Root Rot on Phlox


The versatile phlox plant, which provides a splash of color to a variety of home gardening spaces, is just as susceptible to problems as it is beautiful. Under the threat of destructive root rot pathogens, root rot infections result in cosmetically demolished plants as well as underground problems that lead to the loss of your flowers. Avoid home garden injury through attentive care.

Care of Phlox

Vigorous plants are known for their ability to fight off infection with more success than weakened garden plants. Grow phlox according to the particular species in your home garden. Garden phlox (Phlox paniculata), for example, thrives in locations that offer full sun and moist, well-drained soil rich in organic content, according to the University of Illinois Extension HortAnswers. Moss phlox (Phlox subulata) is similar in its need for full sun exposure and well-draining soil. However, this phlox species prefers moist, sandy soil, according to the Michigan State University Extension. Care is often nearly identical, but paying attention to each detail ensures the well-being of your phlox plant.

Root Rot Causes

Black root rot is a primary invader of phlox plants, caused by the fungal pathogen Thielaviopsis basicola. Black root rot pathogens are soil-borne and invade the roots of your phlox, according to the Michigan State University Integrated Pest Management. Pathogens favor neutral to alkaline wet soil with pH levels of 6 to 7 as well as cool temperatures.

Symptoms and Prognosis

Phlox black root rot leads to problems both above ground, as well as underground. Above ground, look for yellowing foliage as well as overall diminished health and stunted growth. As roots rot, they blacken, as the name suggests, and become incapable of necessary nutrient and water uptake. This disease leads to plant death, according to the Michigan State University IPM.


Avoid planting other susceptible plants when soil-borne pathogens have infected your site. When replanting where dead phlox has been removed, do not replant with another phlox or susceptible selection. Vulnerable plants to avoid include, but are not limited to, hollies (Ilex species), geraniums (Geranium species) and pansies (Viola species), according to the Michigan State University IPM.


For prevention of phlox black root rot, use uncontaminated soil and disease-free plants. Avoid re-using containers or only re-use after sterilizing to prevent disease spread. Consider the addition of organic mulch to your soil to keep temperatures cool and to decrease the pH of alkaline soil. For chemical control, fungicidal drenches also offer preventive control but do not cure root rot. Select a chemical with the active ingredient thiophanate-methyl, according to the Michigan State University IPM. When in need of additional assistance, contact your local county extension agent for professional advice.

Keywords: phlox root rot, black root rot, phlox plant rot

About this Author

Tarah Damask's writing career, beginning in 2003, includes experience as a fashion writer/editor for Neiman Marcus, short fiction publications in "North Texas Review," a self-published novel, band biographies, charter school curriculum, and articles for eHow. She has a love for words and is an avid observer. Damask holds a Master of Arts in English and creative writing from the University of North Texas.