Cover Story
Phytophthora Root and Crown Rot of Boxwood By Dr. Fulya Baysal-Gurel, Assistant Professor of Plant Pathology
Tennessee State University Otis L. Floyd Nursery Research Center Ornamental Pathology Program McMinnville, TN 37110 fbaysalg@tnstate.edu
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hytophthora root and crown rot of boxwood is caused by several species of Oomycetes (known as water molds). Some of the more frequently encountered Phytophthora species include P. nicotianae, P. cinnamomi, P. citrophthora and P. occultans. Many plant species, including American (common), Japanese, and English boxwood, are susceptible to this disease. The pathogen can infect all growth stages of field and container grown boxwood plants. Warm, humid and rainy conditions favor disease development. Phytophthora spp. can survive in soil or potting substrate as resting spores (oospores, chlamydospores), and inside infected plant tissues as mycelium for long periods. Meantime, these resting spores are available to infect other healthy boxwoods or other host plants. When the environmental conditions are favorable, chlamydospores and oospores produce new mycelium or sporangia. Sporangia release zoospores into soil, and the zoospores swim toward healthy roots through saturated soil or irrigation water. The general symptoms presented by infected plants include wilting, stunting, leaf shed, limb dieback, leaf chlorosis, off-color foliage, crown rot (gray to brown color) (Figures 1–4) and root rot (roots appear brown and water-soaked) (Figure 2). Depending on the disease progression in roots, foliar symptoms may be observed on a few branches or the entire plant (Figure 3).
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tennessee greentimes SPRING 2020
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Figure 1. Left: Symptoms of boxwood Phytophthora crown rot. Right: Dark lesions beneath the bark may also be apparent outside infected stems (Right photo credit, Elizabeth Bush, Virginia Polytechnic Institute and State University, Bugwood.org)
