JARS v37n2 - Rust of Deciduous Azaleas

Rust of Deciduous Azaleas
R.K. Jones, R.E. Bir and D.M. Benson
North Carolina State Univ.

Nature of Work:
Deciduous azaleas (Rhododendron sp.) are becoming increasingly important as a nursery crop in North Carolina. One of the main disease problems with this group of plants is rust caused by fungus Pucciniastrum myrtilli ( P. vaccinii ). A serious epidemic of this disease occurred in the fall of 1979 in western North Carolina. This pathogen has been reported on several species of native rhododendron and blueberry from Florida to Maine and Canada. The alternate host for this rust fungus is hemlock. Ten plants each of 17 cultivars were sprayed with various fungicides using one plant of each cultivar for each treatment and five plants of each cultivar for the control and inoculum source. All fungicide applications were made using a back pack sprayer. Plantvax was applied every 14 days while all other fungicides were applied every 7 days. Applications were made from mid-August through September. Plants were evaluated in mid-October on a scale of 0-10 where 0 indicated no trace of rust on the plant and 10 indicated complete coverage of all leaves by rust pustules.

Results and Discussion:
The first sign of rust development was observed on August 22, 1980. All fungicides used provided some control of the disease with no indication of phytotoxicity (see Table 1) Plantvax, Ferbam, and Mancozeb gave the best control. Plantvax systemic fungicide, was the most effective with applications every 14 days while the other fungicides were applied every 7 days. The study will be continued in 1981 to determine the maximum interval between applications of Plantvax.

Table 1.  Evaluation of Various Fungicides for Deciduous Azalea Rust Control.
Rate per Disease
Treatment 100 gal. Frequency A Index B
1. Control None 6.35
2. Wettable Sulfur 4 lb. 7 days 2.06
3. Triforine 18.2 EC 12 oz. 7 days 1.65
4. Ferbam 76 W 2 lb. 7 days 0.60
5. Mancozeb 80 W 1/2 lb. 7 days 0.18
6. Plantvax 75 W 1 lb. 14 days 0.06

A Fungicide applications were started in mid-August and continued through September.
B Disease index (0 = no disease, 10 = rust pustules completely covering all leaves.