© 2006 Plant Management Network.
Host Range of Rust Isolates on Oregano and Mint in Florida
Carol M. Stiles and Patricia A. Rayside, University of Florida, IFAS, Plant Pathology Department, Gainesville 32611-0680
Stiles, C. M., and Rayside, P. A. 2006. Host range of rust isolates on oregano and mint in Florida. Online. Plant Health Progress doi:10.1094/PHP-2006-0417-01-RS.
Urediniospores were collected separately from rust pustules on oregano (Origanum vulgare) and spearmint (Mentha spicata) plants purchased from retail stores in Gainesville, Alachua Co., FL, in April 2004 and March 2005. Morphological characteristics of the urediniospores from both hosts were consistent with descriptions of Puccinia menthae. Spearmint, peppermint (M. x piperita), oregano (O. vulgare), sweet marjoram (O. majorana), and Greek oregano (Oregano heracleoticum) were inoculated with urediniospore suspensions of each rust isolate in a series of cross-inoculation experiments. Rust developed on oregano, sweet marjoram, and Greek oregano only when inoculated with urediniospores from the oregano plant, and this rust isolate did not infect any of the Mentha spp. tested. Rust developed on spearmint when inoculated with urediniospores from the mint plant, but this isolate did not infect peppermint, oregano, sweet marjoram, or Greek oregano. The two isolates of Puccinia menthae appear to represent different populations, “spearmint rust” and “oregano rust.” This is the first report of P. menthae on oregano in Florida.
This report documents for the first time Puccinia menthae Pers.:Pers. on oregano (Origanum vulgare L.) in Florida, although P. menthae has been previously listed on Mentha spicata L. (spearmint), M. aquatica (water mint), Mentha sp., and Monarda punctata (horsemint) in Florida (2). Koike et al. (10) previously reported P. menthae on oregano (O. vulgare) and sweet marjoram (O. majorana L.) in California. Puccinia menthae also has been reported on O. vulgare in other countries, including Bulgaria, Canary Islands, China, Denmark, Germany, Greece, Himalaya, India, Norway, Pakistan, Sweden, and United Kingdom (5). Although summer temperatures may limit rust development in Florida, mint and oregano plants are most commonly available for purchase in the cooler fall, winter, and spring seasons. Rust on oregano and mint plants in commercial garden centers has the potential to reduce product quality if conditions are favorable for disease development.
The rust fungus was first encountered on oregano plants in Florida in April 2004, when leaves with rust pustules were collected from potted plants at a residence in Gainesville, Alachua Co., FL. The sample was collected by a graduate student, Matthew J. Moyer (Environmental Horticulture Department, University of Florida IFAS) and submitted as a collection for the Fungal Plant Pathogens course taught in the Plant Pathology Department, University of Florida IFAS, by the first author (C. M. Stiles). Disease symptoms included small, circular to irregular, brown, necrotic spots, some of which included uredinia (Fig. 1). Uredinia in the symptomatic tissue on oregano were few in number, but a mint plant (Mentha sp.) growing in the same pot also had developed similar rust symptoms with more numerous uredinia. Attempts to re-inoculate new growth on the same plants with urediniospores were unsuccessful, probably due to high greenhouse temperatures (6,7) in summer 2004.
In early March 2005, an oregano plant (O. vulgare) with rust symptoms and pustules was purchased from a local retail store in Gainesville, Alachua Co., FL, by C. M. Stiles. This plant originated from a Florida-based commercial greenhouse operation. Symptoms and pustules were similar to those on the oregano collected one year earlier. About a week later, additional, younger oregano plants at the same store appeared to be free of rust; however, a mint plant (Mentha sp.) with orange-brown rust uredinia (Fig. 2) was found at the store. On mint plants, there was less necrotic tissue compared to oregano, however, uredinia were more numerous, and there was some chlorosis associated with uredinia. Urediniospores (Fig. 3) were similar in size and appearance on both the oregano and mint plant. The mint plant was maintained in a lighted chamber at 21°C and numerous pustules formed on upper leaves over the next two to three weeks. However, in June 2005, mint plants observed at the same retail store appeared to be free of rust, thus, high summer temperatures may limit rust development in Florida as noted for peppermint rust (7) in other areas. Fletcher (6) found that urediniospores of P. menthae failed to germinate at 25°C, and germination was reduced at 20°C.
Ellipsoidal urediniospores (Fig. 3) from both oregano and mint had up to three germ pores, consistent with descriptions of Puccinia menthae Pers.:Pers. (1,10). Urediniospores from mint and oregano plants were measured separately from plants collected in 2004 and 2005. Urediniospores from mint plants ranged from 20 to 30 µm by 15 to 28 µm (mean = 25 by 22 µm, n = 31); urediniospores from oregano plants ranged from 21 to 25 µm by 18 to 22 µm (mean = 24 by 20 µm, n = 19). These size ranges of urediniospores were also consistent with descriptions of P. menthae (1,10). No teliospores were observed on either host.
