Breeding for biotic stress resistance in Rhododendron simsii

Historically, breeding and selection of pot azalea (Rhododendron simsii Planch hybrids) has focused predominately on morphological traits. Today, biotic stress resistance has become a key issue, mainly due to the restrictions on the use of pesticides and the mandatory use of integrated pest management (IPM) in European horticulture. The two main diseases during production are caused by Phytophthora citricola Saw. and Cylindrocladium spp. We have developed bio-assays on rooted cuttings for both diseases. These assays were validated using a set of well-defined susceptible/tolerant genotypes and are currently routinely used in our breeding program. More recently the broad mite, Polyphagotarsonemus latus Banks, has become an economic important pest in azalea. A first screening for genetic variation in Rhododendron simsii hybrids revealed significant differences in susceptibility between accessions. Scanning electronic microscopy of the leaf surface of susceptible and tolerant azalea genotypes showed differences in trichome structure. This might possibly play a role in the defense mechanism of the plant. This observation also opens new possibilities to select for broad mite resistance in the future. After there is more support for this hypothesis.