This first appeared as “Spotlight on Ornamentals: A Thousand and One Arabian Rose Trees?” in the IR-4 Newsletter 46(4):4.
Throughout the world there are more than 1,000 known species of Rhododendron, or “rose trees” as directly translated from ancient Greek. Commonly known in cultivation as azaleas, Rhododendron species are grouped with ericaceous plants (health, heather, cranberry and blueberry) and can be evergreen or deciduous. Rhododendrons are distributed worldwide in temperate to tropical climates. In addition to the large number of species, many inter-species hybrids and cultivars have been bred for improved bloom size, color, fragrance and foliage form.
Together, azaleas and rhododendrons are considered on of the top broadleaf evergreen horticulture crops in the U.S. with over $141 million wholesale value annually (USDA-NASS, Census of Agriculture 2009).
In cultivation, there are three primary groups: evergreen rhododendrons, deciduous hybrid azaleas, and evergreen hybrid azaleas. Evergreen rhododendron species are understory plants in many eastern U.S. forests and are commonly used in landscape plantings in shady spots. (Figure 1). Both deciduous and evergreen azaleas may be place in shady areas but are often transplanted in more sunny locations, which can lead to stress and increased problems with lace bugs. A fourth group (Vireya Rhododendrons) are unusual in that they are epiphytic tender shrubs. Vireya rhododendrons originated in Southeast Asia in tropical regions, but they grow mostly in cool mountainous region as epiphytes on tall trees or low growing shrubs.
Azaleas and rhododendrons are in the same genus which means they have fairly similar characteristics. However, along with other morphological and generic traits, they can be separated based on the number of stamens present in the flowers: azaleas have 5 stamens, and the rhododendrons have 10 stamens.
Main Disease and Pest Problems
Azaleas and rhododendrons are prone to several disease and pests. Root pathogens include Phytophthora, Rhizoctonia, and foliar pathogens include Botrytis, Botryosphaeria, rusts and various fungi causing leaf spots. Pests include root-feeding insects such as the larval stages of black vine and strawberry weevils, Japanese and oriental beetles, and foliage or shoot feeding insects such as the adult stages of previous mentioned insects, lace bugs, aphids, scale and leaf miners. Ir-4 has sponsored efficacy research on some of these pathogens and pests. In particular, many experiments have been conducted to screen efficacy for various Phytophthora species including P. ramorum, a pathogen causing foliar blighting.
Weeds impact production of azaleas and rhododendron pots. Crop safety studies using over the top or directed herbicide applications are necessary for growers to know which products to use during dormant periods or during periods of active growth. Crop safety studies have been conducted with 15 fungicides. 61 herbicides and 10 insecticides. In addition, IR-4 has sponsored research on Rhododendron species with plant growth regulators to increase plant branching. Products containing 6-BA, or cyclanalide, exhibited variable impact from decreased to increased branching and products containing GA4+7 had no impact. In other words, some additional work is needed to determine best rates and/or timing to foster improved branching characteristics with different species and cultivars.
Rhododendron species have been well-studied crops though IR-4 due to their popularity over the years. As priority projects shift to different pathogens and pests, we will continue to work to develop efficacy and crop safety for relevant problems.