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.
Control of Snails and Slugs is achieved by:
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.
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.
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).
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.
|Left: Ink disease. Right: Rust. Photos: Brian Walters|
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.
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.
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.
|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
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.
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:
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.