Black Root Rot in Pansies

While each disease can seriously compromise the health and vigor of the pansy crop, Thielaviopsis basicola, or Black Root Rot, tends to be the most serious pathogen because it can be extremely difficult to control once introduced into the greenhouse environment.

When root diseases occur in pansies, the infection can usually be linked to either over-watering or poorly drained media. Root rot diseases are soil-borne, and they will survive in substrates and on organic debris found on greenhouse floors, benches, and/or reused or unclean containers. Root rot diseases can also be introduced into the greenhouse environment on worker shoes, clothing, gloves, tools, hoses, workers’ hands, infected plugs, and on potting-mix bags that are brought into the production area.

Pansies infected by a root rot disease often take on a purplish hue or may appear chlorotic. Severely infected pansies will appear weak, stunted, or wilted. To diagnose a root rot disease, remove the container and examine the roots. If the root systems appear gray, black, or brown, there is a high probability that a root rot disease is present.

Black root rot (Thielaviopsis basicola) is the most serious root rot disease observed in pansies. Symptoms of black root rot infection include wilting and stunting of the affected pansy plants and leaf yellowing (chlorosis). Roots infected with black root rot develop dark spots or bands on the white pansy roots. Symptoms of early infection in pansies are largely confined to the tips of secondary feeder roots. As the disease progresses, the entire pansy root system will become water-soaked and black in appearance. Black root tends to be the more severe when the substrate pH is over 6.0, and the air temperatures are high.

To prevent black root rot infection in your pansy crop, growers should always practice good sanitation, use only new soilless pathogen-free potting media, and use only new containers when transplanting and growing on your pansy crop. Pansies are also very sensitive to environmental stress. Growers should avoid over-fertilization, moisture stress, and heat stress when raising pansies.

Fertilizers high in ammoniacal nitrogen should also be avoided when growing pansies. Pansies prefer a substrate pH between 5.4 to 5.8. Greenhouse operations that deal with water alkalinity issues may see their substrate pH creep up over time. Consider checking the substrate pH weekly and take immediate action if the pH exceeds 5.8.

Fungicides labeled for use on pansies to manage black root rot include:

• etridiazole + thiophanate methyl (Banrot 40 WP) *Not as effective as stand-alone thiophanate methyl products on Black root rot
• fludioxinil (Medallion WDG)
• polyoxin D zinc salt (Affirm WDG)
• thiophanate-methyl (3336 DG Lite, 3336 F, 3336 EG, OHP 6672 50 WP)
• Trichoderma asperellum + Trichoderma gamesii (Obtego)
• Trichoderma harzianum Rifai T-22 (RootShield G)
• Trichoderma harzianum Rifai T-22 + Trichoderma virens G-41 (RootShield Plus G, RootShield Plus WP)
• triflumizole (Terraguard SC)