Differential Response by Melaleuca quinquenervia Trees to Attack by the Rust Fungus Puccinia psidii in Florida.

Melaleuca quinquenervia (melaleuca) is an exotic invasive tree in Florida, Hawaii, and some Caribbean islands (1,2). Puccinia psidii (rust fungus) attacks melaleuca as well as other plants in a few genera of the Myrtaceae and Heteropyxidaceae, both members of the Myrtales (1,2). Disease occurs on succulent stems and foliage of melaleuca, causing twig dieback and defoliation (3). Melaleuca trees growing under similar field conditions exhibit susceptible or resistant reactions toward this fungus. To document this differential susceptibility of melaleuca to P. psidii, we visually evaluated 331 field-grown melaleuca trees from southeast Florida for occurrence of disease attributes: pustules (susceptible), nonpersistent halos (resistant), or asymptomatic (no macroscopic symptoms) conditions on leaves and succulent twigs during February and March when symptoms were at their peak. Percentages of trees manifesting susceptible, resistant, and asymptomatic responses to this fungus were 85.8, 13.0, and 1.2%, respectively. A screenhouse study was conducted to corroborate these observations by raising plants from composite seed sources and maintaining them in seven 3.8-liter plastic pots that were filled with commercial potting media. Nine to eleven plants per pot (with new foliage) were individually tagged, grown to 30 to 45 cm high, and spray inoculated (during February and March) with uredospores (~2 × 10/ml) obtained from melaleuca trees and suspended in water. Inoculated plants were placed on a screenhouse bench under infected trees and subjected to additional inoculum, thereby simulating field conditions. Evaluations made weekly during a 4-week period revealed that susceptible, resistant, and asymptomatic seedlings constituted 63.3, 33.6, and 3.2%, respectively, of the tagged plants. To assess the stability of these fungal and host attributes over time and space, we multiplied two P. psidii susceptible and two resistant plants from cuttings. We spray inoculated 6 to 13 rooted cuttings from each plant types with uredospores (0.8 to 2 × 10/ml) obtained from diseased melaleuca trees and suspended in water. These plants were incubated in a dew chamber for 72 to 96 h under 100% relative humidity at 19 to 23°C maintained with a 12-h fluorescent light cycle. After incubation, plants were placed randomly on a bench in a screenhouse (21 to 23°C) and evaluated weekly for symptom development during a 4-week experimental period. Noninoculated controls were maintained as well. The experiment was repeated twice. Foliage of the resistant plants developed a few incipient halos whereas 100% of the susceptible plants developed erupted uredinia and were defoliated in both replications. No detectable change in P. psidii virulence and melaleuca susceptibility patterns was observed. Despite wide host range within Myrtales, resistance to P. psidii exists within M. quinquenervia. Other P. psidii susceptible host systems of economic and environmental importance may have host/pathogen relationships similar to that of melaleuca and the selection of resistant individuals from their affected populations may be possible. Additional studies will be needed to ascertain the attributes of virulence or resistance in this rust fungus-melaleuca association. References: (1) M. Glen et al. Australas. Plant Pathol. 36:1, 2007. (2) P. D. Pratt et al. J. Aquat. Plant Manag. 45:8, 2007. (3) M. B. Rayachhetry et al. Biol. Control 22:38, 2001.