Susceptibility Of Selected Deciduous Azalea Cultivars To Azalea Rust
R.E. Bir, R.K. Jones, D.M. Benson
North Carolina State University
Nature of Work: Hybrid deciduous azaleas, sold as Exbury, IIam, Knap Hill and Windsor Hybrids, have increased in popularity in recent years due to their abundance of large and colorful blooms as well as being culturally adapted to much of the Eastern United States. The relatively few growers able to successfully propagate and produce quantities of these hybrids have been able to enjoy consistent demand for plants.
During the 1979 growing season, azalea rust, Pucciniastrum myrtilli, developed to such an extent that many of the plants in one nursery were unacceptable due to the extensive display of yellow-orange pustules on the foliage during the fall shipping season. When a search was conducted for information concerning the disease and its control in the nursery, very little was found.
A test was established to screen fungicides for rust control effectiveness on cultivars of 17 hybrid deciduous azaleas. Ten 2-gallon plants of each cultivar were included in the test, 5 being used as controls to ensure adequate inoculum and evaluate susceptibility. The test was conducted at the Mountain Horticultural Crops Research Station to provide conditions similar to the grower's climatically but allow the disease to develop without risking further infection of the grower's crop.
A 10 point system was used for evaluating levels of infection (0 indicated no infection; 10 complete infection). Plants were evaluated by mid-October.
Results and Discussion: We have arbitrarily divided the plants into 3 categories: those that scored an average of 9 or greater were considered highly susceptible; those with scores of 5 to 7 were considered moderately susceptible; and those with scores of 4 or less were considered to have low susceptibility.
These ratings have applicability to commercial producers and home gardens. We have not seen any problems related to azalea rust in home gardens regardless of cultivar or geographic location, however, an extensive survey has not been conducted.