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White rust (causative pathogen Puccinia horiana) is a destructive disease of commercial chrysanthemum crops. A panel of 19 accessions of commercial chrysanthemum near-relatives (four Ajania species, 11 Chrysanthemum species including five accessions of Chrysanthemum indicum) were screened for their reaction to white rust infection in separate greenhouse trials carried out at two independent sites in eastern China, one in 2010 and the other in 2012. The reaction of the accessions to artificial inoculation ranged from immune to highly susceptible. Accessions of Chrysanthemum indicum, C. yoshinaganthum, C. makinoi var. wakasaense, C. nankingense, C. vestitum, C. lavandulifolium, C. crassum, and Ajania tripinnatisecta were immune, and strong resistance was present in C. japonense, C. 3 shimotomaii, and A. przewalskii. Most of the accessions behaved similarly in the two trials, but two of the C. indicum accessions produced inconsistent results, each being highly resistant in one trial but susceptible in the other. Because wide crosses are relatively easy to achieve in the chrysanthemum complex, these immune and highly resistant accessions represent promising germplasm for white rust resistance breeding. Chrysanthemum white rust (causative pathogen Puccinia horiana Hennings) is a notifiable disease, which infects chrysanthemum plants grown for the cut flower and pot plant trade, largely because they are routinely cultivated under glass or in plastic tunnels where the microclimate favors the growth of the pathogen. The global trade in chrysanthemum has dispersed the disease from the site of its first documentation in Japan in 1895 to China, South Africa, and Europe (Baker, 1967; Firman and Martin, 1968; Whipps, 1993) and by now has become endemic to most chrysanthemum-growing areas (Cook, 2001; Göre, 2008; O’Keefe and Davis, 2012). The pathogen is an obligate biotroph, which colonizes young leaves and flower buds (Yamada, 1956). Under conditions of high humidity and mild temperature, its teliospores, which develop within a pustule on the abaxial leaf surface within 14 to 18 d post-infection (Zandvoort et al., 1968b), germinate to form a promycelium. Each promycelium bears a mean of two infective propagules (basidiospores) (Kapooria and Zadoks, 1973), which can be dispersed by wind over a distance of up to 700 m (Zandvoort, 1968b). Although white rust can be chemically controlled (Dickens, 1990; Zadoks et al., 1969; Zandvoort et al., 1968a), the use of fungicides is associated with both environmental hazards and an increased production cost (Waard et al., 1993); overdependence on their use has already led to the appearance of tolerant strains (Abiko et al., 1977; Cook, 2001). The more sustainable strategy of exploiting genetically determined resistance requires the identification of sources of resistance, as first attempted by Dickens (1968) and Martin and Firman (1970). DeJong and Rademaker (1986) have suggested that resistance is most commonly under monogenic control. Outside of the primary gene pool, a number of related species have been shown to harbor variation of relevance to aphid tolerance (Sun et al., 2012), to Alternaria leaf spot resistance (Xu et al., 2011), and to tolerance of various abiotic stresses (Guan et al., 2010; Yin et al., 2009). Several Chrysanthemum spp. have been reported as resistant to white rust (Dickens, 1968; Hiratsuka, 1957) and an intergeneric hybrid between chrysanthemum and Artemisia sieversiana has been shown to be more resistant than the chrysanthemum parent (Furuta et al., 2004). Hybridization between commercial chrysanthemum and various Ajania and Chrysanthmum spp. has been repeatedly achieved (Cheng et al., 2011; Deng et al., 2011), so wide crossing could have considerable potential for white rust resistance breeding. Here, we report the evaluation for the white rust resistance shown by a panel of 19 Ajania and Chrysanthemum spp. involving two independent trials. To our knowledge, this represents the first such survey of the potential of Ajania and Chrysanthemum spp. to provide a source of genetic resistance to white rust. Materials and Methods Plant materials and inoculation. The set of 19 accessions including species from Ajania (four species) and Chrysanthemum (10 species with five accessions of Chrysanthemum indicum) (Table 1) was obtained from the Chrysanthemum Germplasm Resource Preserving Center, Nanjing Agricultural University, Nanjing, China. The commercial cultivar Jinba served as a white rust-susceptible control to indicate inoculations were succeeded or not. Screening was carried out on plants grown in two different plastic greenhouses in eastern China, one in 2010 (‘‘Trial 1’’) and the other in 2012 (‘‘Trial 2’’). For Trial 1, morphologically uniform 3-week-old rooted cuttings (eight to 10 leaf stage) were grown at a site located at lat. 34.06 N, long. 118.28 E. The plastic house was initially treated with