Kangaroo Paws: Pests and Diseases

Keith Oliver

The following article is reproduced from the December 1992 issue of the Society's journal Australian Plants.



I write on this subject not because I am an expert on pests and diseases, but to summarise what is known to me and what I have observed in my own gardens over a period of 28 years and to indicate areas that need further investigation. In particular I appeal for more information relating to the new phenomenon called "Flower Bleach Deformity" or "Flower Abortion" that has appeared in kangaroo paws in recent years.

The kangaroo paws consist of 12 species of perennial herbs native to southwestern Australia. In their natural habitat most of the species are fire opportunists and colonisers of areas of secondary regrowth. They are not long-lived but produce large quantities of seed which is shed each summer and tends to remain dormant in the soil until the area is burned. The autumn after a fire the seed germinates to produce large numbers of seedlings which proliferate for several years before being overgrown by larger and longer lived species. Under these conditions pests and diseases of kangaroo paws tend to be rare. However, in cultivation, in row cropped monocultures or in gardens, where summer watering and the use of fertilisers is normal, and where the plants are exposed to introduced species of snails, slugs and other pests, many problems are encountered.

The species Anigozanthos flavidus is an exception in that it is not short lived. It can live for at least 30 years in cultivation, possibly for much longer and it is an exceptionally vigorous, hardy and disease free species that has been widely used in breeding the hybrid kangaroo paws that are now widely grown, both within Australia and in other countries1. Careful selection from the hybrid progeny of A.flavidus yields plants that have good disease resistance and which are little eaten by snails. They are also hardy and vigorous and live much longer than most species of kangaroo paws.

Slugs and Snails

Control of Snails and Slugs is achieved by:

  • The slash and burn method of growing (see below) may give at least partial control of snails and many other pests and diseases of kangaroo paws.
  • Elimination of potential breeding sites, rockpiles, hedges, ground hugging creepers, rubbish and rough weedy areas, etc.
  • Khaki Campbell or Indian Runner ducks give good control.
  • Beer traps and sawdust barriers etc., are useful in small areas.
  • Baits of Methiocarb (pellets) and metaldehyde (pellets, powder or paste).
  • Sprays such as Bordeaux mixture, methiocarb or metaldehyde.

Caterpillars

The damage caused by caterpillars can usually be distinguished from snail damage in that the caterpillars do not eat into the leaf deeply enough to expose the vascular bundles, as snails do, and that snails tend to eat away long strips of tissue, not irregular circular or oval areas. Hand plucking may suffice for small areas. A selective insecticide for leaf eating caterpillars is Dipel® (Bacillus thuringiensis) which is a highly effective stomach poison for caterpillars but is non-toxic to other insects, fish, birds and mammals.

Other stomach action insecticides include Endosulfan, Fenthion (Lebaycid®) and Rotenone (Derris®). These must be eaten by the pest before they are active, so are useful against foliage chewing insects.

Aphids

It has been reported that kangaroo paws can become infested with aphids during the spring or autumn2. Control by spraying with Metasystox® or Rogor® is recommended. I personally have rarely seen aphids on kangaroo paws and tend to think that they may only occur on excessively soft growth.

Leaf Miners

These are also outside of my experience but have been reported on kangaroo paws2. Control is with Metasystox® or Rogor® when the larvae are active (winter).

Ink Disease or lnkspot Disease

  
Further Notes on Ink Disease

Work done by Lisa Gillespie-Sasse (pers. Comm.) at the University of Western Australia in 1989 indicates that ink-spot disease is a result of a plant-toxin interaction and that different isolates of Alternaria alternata taken from kangaroo paws can be divided into different races according to the types and levels of toxins that they produce and further that the symptoms associated with ink-spot disease may be associated with more than one fungus.

In view of the number of races of Alternaria afternata, the possible involvement of other fungi, and the variability in disease expression, great care must be taken before claiming resistance or tolerance to ink-spot disease for any particular clone of plants. It has come to my notice that in one case disease resistance was claimed for plants that failed to develop symptoms within six months when not protected by fungicides. Such claims should be treated with great caution.

