This is a question we have been asked frequently this fall. Though every situation is different, we’ll try to explain the most common causes for pansies to die in the fall. Note that though we say pansies, the same applies to violas.
So, we’ve had a pretty wet fall here in the Triad, with significantly more than average rainfall September through November. While above-average rainfall can make some plants very happy (fescue lawns, particularly those reseeded this fall, are doing very well, thank you very much), some plants can be susceptible to disease when there’s extra moisture.
One of those plants is pansies. While they appreciate even moisture, excessive rainfall (or irrigation) can cause several plant diseases that are commonly present in the soil or in the air to become active. Many diseases slow or stop during freezing weather, so during periods of unseasonably warm weather the disease cycle continues much longer than it would. Seemingly healthy plants can suddenly decline and die rapidly. Here are the symptoms for the most common and destructive diseases of pansies in the landscape.
Black Root Rot (Thielaviopsis) destroys the roots, initially causing stunting, yellowing and wilting of the plant. The disease often progresses enough to kill the plant. Root tips, then eventually the entire root system of affected plants turn black. The spores are widespread in the soil and can be spread by splashing raindrops.
Botrytis blight causes flowers to rot, often covered in grey, fuzzy or webby mold. In wet weather, the rot can spread to seemingly healthy flowers and leaves. It can eventually affect a large portion of the plant. Botrytis is less of a problem when pansies are regularly deadheaded as the airborne spores (or occasionally water-borne) often gain an initial foothold on dead flowers.
Crown rot (Phytophthora) is generally active in warmer weather and can cause plant collapse in pansies. The fungus can attack the crown just above the soil line or cause a leaf-spot disease. The plant will begin to collapse, and can often be easily separated from the roots with little effort. It’s not generally the problem in late fall when the weather is cooler, but it should be suspected when pansies fail in warmer wet weather. Spores are spread both by splashing water droplets and through water movement in the soil.
Depending on the conditions, you may lose just one or two plants, or larger areas may be affected.
Unfortunately there is little treatment for any of these fungal diseases once they are noticed. Providing good air movement by not overcrowding when planting, removal of dead leaves and flowers, and taking care not to mulch deeply around the plants can reduce the risk somewhat. Diseased plants should be removed carefully to minimize spreading disease spores and bagged and disposed of as trash.