysiaspecies. Generally the fungus produces small dead circular patches in the lawn.

Non-chemical Control
Fusarium diseases are extremely difficult diseases to control primarily because of a lack of a chemical control option and because the fungus is able to persist in soil for long periods of time. Crop rotation to non susceptible plants is advised and consequently it will be important to have the disease accurately diagnosed to ensure the identity of the pathogen and the possible rotation options.
Remove and destroy any infected plants. In the case of large trees or palms, it will be necessary to remove the infected plant, this not only means cutting down the above ground parts, but also removing the roots of the plant. These will then need to be removed and destroyed.
Fusarium Patch can be minimised by reducing thatch and aerating the soil regularly or improving the drainage.

Chemical Control
There are no effective chemical control options currently available for plant diseases caused by Fusarium species.
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