The woody monarch of Hawaiian forests, Acacia koa Gray (koa), hosts at least five distinct rust diseases in Hawaii. The name rust derives from the uniquely powdery texture and brownish to orange coloration of fungal spore masses that appear on plants, and in the case of koa, mainly brownish spore masses on diseased phyllodes, flowers and fruits. 

 

A general diagnosis of "koa rust" is readily determined by the presence of these distinctive and typical spore masses. To narrow the diagnosis to one of the specific rust diseases of A. koa requires an understanding of symptom expression and the fungal morphology associated with a disease and its pathogen.

 

One of the most widespread and important rust diseases of A. koa in Hawaii and one of the most important of all diseases of Acacia mangium in Java, Sumatra and Indonesia is caused by the rust fungus, Atelocauda digitata. This is one of the rare macrocyclic rust fungi in Hawaii, meaning that it produces all five spore stages that are typical of macrocyclic rust fungi and diseases in general. 

Atelocauda digitata occurs on all major islands, except it has yet to be reported from Lanai.

 

The disease caused by A. digitata is known as phyllode rust or gall rust, and it can cause witches' brooms on some Acacia species. The brooms are the result of the rust pathogen's systemic infection of A. koa. This endophytic relationship causes hormonal changes in A. koa organs (phyllodes, flowers, fruits) which result in unusually swollen or galled tissues and abnormal branching and floral reversion. Many rust diseases of plants do not infect their hosts in this systemic fashion, so this is an unusual disease.

 

 

Much of what we know about the koa rusts in Hawaii stems from the careful research of Don Gardner and his colleagues. In the 1970s, 80s and 90s Gardner working in Hawaii and his colleagues studied and classified the rusts.  They found the "koa rust" in Hawaii was not caused by just one or two species, but by several species, each unique and causing unique disease symptoms and producing unique fungal life cycles. If you want more information than that which appears in this brief bulletin, there is a list of references below that is worthwhile reading. 

 

Here we discuss one of the several important koa rusts in Hawaii, a gall rust caused by Atelocauda digitata. We also describe how rusted koa organs attract fungal-feeding rust parasites, generating extraordinarily interesting micro-ecosystems on plant surfaces.

 

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Disease name(s): Atelocauda rust, phyllode rust, gall rust, Acacia rust, koa rust

 

Hosts: a wide range of Acacia species, including:

Host common name: koa (the focus of this bulletin)

 

Pathogen name: Atelocauda digitata (G. Winter) Cummins & Hiratsuka. However, based on a revision of the genus by Walker (2001), the pathogen has been renamed Racospermyces digitatus (G. Wint.) J. Walker comb. nov. We chose not to use that new name in this bulletin.

 

Pathogen synonyms: Racospermyces digitatus (G. Wint.), J. Walker comb. nov.Uromyces digitatus Winter; Uromyces phyllidiorum (B. and Br.) McAlp. 

 

Phyllode rust of Acacia mangium (caused by Atelocauda digitata): this is a disease with potentially devastating effects in Acacia plantations in Java, Sumatra and Indonesia. The pathogen can cause severe damage to foliage and young stems in nurseries and young plantations. Seeds pods may be heavily damaged and deformed. Severe infections result in foliar deformation, defoliation, reduced growth and stunting. On Acacia mangium, infected phyllodes, shoot tips, petioles and even fruits may suffer gross malformation or have galls or blister-like swellings. Acacia mangium trees in young stands cam become severely infected. Young shoots may be malformed and branches may develop multiple stems. Severely infected plants are prone to premature defoliation. Where the disease occurs in A. mangium nurseries, the entire stock of plants may be infected and subject to being destroyed by the grower. The surface of older lesions may be colonized by fungal parasites (e.g., fungi, insects).

 

Historical remarks: Atelocauda digitata was first recorded from Australia (Berkeley and Broom, 1881 - 1887).

 

Species variability: Reports indicate that Atelocauda digitata is a genetically variable fungal species. 

