Roses are the largest crop group in the floriculture sector of the Netherlands. To maintain this position, new, disease-resistant rose varieties are needed. Most commercial rose varieties are susceptible to several diseases and pests, currently resulting in a substantial use of chemical plant protection products in rose cultivation. Yet, consumers and governments are increasingly demanding a significant reduction in the use of chemical plant protection products with many products becoming prohibited. Disease-resistant rose varieties would require less plant protection products and thus provide a sustainable alternative. Therefore, rose breeders have shifted part of their focus to resistance breeding. In the past years, disease resistance genes have been identified in rose for several, but not all, important rose diseases, including Black Spot, Powdery Mildew, and Agrobacterium. One important disease that has remained understudied is Downy Mildew. Resistance to Downy Mildew will be the focus of this study. Furthermore, the use of DNA information in breeding of polyploids (such as rose) has recently become feasible, providing opportunities for breeders to make resistance breeding more efficient and faster. However, many resistance alleles have been found in wild, diploid rose species, thus incorporation of these alleles into elite germplasm requires additional steps. Furthermore, elite rose germplasm is divided into various market segments (cut, garden, pot roses), each with its own characteristics, and transfer of resistance alleles between germplasm of different market segments is not yet straightforward. This project will 1) create a disease assay for downy mildew, an important yet understudied disease in roses, 2) genetically characterize sources for resistance against downy mildew, including the identification of informative markers linked to disease resistance QTLs, and 3) develop breeding strategies for efficient introgression of disease resistances against the four main diseases into elite tetraploid rose germplasm. The results of this project will be a needed step towards disease-free roses by enabling breeders to produce new varieties that are resistant to multiple diseases and are fit for sustainable production of roses in terms of plant protection

The ultimate objective of the project is a more sustainable rose production by enabling breeders to create new, disease-resistant roses for various market segments. Rose is the most important ornamental crop worldwide and, in the Netherlands, it is the largest crop group in the floriculture sector. Different rose backgrounds exist, each with their own use. It is grown for cut-flowers, pot and garden uses as well as for the oil and perfume industry (rose petals) and as food component (rosehips). Unfortunately, rose production requires regular application of chemical plant protection products as rose is susceptible to various diseases, such as powdery mildew, black spot, and downy mildew (Table 1 of Annex 2). Governments are limiting both which and how much of these products can be used. The increased energy costs for greenhouse heating and subsequent decrease of energy-usage has led to more favourable climate conditions for diseases such as botrytis and downy mildew. In open and unheated production areas, climate change has also led to more unfavourable conditions with losses of whole flushes mainly due to downy mildew. Cultivation of disease-resistant roses would make the sector more sustainable, yet such cultivars are scarce. Resistance sources have been identified for some important diseases (e.g., black spot and powdery mildew; Table 1 of Annex 2) but not all (specifically downy mildew). Furthermore, the implementation of available disease resistance sources has been slow as they are usually found outside the target background. Thus, breeding is hampered by a lack of information on the (genetic) sources for some diseases such as Downy Mildew and the complexity of introducing resistances into a specific background (e., cut roses or garden roses).

This project contributes to ST2, Biotechnology and breeding, as it aims to generate the knowledge and tools necessary to develop rose varieties with resistance against downy mildew as well as develop ideas, tools, and strategies to improve the efficiency of introgression breeding for resistance traits in different genetic backgrounds of tetraploid roses. Knowledge generated in the project will form the basis for the creation of novel varieties that have multiple natural resistances, and as such contributes to “Missie 2C” (Weerbare plantaardige productie op een vitale bodem of substraat) resulting from resilient varieties aiming at sustainable production of roses in terms of plant protection.

Objective 1: Creation of a Downy Mildew disease assay
Objective 2: Genetic characterization of Downy Mildew resistance sources
Objective 3: Conceptualize breeding strategy for resistance introgression in tetraploid roses