Home & Garden Tips
Root Rot Management for Annuals
By Roosevelt McWilliams County Extension Agent

Fungal root rot disease on annuals such as petunia, geranium and other annual flowers has been a common diagnosis on annuals that I have seen over the last month.

Symptoms of a plant with root rot are stunted growth, marginal leaf burn, and wilting, yellowing and general decline. These symptoms can be caused by other root problems such as nematodes, drought, low fertility and/or improper planting so a plant will have to be removed from the soil and examined to diagnose the problem.

The root system of affected plants is usually smaller than normal. Roots are fragile, they break easily when touched. Often the outer portion (cortex) of the root will strip easily away from the inner portion (stele).

Depending on the pathogen involved the color of the roots may also be diagnostic. Roots are usually a pale gray or honey color if infected with Pythium SP, tan to brown if infected with Rhizoctinia SP, or streaked with black if with Thielaviopsis SP.

If a root rot is suspected these diseases are generally favored by over watering, poor drainage and sometimes-high levels of fertilizer. Once a plant is displaying advanced symptoms it should be removed. These plants cannot be cured. The control for root rot should be preventative measures first.

Proper planting by breaking up the root ball and planting into a well-prepared soil reduces the likelihood of root rot disease. Incorporating composted pine bark mulch can improve drainage in wet areas and reduce the incidence of root rot disease.

Elevating the grade of the bed by adding additional soil is helpful. Watering practices are also important. Irrigation should be thorough but as infrequent as possible to provide adequate moisture. Avoid frequent shallow irrigation that continually keeps an area wet. Avoid rotations of annuals that favor disease buildups.

Depending on the fungal pathogen involved fungicides may be of some benefit to reduce inoculums and protect healthy plants from infection. However, they must be applied at transplanting and are not effective to cure existing plants already in decline. Root rot diseases are best controlled through good cultural practices and managing growing conditions not to favor root disease.