 

Pathogen dispersal: wind; although the pathogen infects seed pods and seeds, A. digitata is not regarded as seed borne, however the large number of spores produced on lesions and the capacity of the pathogen to infect flowers and fruits makes contamination of seed lots a possibility.

 

Sites of infection: shoots, phyllodes, flowers, seed pods, juvenile leaves.

 

Type of rust disease: systemic infection creating deformity and witches' brooms at branch terminals. The pathogen forms an endophytic system within the host and affects the host hormonal balance.

 

Host phenology and disease seasonality: Spermagonia and aecia of the koa rust pathogen A. digitata coincide with the seasonal initiation of flowering and shoot growth of Acacia koa in Hawaii. This depends upon elevation and may vary among years. The aeciospores appear on the witches brooms in the several months after flowering and shoot growth begin.  

 

Pathogen life cycle: Atelocauda digitata in Hawaii is a macrocyclic rust, with five distinct spores stages, all occurring on A. koa.

 

Table 1. Spores types and generally associated symptoms for koa rust caused by Atelocauda digitata in Hawaii.

 

Disease cycle: New cycles of disease probably occur every year on a given koa tree, and for the life of that tree. Yearly disease cycles always occur on fresh host growth. Infection and symptoms occur on the growth at the branch terminals (growth tips, phyllodes, flowers, seed pods). New cycles of disease derive from the systemic infection of branches; when new growth blooms the systemic infections become active and move into the young tissues, distorting them and sporulating upon them. Initial infections of new trees and/or branch terminals or phyllodes or juvenile leaves is most likely by windborne aeciospores or urediniospores. 

 

Effects of this disease on A. koa plants: This rust disease is not fatal to koa trees. Severe symptoms (witches' brooms) are mainly confined to several or more branch terminals. Seed pods can be severely affected. The rust can destroy all seed pods on a given tree, and do so on a yearly basis. The disease  symptoms can vary significantly among koa genotypes and locations.

 

Disease management:

 

Note - Trimming off witches' brooms does not cure the plant of infections; the disease is systemic in branch terminals. However, trimming of witches' brooms can help to reduce the number of rust spores in the environment.

 

(5) Gardner, D. E. 1991. Atelocauda angustiphylloda n. sp., a microcyclic rust on Acacia koa in Hawaii. Mycologia 83:650-653.

 

 

(7) Gardner, D. E. Rusts on Acacia koa: Atelocauda digitata (University of Hawaii at Manoa website, Accessed April 15, 2008).

(10) Hodges, C. E., and Gardner, D. E. 1984. Hawaiian forest fungi. IV. Rusts on endemic Acacia species. Mycologia 76:332-349.

 

(11) Lee, S. S. 2004. Diseases and potential threats to Acacia mangium in plantations in Malaysia. Unasylva 217:31-35.

 

(12) Old, K. M., Vercoe, T. K., Floyd, R. B., Wingfield, M. J., Roux, J., and Neser, S. 2002. Acacia spp. - FAO/IPGRI Technical Guidelines for the Safe Movement of Germplams No. 20.

 

(13) Ramsden, M., McDonald, J., and Wylie, F. R. 2002. Forest pests in the South Pacific region: a review of major causal agents of tree disorders. Department of Primary Industries, Agency for Food and Fibre Sciences, Forestry Research, Queensland, Australia.

 

(14) Walker, J. 2001. A revision of the genus Atelocauda (Uredinales) and description of Racospermyces gen. nov. for some rusts of Acacia. Australian Mycologist 20:3-28.

 

Acknowledgements: Don Gardner, for correspondence regarding the rust disease and Tuberculina maxima; Pat Conant (HDOA) for identifying an insect that feeds on the aeciospores of A. digitata on the spermagonial-aecial witches' brooms; Brian Bushe (UH-CTAHR) for assistance with microphotography; J. B. Friday (UH-CTAHR) for photography.