Both evergreen and deciduous azalea shrubs can suffer from root rot caused from the soil-borne fungus Phytophthora cinnamomi. The thick-walled spores of the fungus live within the soil and around infected plant root systems. The fungus spores have mobility and can easily travel short distances through the soil to the roots of newly planted, healthy azalea shrubs.
The fungal spores infect the fine roots of the azalea shrub first, according to the University of Minnesota. The roots take on a dried, brown appearance when the shrub is infected. If the shrub is dug up the roots will feel brittle to the touch and easily break when infected. The fungus makes its way up the root system of the plant to the crown. It quickly girdles the crown of the azalea. The stem of the shrub will take on a brownish, black appearance in sections. Canker spots appear.
An azalea afflicted with root rot will develop drooping foliage that will begin to turn yellow. The yellow leaves begin to roll downward on the shrub. Stunted growth often develops with complete stem dieback manifesting over time. The azalea will become stressed as more and more of its root system suffers infestation. Soon the shrub perishes from the widespread fungal root rot.
Phytophthora root rot fungus requires moist soil conditions to thrive. It prefers a soil temperature above 80 degrees Fahrenheit, according to Virginia Tech. The fungus does not flourish in sandy soil. It prefers soil that is rich in humus or clay based. Once an azalea is infected with root rot there is no cure. The infected plant will need to be lifted and destroyed. The surrounding soil medium should be cleared away and disposed of.
If an azalea is infected with root rot the neighboring shrubs should be treated to help prevent them from becoming infected. Drench the soil using a fungal solution that contains mefenoxam, etridiazole, etridiazole + thiophanate methyl or fosetyl-al, according to Virginia Tech. During the summer months the soil will require treatment every four weeks to make sure that neighboring azalea shrubs do not become infected.
Planting and Care
Plant azalea cultivars that offer root rot resistance. Eikan, New White, Hampton Beauty and Polar Seas azaleas have been specifically bred to not be susceptible to fungal root rot. Planting azaleas in raised flower beds and adding sand to the soil also helps prevent root rot from developing. Spraying the foliage of azaleas during the summer months when the temperature tops 70 degrees Fahrenheit with fosetyl-al or metalaxyl also helps prevent the shrub from becoming infected, according to the University of Connecticut.