November 2015 Plant Disease Advisory

Container lilac showing symptoms of off-color foliage and wilt caused by Phytophthora root rot.
Figure 1. Container lilac showing symptoms of off-color foliage and wilt caused by Phytophthora root rot.

“Phytophthora” is derived from two Greek words that translate as “plant” and “destroyer”. It is an aptly named plant pathogen because it causes serious disease on a wide range of plant species in many locations throughout the world. Phytophthora is a “water mold”, which produces motile spores that swim through water and are chemically attracted to root tissue. Thus, it is well adapted to cause root rot on plants in wet soil.

Phytophthora root rot is a common problem on many woody ornamental plants in both the nursery and landscape. Early aboveground symptoms of the disease may include an off-color appearance and/or slow growth. As root rot progresses, wilt occurs, followed by dieback and eventually death. By the time the aboveground symptoms are observed, loss of roots is significant and control is no longer possible.

A hedgerow of yews showing a progression of Phytophthora root rot symptoms, from off-color foliage to dieback to death.
Fig. 2. A hedgerow of yews showing a progression of Phytophthora root rot symptoms, from off-color foliage to dieback to death.

The best approach to Phytophthora root rot control is avoidance. Since the disease can be brought into the landscape on nursery plants, be sure to examine plants carefully before purchase and purchase only plants that appear healthy. It is always advisable to purchase plants from a reputable vendor. Do not plant woody ornamentals in sites with poor soil drainage or where water tends to collect during heavy rains, since overly wet soil favors sporulation, movement of spores and infection by Phytophthora. Also, avoid over-irrigating plants. Fungicides labeled for control of Phytophthora can be used preventatively for Phytophthora root rot, but repeated applications will be necessary and fungicides will not be effective in locations with chronically wet soil conditions.