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Trials: Black Spot (Diplocarpon Rosae Wolf) Resistance and Ploidy

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Trials: Black Spot (Diplocarpon Rosae Wolf) Resistance and Ploidy HORTSCIENCE 45(12):1779–1787. 2010. et al., 2009). Pesticides are not used during the trials and cultural management practices use techniques that support environmental Evaluation of Roses from the stewardship. This includes pre-plant incorpo- Ò ration of compost, maintenance of a 7.6- to Earth-Kind Trials: Black Spot 10.2-cm layer of organic mulch, and irrigation methods that conserve water. Before a rose (Diplocarpon rosae Wolf) can earn regional Earth-Kind designation, it must exhibit consistent, superior performance across multiyear and multilocation trials rep- Resistance and Ploidy resenting different soil types and other envi- David C. Zlesak1 ronmental conditions typical in the region. A University of Wisconsin–River Falls, 410 S. 3rd Street, River Falls, WI 54022 large number of cultivars are advertised as low maintenance by nurseries wanting to capital- Vance M. Whitaker ize on the popularity of lower maintenance University of Florida Horticultural Sciences, Gulf Coast Research and roses. It is difficult for consumers to know which roses truly possess the highest levels Education Center, 14625 CR 672, Wimauma, FL 33598 of pest and environmental tolerances. Earth- Steve George Kind designation gives consumers confidence that they are choosing roses that will have AgriLife Research and Extension, Texas A&M, 17360 Coit Road, Dallas, TX a high likelihood of success when basic plant 75252 care is provided. Disease susceptibility poses a major chal- Stan C. Hokanson lenge to roses and limits their success as Department of Horticultural Science, University of Minnesota, 1970 Folwell low-maintenance landscape shrubs. Fungi Avenue, St. Paul, MN 55108 that attack roses include Diplocarpon rosae (Wolf) (causal agent of black spot), Podos- Additional index words. rose chromosome count, rose disease, fungal isolate, Diplocarpon phaera pannosa (Wallr.: Fr.) de Bary (causal rosae pathogenic race agent of powdery mildew), and Cercospora Abstract. Regional, replicated cultivar trials of landscape roses are an ongoing compo- puderi B.H. Davis (one of the causal agents nent of the Earth-KindÒ program, which was started at Texas A&M University in the of rose leaf spot) (Horst and Cloyd, 2007). Of 1990s to support environmental landscape stewardship. The rose trials within the Earth- these, black spot is the most serious in the Kind program identify and promote the most regionally adapted rose cultivars and are outdoor landscape across most regions as a re- conducted without fertilizers or pesticides and greatly reduced irrigation. Black spot sult of the potential for rapid disease develop- (caused by Diplocarpon rosae Wolf) is the most serious disease of outdoor-grown roses ment that typically leads to leaf yellowing and worldwide as a result of the potential for rapid leaf yellowing and defoliation. Earth-Kind defoliation (Dobbs, 1984). Black spot has been designated cultivars for the south–central United States and roses under trial in other the most prevalent and widespread disease in regions or considered for future Earth-Kind trials (n = 73 roses) and two susceptible the Earth-Kind rose trials (Mackay et al., control cultivars were challenged with North American Races 3, 8, and 9 of D. rosae, 2008). Plants repeatedly defoliated from black which were previously characterized at the University of Minnesota. Young expanded spot become weakened and quickly fall out of leaves were inoculated using detached leaf assays. Lesion length (LL) was measured for contention for Earth-Kind designation. susceptible reactions and cultivar ploidy was determined using root tip squashes. Diploid, Diplocarpon rosae is capable of infecting triploid, and tetraploid cultivars (n = 20, 30, and 23, respectively) were identified, and only the genus Rosa. Asexual spores (condia) race-specific resistances and partial resistances were also identified. Race-specific overwinter on stems and fallen leaves and are resistance was generally more prevalent in newer rose cultivars and rose cultivars more transported to new growth in the spring through recently included in Earth-Kind trials. Nine cultivars were resistant to all three races water droplets. If free water remains present, (Brite Eyesä, ‘Grouse’, Home RunÒ, Knock OutÒ, Paprikaä, Peachy Creamä, Pink the conidia form germ tubes that penetrate the Knock OutÒ, Rainbow Knock OutÒ, and Yellow Submarineä). Blushing Knock OutÒ, leaf epidermis. Lesions may appear in as little a sport of Knock OutÒ, was susceptible to Race 8. Partial resistance rank for LL was as 4 d as sub-cuticular mycelia radiate from the generally consistent across races for roses susceptible to multiple races. The application point of infection. Condia-bearing