carbendazim and mancozeb, after which a minimum of 30 cuttings per accession were planted in a single 10-m row with an interplant spacing of 20 to 30 cm and an interrow spacing of 0.5 m. The experiment was arranged as a randomized complete block with three replications (McIntosh, 1983). Overhead irrigation was provided with a sprinkler. No fertilizer or fungicide was applied during the whole experimental process (1 May to 30 June). Seven days after planting, the cuttings were inoculated with teliospores collected from diseased cv. Jinba plants. To minimize disease escape, they were inoculated first using the fraction method elaborated by Zhu et al. (2011), then a day later using a spray method derived from Zandvoort (1968a); briefly, the infected leaves containing white rust pustules were cut into small pieces and were dispersed in deionized water and filtered through medical gauze to remove any plant debris. The concentration of the pathogenic spore suspension was then adjusted using a hemacytometer slide to a concentration of 1 · 10 zoosporangia/mL with deionized water containing one drop of Tween 20 before application to the plants until runoff using a handheld sprayer. The plants were covered with a black polythene sheet for the first 48 h postinoculation both to maintain a high relative humidity and to exclude light (Yamada, 1956). Thereafter the temperature was kept within the range 15 to 25 C, and supplementary light was given to provide a 16-h photoperiod. The relative humidity was kept above 80% by the use of 360 rotating sprinklers for overhead sprinkling, in which water pressure Received for publication 17 Apr. 2013. Accepted for publication 28 July 2013. This research was financially supported by the Chinese Ministry of Science and Technology 863 project (2011AA100208), the Chinese Ministry of Education for New Century Excellent Talents in University Program (NCET-10-0492), the Fundamental Research Funds for the Central Universities (KYZ201112), the Jiangsu Province Science and Technology Program (BE2011325, BE2012350), and the Natural Science Fund of Jiangsu Province (BK2011641). We thank Suqian Richangsheng Gardening Co., Ltd. and Xianhuashengchanjidi Co. Ltd. for the use of greenhouse facilities. We appreciate the constructive suggestions made by Dr. Pengfang Zhu. To whom reprint requests should be addressed; e-mail chensm@njau.edu.cn. HORTSCIENCE VOL. 48(10) OCTOBER 2013 1231 was 200 Kpa, flow was 27.8 L·h, and maximum range was 3.5 m (Lemiao Irrigation Equipment Factory, Zhejiang, China) every 2 h during the day. The Trial 2 plastic house was sited at lat. 34.32 N, long. 118.12 E. Planting period, plant layout, inoculation procedure, and field management were as for Trial 1. The fungal inoculum used in this trial was collected from diseased cv. Iwanohakusen. Disease monitoring and classification. The latent period, infection type, disease severity, and disease incidence were recorded at the time when teliospore-containing pustules had become well developed. Monitoring was continued until 52 d post-inoculation (dpi). The latent period was defined as the number of days elapsed between inoculation and the first appearance of symptoms (Browne and Cooke, 2004). Infection type was represented by a 0 to 5 scale, which was observed from the majority (15 or greater) of individual data, adapted from Pathan and Park (2006) and Zhu et al. (2011), in which ‘‘0’’ indicated no visible symptoms, ‘‘1’’ indicated rare visible yellowish hypersensitive flecks are discernible, ‘‘2’’ indicated a few small yellowish flecks and very little telia on the back, ‘‘3’’ indicated more small or few large yellowish necrosis and clear telia on the back, ‘‘4’’ indicated large and continuous yellowish necrosis and clear telia on the back, and ‘‘5’’ indicated massive large and continuous yellowish necrosis and diffusible telia on the back and some leaves even roll or rot. Disease incidence (I) was given by the ratio between the average number of diseased and nondiseased leaves on a plant from 30 individuals per accession. A mean disease severity measure (Ŝ, given by the mean proportion of leaf area infected) was calculated from 30 infected leaves per accession; both the area of diseased leaf surface and the overall leaf area were obtained from scanned images of the leaves following Igathinathane et al. (2006). A disease index (DI) was then derived from the expression I · Ŝ · 100 based on rule for resistance evaluation of wheat to leaf rust (Puccinia triticina) (Ministry of Agriculture of the People’s Republic of China, 2007). Statistical analysis. SPSS 17.0 software (SPSS Inc., Chicago, IL) was used for all statistical calculations. A one-way analysis of variance, in conjunction with the Duncan multiple range test, was used to assess whether accessions differed significantly from one another for their reaction to infection.

Variation for Resistance to White Rust (Puccinia horiana) among Ajania and Chrysanthemum Species