JARS v46n3 - Phomopsis Dieback of Azalea

Phomopsis Dieback of Azalea
Donald R. Roberts, Jr. and J. T. Walker
Department of Plant Pathology
University of Georgia, Georgia Station, Griffin, Georgia

(Following is a summary of the authors' research paper "Phomopsis Dieback of Azalea - Possible Modes of Infection and Growth Inhibition of Fungicides.")

A disease known as Phomopsis die-back occurs on landscape azaleas in the Southeast. It is not a new disease, having been observed in the Northeast during the 1930s, but has been more prevalent during the environmental conditions of the 70s and 80s.

An early disease symptom is leaf chlorosis with the leaves turning brown and wilting but remaining attached to the stems. When defoliation eventually does occur in late summer or early fall the dead bare branches accurately describe the name given the disease (see photo). The interior of the diseased stems often is reddish-brown in color usually extending from the branch tip downward. The fungus most frequently isolated from diseased tissue by plant pathologists in South Carolina and Georgia is a species of Phomopsis whose specific epitaph remains uncertain.

In our attempts to better understand the disease cycle different tissues from the azalea cultivar 'George Lindley Tabor' were inoculated with fungal fragments (mycelium) or fungal spores (alpha conidia). The optimum temperature for conidial germination also was investigated as were studies on the effects of several dosages of fungicides on pathogen growth.

Mycelial and conidial inoculations of wounded stems resulted in vascular discoloration within two weeks of inoculation. No stem discoloration occurred after flower inoculations, yet the fungus could be recovered from symptom less tissue (ovaries) of the inoculated flowers. Inoculated apical buds exhibited some water-soaking, but the infection did not progress more than 5 mm down the stem in two weeks. The fungus could be recovered from old and new leaf scars following conidial inoculations. Leaves did not develop the disease at any stage.

The Phomopsis fungus produces ascospores and two types of conidia, alpha and beta, on dead azalea twigs. Germination of the alpha conidia occurs rapidly (60-90% in 48 hrs.) in free water at temperature ranging from 77-94°F. They do not germinate well at 95% or 100% relative humidity. The ascospores germinate in less than three hours in free water at room temperature. Germination of the long slender beta conidia was not observed. They may not play any role in the disease cycle.

Various fungicides were inhibitory to growth of the fungus in culture, some at very low concentrations of active ingredient. Those which completely prevented fungal growth were Benlate 50 WP, Prochloraz/Mn 50 WP and Tilt 3.6 EC. These fungicides were not phytotoxic to five azalea cultivars when two spray applications were made at twice the concentrations which inhibited the fungal growth. (These materials are not recommended for use unless cleared and labeled for azalea.) To our surprise the copper-containing fungicide Kocide was not inhibitory to the fungus.

Although we suspect that cultivars vary in their susceptibility to Phomopsis dieback, our studies suggest that removal of dead stems through fall pruning may be appropriate for reducing inoculum. Where the disease has been prevalent, three days of rain or a water film coupled with 80°F temperatures may be sufficient to initiate the disease cycle. If fungicides are deemed necessary, then many are available which are effective. In any case fungicide applications should only be made after consultation with your local extension agent about materials cleared and labeled for such purposes.

The authors express their appreciation of the American Rhododendron Society Research Foundation for support of this project - JTW.

Editor Note: Copies of the full research report are available from the Journal.

Benlate (benomyl), prochloraz (brand name Omega), Tilt (propicon-azole) are not restricted; use is mainly commercial.