Cases are known where clones of plants have been unaffected for many years only to quickly succumb and die when infected with other races of pathogens. Nevertheless clones of plants are known which have survived for many many years without any protection from fungicides and which have undoubtedly been exposed to many races of ink-spot and rust pat hogens and which have never become infected with either disease. These are clearly valuable plants to breed from as resistance is known to be inherited. Nevertheless, no absolute guarantee can be given that these plants or their progeny will not succumb to an unknown or new race of pathogen some time in the future, but for the present they may be said to be disease resistant.

     
  
Rust Immunity?

Rust disease of Kangaroo Paws is evident as small (approx. 2 mm diameter) reddish brown pustules on the leaves, which usually become more and more dense until the plant is killed. Rust is rare on, if not absent from, wild populations of kangaroo paws, but is a serious disease of cultivated kangaroo paws.

The story is told of some researchers in an eastern Australian state who innoculated plants of Macropidia fuliginosa with rust taken from Anigozanthos manglesii, and finding no infection then concluded that the former species is immune to rust disease. This is clearly untrue, because at this same time rowcropped M.fuliginosa in Western Australia was being devastated by rust. Also from my own experience I had hybrid plants that had survived trouble free for about 12 years, but which were quickly killed when a suitable race of rust attacked them.

This has been described as a hypersensitive host reaction to several species of fungi, early workers implicating Mystrosporium adustum3 or Drechslera iridis4 but later work has attributed the disease primarily to Alternaria alternata5,6,7.

Species of kangaroo paws differ in their susceptibility to this disease. Anigozanthos manglesii and A.gabrielae are usually killed by the disease while A.viridis, A. pulcherrimus and A.rufus are less affected and A.flavidus is highly resistant. Other species are susceptible in varying degrees.

It is important to note that different isolates (or races) of Alternaria alternata from kangaroo paws differ in their virulence and that each species of kangaroo paw varies in its susceptibility to many of the different isolates. This complicates the breeding of resistant hybrids or varieties as although the fungus is ubiquitous, apparent resistance in particular plants may not persist against a wide range of fungal races.

The symptoms of ink-spot disease vary from brown sunken lesions with some chlorosis and a black rim, to black lesions, or to large black lesions (the rare "big blotch" form) that is seen even on the stems of inflorescences, causing withering of the stem and destruction of the flowers. These symptoms are easily confused with other blackening or the leaf tips or spots on the leaves due to such things as water stress, damage by insects and by other physiological and environmental stresses, so that great care is needed in diagnosing ink-spot disease.

Ink disease   Kangaroo Paw rust
Left: Ink disease. Right: Rust. Photos: Brian Walters

Rust

Unlike the ink-spot fungus, the rust fungus Puccinia haemodora cannot live as a saprophyte (i.e. it cannot live on dead plant tissue) for a part of its life cycle but must always be a parasite (i.e., live on living plants). It has different races and these are often specific to a particular species.

Anigozanthos flavidus seems to be immune to rust attack. A.pulcherrimus and A.rufus may also be resistant8, but most of the other species are susceptible. In my experience A.humilis seems to be particularly susceptible. When A.flavidus is crossed with other species the resultant hybrids are variable but many seem to be highly resistant to rust, but A.humilis x A.flavidus hybrids are often susceptible to rust.

Control of Ink-spot and Rust Diseases of Kangaroo Paws

Spraying with Mancozeb® is recommended for ink-spot disease and spraying with Baycor® is recommended for rust. These may not be fully effective in preventing the diseases. Growing disease resistant hybrid plants is a good solution but there may be a disadvantage from irritant hairs on the stems of A.flavidus hybrids if large areas of flowers are to be picked.

Control of Pests and Diseases by the Slash and Burn Method

Slashing and burning of plants after flowering in late summer to autumn will reduce the innoculum load and give the plants a "clean start" for the following year.

Burning Anigozanthos    Burning Anigozanthos    Burning Anigozanthos
Rejuvenation of a hybrid kangaroo paw which had been severely attacked by ink disease (note the barriers erected to protect nearby plants). Fresh foliage was produced by the plant
over the following weeks.
Photos: Brian Walters

  
Slash and Burn!