acervuli of these data includes: characterizing the minimum resistance level needed for roses to burst through the leaf cuticle followed by leaf warrant inclusion in Earth-Kind field trials, the identification of additional race-specific abscission in susceptible cultivars (Horst and resistance genes, identifying resistance-breaking isolates of D. rosae, understanding race Cloyd, 2007). composition in field trials based on infection patterns of key cultivars, selection of parents Multiple studies have been conducted to for resistance breeding efforts, and continued comparisons between LL and growing characterize the pathogenic race structure of bodies of Earth-Kind field resistance data. D. rosae and then to use the characterized races to identify genes conferring host resis- tance (Debener et al., 1998; Whitaker et al., Garden roses (Rosa sp.) are among the toward pesticides, emerging legislation put- 2007a, 2007b, 2010a; Yokoya et al., 2000). most popular flowering shrubs in the world. ting greater limits on pesticide availability Isolates are distinguished from one another Diversity for traits such as form, color, and and use, busy lifestyles, and greater avail- based on their differential ability to infect a fragrance of flowers, plant habit, size, envi- ability of lower maintenance rose cultivars common set of rose genotypes. Those isolates ronmental adaptability, and extended season (Harp et al., 2009). with the same host infection pattern are des- of flowering all contribute to their widespread Earth-KindÒ Rose Trials are a component ignated as a race. Collections that have been cultivation and versatility (Zlesak, 2006). Lower of the overall Earth-Kind program, started at preserved for continued research include six maintenance cultivars that can tolerate regional Texas A&M (the term Earth-KindÒ and asso- races discovered in Germany (Debener et al., environmental conditions without routine de- ciated logo are trademarks of the Texas Agri- 1998), four from Great Britain (Yokoya et al., pendence on pesticides and excessive care Life Extension Service, Texas A&M System) 2000), and three races from a genetically di- are especially increasing in popularity (Harp and help to serve the horticulture community verse set of 50 isolates originating in east- et al., 2009; Lonnee, 2005). Factors fueling by identifying the most adaptable landscape ern North America (Whitaker et al., 2007a, this trend include negative consumer attitude roses through regional cultivar trials (Harp 2007b). Whitaker et al. (2010b) evaluated HORTSCIENCE VOL. 45(12) DECEMBER 2010 1779 this international collection, identified 11 Using a collection of races to characterize of cultivars were obtained in the summer of unique races among them, and standardized roses marketed for low-maintenance land- 2007 and maintained on an outdoor gravel the race nomenclature. These studies and others scape use would be helpful beyond highlight- pad. They were transferred to a cold room and have uncovered genetic resistance within rose ing the resistance of roses for growers and provided with a 4 °C vernalization treatment species and cultivars that is race-specific. Such consumers. Knowledge of the genetic resis- for 10 weeks before being pruned, repotted race-specific resistance can only be character- tance of these roses would guide the selection as necessary, and brought to the greenhouse ized with controlled inoculations of roses with of cultivars that could serve as controls to help (23 ± 4.0 °C; St. Paul, MN; lat. 45° N) in mid- individual races as opposed to a field setting differentiate the presence of known races in Jan. 2008. Day extension with supplemental where the presence and prevalence of specific field trials where race composition is not light was used until 2200 HR (400-W metal races are not known. Using characterized races known. Additionally, when cultivars with re- halide lamps; 150 mmolÁm–2.s–1 at plant for inoculations under controlled conditions is sistance to all known races become infected, level) to encourage strong, vigorous growth. also advantageous when surveying for partial pathologists may characterize the infective Sulfur was burned in the greenhouse from resistance (Whitaker and Hokanson, 2009a; isolate(s) in search of new pathogenic races. 22:00 to 02:00 HR nightly to discourage Xue and Davidson, 1998). A widespread cultivar screen with known powdery mildew. Sulfur burners were turned Detached leaf assays have been an effi- races would also allow breeders to identify off a minimum of 3 d before leaves were cient tool to characterize the resistance of desirable genetic resistances among a core set collected for detached leaf assays. rose seedling populations, and they have been of highly resistant cultivars. If knowledge of In late May 2008, a subset of roses needing found to be strongly correlated with whole resistance
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