Neofusicoccum parvum causes a lethal dieback of Syzygium paniculatum in Florida

Syzygium paniculatum (Myrtaceae), a native of Australia, is an important plant in the South Florida ornamental trade where it is used in hedges and trimmed as topiaries. It was relatively free of diseases before Hurricane Wilma (2005), but has since been affected by a serious dieback disease, primarily in production nurseries. Symptoms include wilting and death of terminal and lateral branches (Fig. 1), and extensive vascular discolouration (Fig. 2).

Since 2006, the Florida Extension Plant Diagnostic Clinic in Homestead, FL has received a total of 29 samples of S. paniculatum with the above symptoms from 20 different nurseries in Miami-Dade County. Isolations were made from discoloured and asymptomatic vascular tissues (5 mm²) plated on half-strength potato dextrose agar and incubated in the dark at 25^°C for about one week. To date, a sterile, steel grey fungus with conspicuous aerial mycelium has been isolated from every sample; other fungi (Alternaria and Pestalotiopsis spp.) have only been recovered from asymptomatic tissues.

Ten strains of the grey fungus were examined further. None produced a teleomorph under different light regimes, either on artificial media or on sterile pine needles, toothpicks or twigs of S. paniculatum; however, two produced unicellular conidia in pycnidia on pine needles. DNA was isolated from the strains, amplified with the ITS1/ITS4 primer pair (White et al., 1990), sequenced, edited and compared via BLAST searches to sequences deposited in GenBank; all strains were 99% (nine strains) or 98% (one strain) similar to several accessions of Neofusicoccum parvum. Sequences for two sterile strains (276 and 280) and one that produced conidia (1-4) were deposited in GenBank as EU872493, EU882161 and EU882162, respectively.

In a glasshouse, eight strains of N. parvum (six sterile and two that produced conidia) caused the above-described symptoms when colonized toothpicks or mycelium from PDA cultures were used to inoculate stems of healthy, two-to-six month-old S. paniculatum ‘Monterrey Bay’ (four replications/treatment in each of three experiments). No disease developed on plants that were wounded, mock-inoculated (sterile toothpicks or PDA), or inoculated with Alternaria and Pestalotiopsis spp. recovered as above. Neofusicoccum parvum was recovered from N. parvum-inoculated, but not mock-inoculated, plants after four weeks.

Neofusicoccum parvum has been reported as a pathogen on several species in the Myrtaceae, but S. paniculatum is a new host record (Alfieri et al. 1994; Pavlic et al. 2007; Slippers & Wingfield, 2007).