165 Geranium Bronze Tolerance in Diploid and Tetraploid Ornamental Geraniums M. Alonso and M. Borja Departamento de I+D Fundación PROMIVA Boadilla del Monte 28660 Madrid Spain S. Herrero and J. Ferré Departamento de Genética University of Valencia Burjassot 46100 Valencia Spain P. Ellul and V. Moreno Laboratorio de Células y Tejidos Vegetales IBMCP-CSIC 46022 Valencia Spain Keywords: Breeding, Pelargonium, Cacyreus, ploidy, germplasm, pest management Abstract The ornamental geranium, Pelargonium × hortorum Bailey, is a traditional ornamental plant largely cultivated in Europe and Northern America. Recently, a lepidopteran pest, the geranium bronze (Cacyreus marshalli Butler), is threatening the popularity of geraniums in Europe. An inventory of ornamental geranium samples present in remote rural areas and National Parks is currently being established to build a national germplasm bank [the Spanish Pelargonium Germplasm Bank (SPG Bank)] of garden geraniums. The objective of this study was to analyse twenty representative taxa from the SPG Bank and seven commercial cultivars of Pelargonium × hortorum to assess their ploidy level and test them for potential resistance to the geranium bronze. The ploidy level was determined by flow cytometry. All commercial cultivars tested for geranium bronze tolerance had an elevated degree of susceptibility compared to the most tolerant SPG taxa. This study indicates that there is enough variability among the SPG Bank taxa to be used as potential sources for geranium bronze-tolerant germplasm for the development of commercial Pelargonium cultivars. INTRODUCTION The ornamental geranium, Pelargonium × hortorum Bailey, has a complex heritage (Laughner, 1993). It is thought to derive mainly from P. zonale and P. inquinans (Ciconium section), with P. hybridum, P frutetorum, P. scandens, and P. acetosum contributing traits to a lesser extent (Clifford, 1956; Harney, 1966; Chow and Harney, 1970). Geraniums are universally popular, with over 100 million plants produced and distributed commercially in 1998 in the United States (2000 Floriculture Crops Summary, National Agricultural Statistics Service, USDA) and an equivalent production and distribution in Europe. The genus Pelargonium consists of more than 250 species in 13 sections (Van der Walt et al., 1988), all but 33 originated from South Africa. Seven basic chromosome numbers (x=4, 7, 8, 9, 10 11, and 17) are found in the genus (Van der Walt, 1992), with ploidy leading to somatic numbers from 8 to 104. Crossability is mostly restricted to species having the same chromosome number and ploidy level (Horn, 1994). Today, most asexually produced, commercially successful taxa are tetraploid (Laughner, 1993). Currently two species are largely cultivated in Europe and northern America: Pelargonium × hortorum Bailey ("zonal geranium", 2n=2x=18 or 2n=4x=36) and Pelargonium × hederafolium Bailey ("ivy leaf geranium", 2n=2x=18 or 2n=4x=36), and more than a hundred commercial taxa are available. Geraniums are mostly clones, but F 1 - hybrids are also propagated though seeds (Renou et al., 1997). Today geranium is one of the traditional ornamental plants in Southern Europe, where it is grown as a perennial plant. Recently, a lepidopteran pest, the geranium bronze (Cacyreus marshalli Butler), is threatening the popularity of geraniums in Europe, since these plants are no longer easy to grow because of the biweekly insecticidal treatments that must be sprayed in order to maintain the plants free of the pest. Because of the biology of this pest, its control with conventional insecticides is not easy. The butterfly lays its eggs in the flower buds or in Proc 21 st IS on Breeding Ornamentals, Part II Eds: G. Forkmann & S. Michaelis Acta Hort 651, ISHS 2004