Cross-Inoculation Experiments with Oregano and Mint Rust Isolates
Urediniospores were collected from pustules on the oregano plant using a vacuum-operated spore-collecting tool. Spore suspensions were prepared (~ 1.5 x 105 spores / ml water) using Tween 20 (1 drop / 300 ml suspension) to disperse spores. Using a Crown Spra-tool propellent sprayer (Crown Industrial Products Co., Hebron, IL), the inoculum suspension was sprayed onto leaves (upper and lower surfaces) of spearmint (M. spicata), peppermint (M. × piperita), and oregano (O. vulgare). Plants were placed in a dew chamber at 18°C for about 24 h, then removed to a lighted growth chamber (15 h fluorescent light) at 21°C. Each plant was covered with a plastic bag for another 24 h to maintain humidity. Plants were then maintained in the lighted chamber at 21°C and observed for development of symptoms and uredinia. No rust developed on the spearmint or peppermint plants that were inoculated with urediniospores from the oregano plant. However, after about two weeks, on the oregano, small chlorotic and necrotic spots and rust pustules formed on both leaves and stems (Fig. 4). Urediniospores were similar in size and shape to the original collection.
To determine the host range of these isolates, urediniospores were collected from the newly formed pustules on the inoculated oregano plant as well as from the spearmint plant that was purchased from the retail store. Separate spore suspensions from the two plant species (isolates hereafter termed “oregano rust” and “spearmint rust”) were made and inoculated as described above. Due to the limited quantity of initial inoculum, cross-inoculations were completed in a series of four experiments onto oregano, Greek oregano (O. heracleoticum L.), sweet marjoram, peppermint (M. × piperita), and spearmint plants. One or two pots, with multiple plants and stems, of oregano, Greek oregano, and sweet marjoram were inoculated with each isolate in three of the four experiments, while peppermint and spearmint were included in all four experiments. In all experiments, inoculated and non-inoculated plants were maintained in the same incubator at 21°C. No rust pustules were observed on the non-inoculated plants when plants were observed two to five weeks after inoculation.
Inoculation with spores from oregano again resulted in necrotic lesions with numerous pustules on oregano. On sweet marjoram inoculated with the oregano isolate, numerous pustules formed; however there was less necrosis. Instead, a chlorotic halo was sometimes observed around uredinia (Fig. 5), also observed by Koike et al. (10). Infection also occurred on Greek oregano; however in two of the three experiments, fewer lesions formed on leaves and fewer pustules formed in the lesions, thus, disease development appeared to be less severe or delayed on this species (Table 1). In one of the three experiments, only three pustules were found on a total of 111 lesions observed on Greek oregano three weeks after inoculation. In another experiment, only one of eight stems of one plant had a few lesions on leaves three weeks after inoculation, while oregano plants exhibited abundant lesions and pustules formed on leaves of all ages and on all stems of plants. This suggests the oregano rust isolate may have a longer latent period on Greek oregano, as has been reported when mint rust isolates are inoculated on some related mint hosts (8).
Table 1. Pathogenicity of oregano and mint rust isolates on various oregano, marjoram, and mint species. Urediniospores from oregano and spearmint were collected separately and inoculated onto each of the plant hosts listed in a series of cross-pathogenicity tests. Number of + signs indicate relative abundance of lesions and pustules on the various hosts; “–” indicates host was not infected.
* On Greek oregano, fewer lesions formed on leaves and fewer pustules formed in the lesions, in two of three experiments, compared to oregano and sweet marjoram.
Inoculation with spores from the mint plant resulted in rust pustules only on spearmint, but not on peppermint (Fig. 6), oregano, Greek oregano, or sweet marjoram. When spores from the oregano rust isolate were inoculated onto peppermint and spearmint, neither mint species developed symptoms or formed rust pustules. Results of these inoculation experiments are summarized in Table 1.
Host Specialization of P. menthae on Oregano and Mint Species
Cross-inoculation experiments in this study indicate there is host specialization of P. menthae in Florida. The rust isolate from oregano did infect oregano, Greek oregano, and sweet marjoram. This finding is consistent with the report by Koike et al. (10), who found that California isolates of P. menthae from oregano and sweet marjoram infected other types of oregano and marjoram, but did not infect spearmint. However, the oregano isolate in these studies also did not infect any of the other mint species tested.
The rust isolate from spearmint did not infect oregano or the other related plant species tested, which is consistent with an earlier study by Fletcher (6). The spearmint isolate also did not infect peppermint, which is consistent with studies of mint rust in the midwestern U.S. (3), the Pacific Northwest (8), and in Australia (4). These other studies indicate there are two populations of mint rust, indicated as ‘spearmint rust’ and ‘peppermint rust’ (4,8). Scanning electron studies of urediniospores (9) indicate morphological differences between the two populations of mint rusts. Morphological and molecular studies of Australian isolates (4) suggest that the spearmint rust and peppermint rust isolates could be taxonomically separated at the varietal or species level. Of the previous studies, only Fletcher (6) attempted to inoculate oregano with isolates of rust from spearmint; in that study, none of the eight isolates produced a disease response in oregano. In the current studies, the rust isolate from oregano did not infect any of the mint species tested, thus the oregano rust population may be separate from either the spearmint rust or peppermint rust. Further collections and inoculation experiments would be needed to determine the complete host range of P. menthae in oregano and mint species in Florida, and to determine whether races exist within populations of P. menthae, as reported in the U.S. Pacific Northwest (8), the midwestern U.S. (3), and other locations.
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