UF/IFAS Pest Alert

Contact:
Hank Dankers, senior Biological scientist (850) 875-7140
wadan@mail.ifas.ufl.edu

Tim Momol, Extension Plant Pathologist (850) 875-7154
UF/IFAS, NFREC, Quincy, FL
tmomol@ufl.edu

Myrcianthes fragrans (Simpson's stopper) and Callistemon citrinus (bottlebrush) samples were received by the Plant Diagnostic Clinic in Quincy from a commercial nursery in southwest Florida in mid-December of 2003 with rust symptoms. Microscopic examination showed typical rust pustule eruptions (Fig. 1 and 3) and urediniospores (Fig.2 and 4). Urediniospores found on the leaves were identified as those of Puccinia psidii. This pathogen has an extensive host range in the Myrtaceae family but until recently was only found on Pimenta dioica (allspice) in Florida (Leahy 2004). As it has increased its host range it has become an increasing problem in Florida. A Salix babylonica (weeping willow) (Salicaceae family) sample was also received from the same area at the same time with rust symptoms (Fig. 5). However, these urediniospores (Fig. 6) have been tentatively identified as Melampsora epitea. This fungus is a heteroecious rust whose alternate host are usually among the Pinaceae.

Fig. 1. Rust symptoms on leaves of Myrcianthes fragrans
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Fig. 2. Urediniospores of Puccinia psidii from Myrcianthes fragrans leaf
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Fig. 3. Rust symptoms on leaves of Callistemon citrinus
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Fig. 4. Urediniospores of Puccinia psidii from Callistemon citrinus leaf
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Fig. 5. Rust symptoms on leaves of Salix babylonica
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Fig. 6. Urediniospores of from Salix babylonica leaf tentatively identified as Melampsora epitea
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Puccinia psidii is a tropical rust fungus which infects leaves and young shoots of members of the Myrtaceae family. P. psidii is the causal agent of what is commonly referred to as guava rust. It produces urediniospores and teliospores, the former of which represents the infective stage of the rust. Teliospores have not been observed in Florida. The urediniospores perennate on the infected host plant. No alternate host plant for this rust fungus has been identified.

Historically, this rust was first found in Florida in 1977 on allspice (Pimenta dioica). Although P. psidii has an extensive host range in the Myrtaceae family, allspice, represented, almost exclusively, the host range in Florida until the 1997 - 1998, when a natural epiphytotic on Melaleuca quinquenervia (punk tree) occurred in Broward and Dade counties. The punk tree is an introduced species which has become one of the worst noxious weeds threatening south Florida acreage and the Everglades.

The natural infection of melaleuca by P. psidii spurred a lot of interest in this rust as a potential biological control agent. USDA and University of Florida IFAS personnel began researching P. psidii and implemented an extensive host range study for this rust. Research has shown that some host specialization occurs within P. psidii - Isolates from one host genus may or may not infect other genera in the Myrtaceae.

After the initial interest in P. psidii as a mycoherbicide, and beginning in 2001, the FDACS - DPI detected this rust on Eugenia sp. - a new host record for Florida. In 2002 detections of P. psidii on other Myrtaceous hosts began to escalate, including Myrcianthes fragrans, Myrtus communis, and Syzygium jambos. Pathology reports for 2003 indicated widespread rust infection of these hosts as well as Melaleuca spp., including the ornamentally popular bottlebrush tree (Callistemon viminalis and C. citrinus, now transferred to the genus Melaleuca) . Two other species of Syzygium, S. paniculata and S. camini, also developed rust disease during the year.

In summary, a rust disease which has for a few decades , been found almost exclusively on one particular host in Florida, has increased it's host range dramatically in the state and has become an increasing problem for commercial nurseries, homeowners and native stands of certain Myrtaceous host plants.