Some Rhododendron Diseases
Notes taken covering remarks made by Dr. Denzell Gill, Tifton, Ga.
Moderator of the Panel on Rhododendron Diseases
Cylindrocladium blight attacking both above and below ground parts of plants is becoming increasingly destructive of azaleas and rhododendrons. The disease is more destructive under high moisture and temperature conditions (85° F.). Sanitation of propagation areas and stock block will aid in control. Soaking cuttings 30-45 minutes before setting, spraying cuttings after setting, and spraying cuttings under mist at least weekly have reduced the disease in propagation. Drenching newly set liners with 1 pint per square foot of fungicide has reduced this stage of the disease. Fungicide applications to stock blocks and growing plants will also reduce the blight stage. Fungicides of value have been Benlate 1 lb. 100 gals., Daconil 1 lb. 100 gals., Polyram 1½ lbs. 100 gals. and Thiram 1½ lbs. 100 gals.
Pythium sp. infecting roots of nursery plants is frequently found in addition to the more common Phytophthora cinnamomi. Good drainage and sanitation in the culture should be of value. Pythium capable of attacking roots has been obtained from water sources. Fungicidel drenches of plants with Dexon (½-1 lb. 35% to 100 gals.) or Morsodren (1 oz. 35 gals.) have been of value.
Several pathogenic nematodes may be associated with the root-rots or may attack azaleas separately. Nematode free plants planted in soil treated with nematicides gives control. Re-infestation should be prevented.