Landscape - Phytophthora in Rhododendrons

Overly wet conditions will mean an increase in Phytophthora diseases in the landscape. One plant very susceptible is Rhododendron. The following is more information.

Watch for Phytophthora shoot dieback and canker on Rhododendrons in shaded wet areas. Leaf spotting and blighting progresses to a dieback of the branch tip. Often the petiole will be brown or black and a V-shaped wedge of blackened leaf tissue will be evident where the petiole attaches to the leaf. Dead shoots are often seen close to the ground first because the fungus will splash from the soil to the leaves. Prune out any infected shoots, cleaning the shears between cuts with rubbing alcohol, 10% bleach or other disinfestant. Professional landscapers can apply Subdue or Aliette; homeowners should use mancozeb or a copper fungicide. Prune rhododendrons so the lowest branches are not touching the soil.

Rododendron wilt, otherwise known as Phytophthora root rot, is caused by the fungus Phytophthora cinnamomi. This fungus infects rhododendron, azalea, mountain laurel, blueberry, heather, Irish-heath, Leucothoe, spike-heath, wintergreen, Arbutus unedo, bear-berry, and Pieris.

In infected plants the leaves become pale, droop, roll downward parallel to the midrib, and wilt. The leaves eventually die. Death may occur in as few as 14 days in susceptible young plants, or death may not occur for up to a year in older plants or less susceptible varieties. Individual branches may die. These branches usually have a canker at their base if the plant is young. Underground, small roots become infected first and turn brown. The roots die, although the plant may grow more roots if the plant is older and established. The fungus grows into the stem and when the bark is cut away, the region where the bark attaches to the wood is dark brown.Some less susceptible cultivars may not wilt until all the roots have died. The youngest leaves may turn yellow between the veins, near the midrib. These areas eventually turn brown. Large, well-established plants often have few aboveground symptoms.

Some varieties of Rhododendron are resistant, including Caroline, Professor Hugo de Vries, and Red Head. A few others are moderately resistant. Some azaleas are also resistant to this disease, including Formosa, Fakir, Corrine Murrah, and the Indica hybrids. It is important to use resistance whenever acceptable, particularly in sites where P. cinnamomi is known to exist.

Information from Bob Mulrooney, Extension Plant Pathologist, UD and the University of Connecticut IPM Website.