In the cemetery at Gingin there are populations of A.manglesii and A.humilis which have been slashed and burned in late spring or autumn every year but one since 1973 and probably since the early 60's (Stephen Hopper, pers. comm.). Under these conditions these crowded populations of kangaroo paws have kept remarkably free of pests and diseases. When I thoroughly checked the area in September 1990, I could find no trace of disease and almost no pests, only a very few flowers showing some insect damage.

Dixon8 recommends similar treatment for many species but notes that a slow hot burn can kill the plants and also that in the case of plantation grown M.fuliginosa slashing and burning can cause heavy losses. A.flavidus hybrids respond well to slashing but there is no data concerning their response to burning.

Flower Bleach Deformity of Kangaroo Paws

The first symptom is some loss of pigmentation in the developing flower buds, followed by total loss of pigmentation, over all of, or over large areas of the buds, giving them a white colour. This is accompanied by abortion of the entire inflorescence. Usually only some of the stems on any one plant are affected and only more or less randomly scattered plants are affected. Following the abortion of the primary inflorescence a secondary inflorescence will often develop from a lower node of the primary inflorescence stem and so far as has been observed these nearly always develop normally.

Many reasons for this strange disorder have been suggested or advocated, some of these are: adverse weather, too hot, too cold or too changeable; nutritional imbalance, manganese toxicity, copper deficiency; genetic faults in plants; weakness induced by tissue culture, etc. In my opinion, these are all untenable. There has always been adverse weather on occasions and plants have been grown on many soils for many years, but no flower bleach has occurred. Also flower bleach has occurred on seedling plants and on plants dug up in the wild as well as on tissue cultured plants.

Most biologists are fairly certain that it is not caused by a fungus or a bacteria and after discussion, I have formed this hypothesis:

  1. It is caused by a pathogen that is probably new to Western Australia, or even to Australia;
  2. The pathogen is probably a mycoplasma-like organism (MLO)***.
  3. The vector is possibly a green leaf hopper that is occasionally seen sucking sap from soft young flower stems.

Plants affected one year may be clean the next year, only some stems on a plant are affected and clean shoots grow from a lower node of affected stems. All of this indicates that the MLO is not systemic.

Spraying with copper oxychloride is reported to cure the problem although the work did not include unsprayed control plants. As there is circumstantial evidence against the disorder being caused by a copper deficiency and as it is reported that the copper must be sprayed onto the leaves and not added to the soil it is thought that the copper may make the stems either unpalatable or poisonous to the insect vectors, or that in some other way the copper prevents infection by the MLO.

References

  1. Oliver, K. R. (1991). Hybrid Kangaroo Paws. Australian Plants 16: 116, 53-60.
  2. Reid, A. (1988). Cultivation of Kangaroo Paws. Farmnote No. 54/88, Western Australian Department of Agriculture.
  3. Smith, W. T. (1963). The Ink Disease of the Paws. Australian Plants 2:16, 115.
  4. McMaugh, J. (1985). in What Garden Pest or Disease is That? Weldon Publishing, Willoughby, New South Wales.
  5. Sivasithamparam, K. and Watkins, P. A. (1982). Alternaria alternata as a causal organism of ink-spot disease of Anigozanthos spp. in Western Australia. Australian Plant Pathology 11:18.
  6. Sivasithamparam, K. (1985). Diseases. In Horticulture of Australian Plants. (eds B. Lamont and P.Watkins). Pp. 99-102. Western Australian Department of Agriculture, South Perth.
  7. Verhooght, M. M. and Sivasithamparam, K. (1986). Ink-spot disease of 'Kangaroo-paws' (Anigozanthos Labill. and Macropidia Drumm. ex Harv.) in Western Australia. Crop Research 26: 49-55.
  8. Dixon, I. B. (1991). Kangaroo Paw. Australian Plants 16: 116, 77-87.
  9. Ulrike Schaper and R. H. Corverse (1985). Detection of Mycoplasma-Iike Organisms in Infected Blueberry Cultivars by the DAPI Technique. Plant Disease 69: 193-196.

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