Disease of the month: Dieback of Eugenia

October's disease of the month is dieback of Eugenia, caused by the fungus Neofusicoccum parvum. Eugenia (scientific name=Syzigium paniculatum) is a popular ornamental that is often used in topiaries. It is a tree that is native to the rainforest, and is grown in the US in zones 10 and 11. Nurseries in Miami-Dade County produce an estimated 600,000 eugenia plants per year.

After Hurricane Wilma in November 2005, a serious dieback disease of eugenia first appeared in South Florida. Affected nursery plants had wilted, defoliated branches, with the dieback symptoms affecting anywhere from a few affected branches to the entire plant canopy. When diseased branches were cut longitudinally, the vascular tissue had a red discoloration (right).




Causal agent
Work by Aaron Palmateer and Randy Ploetz has determined that a fungus, Neofusicuccum parvum, causes the dieback. Neofusicoccum parvum (=Fusicoccum parvum, teleomorph Botryosphaeria parva) is actually a complex of closely related species that have a wide host range, with 43 different hosts reported in the USDA-ARS Systematic Mycology and Microbiology Fungal Database. It also causes a dieback on Syzigium cordatum in South Africa, a native ornamental tree in that country. Neofusicoccum parvum has been associated with dieback on many other tropical and subtropical hosts, including avocado, guava, citrus, eucalyptus, and mango. The fungus grows rapidly in culture, producing fluffy grey colonies that darken with age. It can produce unicellular conidia that form septa and become pigmented with age, but many isolates grow vegetatively without producing spores.

Disease Cycle and Epidemiology
Dieback occurs mainly in the late summer, with the onset of high temperatures. The severity of external and internal symptoms increase as temperature increases. Sunlight does not impact disease severity, so the production of eugenia in full sun is not a contributing factor to the disease.

Further research is needed to fully characterize the disease cycle and to determine the environmental factors that contribute to disease development. We have a greenhouse experiment underway (right) to evaluate the role of water stress and soil fertility in causing dieback symptoms. In the future we plan to investigate how the pathogen is spread from plant to plant.

Management
Currently it is recommended to use good sanitation measures to manage this disease. This includes pruning symptomatic branches and removing the diseased cuttings from the nursery. Sanitize all tools used to prune or work with plants before each use. It's best to sanitize pruning shears between each plant. Some growers handle this by dipping shears in bleach and then dunking in oil after drying (to minimize rust). Examples of disinfectants for tools include: 1) 25% chlorine bleach (3 parts water and 1 part bleach; 2) 25% pine oil cleaner (3 parts water and 1 part pine oil); 3) 50% rubbing alcohol (70% isopropyl; equal parts alcohol and water); 4) 50% denatured ethanol (95%; equal parts alcohol and water); 5) 5% quaternary ammonium salts. Soak tools for 10 minutes and rinse in clean water. Do not mix quaternary ammonia with bleach.

Chemical control options include applying a copper based fungicide according to the manufacturer's label. Recommendations include applying fungicides after pruning to minimize infection of the freshly cut tissue. Be certain to achieve good coverage especially on new wounds made during pruning. Because it is likely that plant stress contributes to disease outbreaks, the maintenance of plant health is likely important for dieback management. Although information on this new disease is relatively limited, an extension publication is in preparation, and the following publications give more information on the pathogen and disease:

• Ploetz RC, Pérez-Martíez JM, Palmateer AJ, Cating RA. 2008. Neofusicoccum parvum causes a lethal dieback of Syzygium paniculatum in Florida. BSPP New Disease Reports Vol 18, Aug 2008 to Jan 2009. (free full-text)
  
• Ploetz RC, Pérez-Martíez JM, Palmateer AJ, Tarnowski TL. 2009. Influence of temperature, light intensity, and isolate on the development of Neofusicoccum parvum-induced dieback of eugenia, Syzygium paniculatum. Plant Disease 93:804-808. (subscription or fee)
  
• Pavlic D, Slippers B, Coutinho TA, Wingfield MJ. 2007. Botryosphaeriaceae occurring on native Syzigium cordatum in South Africa and their potential threat to Eucalyptus. Plant Pathology 56:624-636. (free full-text)
  

VIDEO: Check out our video from a trip to a topiary nursery on the west coast of Florida.