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Natural Epiphytotic of the Rust Puccinia psidii on Melaleuca quin-quenervia in Florida

Melaleuca quinquenervia (Cav.) S. T. Blake (melaleuca), a tree of Australian origin, is an invasive weed of natural areas in southern Florida and has been listed as a federal noxious weed. During January 1997, severe incidence of a rust disease was detected on new growth of about 70% of the melaleuca trees over a 2-km strip in Broward and Dade counties. These top-pruned trees were 3 to 5 m tall with bushy appearance and had many new shoots. The rust was observed on melaleuca saplings and trees in a 20-km radius in January through March 1997. Leaf lesions began as chlorotic flecks that expanded, produced spores, and developed into necrotic spots. Infected leaves were severely distorted. Branches were severely defoliated and succulent twigs were often girdled by lesions, causing dieback of the new growth. Yellow uredinia were observed on all young leaves and some petioles and twigs. Urediniospore morphology and dimensions (17 to 27 × 15 to 24 µm) are consistent with the description of Puccinia psidii G. Wint. (1) and the University of Florida's herbarium material of P. psidii on Pimenta dioica (L.) Merr. (allspice) (2). An inoculation test was conducted with 40-cm-tall melaleuca seedlings. Fully expanded leaves and terminals of these seedlings were brushed or sprayed with freshly collected urediniospores, covered with plastic bags, and placed in a growth chamber maintained at 16°C (night) and 26°C (day) with a corresponding 12-h light cycle for 72 h. The plastic bags were then removed and the seedlings maintained in high humidity and ambient temperatures in a shadehouse. Typical symptoms and sporulation occurred after 10 and 12 days, respectively, following inoculation. Although P. psidii has been recorded on 11 genera in Myrtaceae in the Americas (1,2), including melaleuca, an epiphytotic of this magnitude on melaleuca has not been reported. A different race of P. psidii has been suspected to cause sudden epiphytotics on Pimenta officinalis Lindl. in Jamaica (1). Further research related to host range is warranted to determine the specificity of P. psidii, as this rust may have potential as a microbial biological control agent of melaleuca.

References: (1) G. F. Laundon and J. M. Waterson. C.M.I. Descriptions of Pathogenic Fungi and Bacteria No. 56, 1965. (2) R. B. Marlatt and J. W. Kimbrough. Plant Dis. Rep. 63:510, 1979.