Ag Notes: Diseases can wipe out flowers

June 01, 2004|JERRY LITTLE

Annuals and herbaceous perennials transplanted into flower beds add color to many Kentucky landscapes. Unfortunately, when these plants fail to make good growth or die in the landscape, time and money are wasted and the anticipation of colorful splendor is lost.

Last summer, some beds of petunias, impatiens, vinca, geranium, begonia, and many other bedding plants died out due to one or more root and stem rot diseases. Most parts of Kentucky had excess rainfall in late spring and early summer last year, just after flower beds were established, thus contributing to loss of plants in the beds.

Root and stem rot pathogens, including Rhizoctonia, Thielaviopsis, Phytophthora, and Pythium were often involved, but sometimes abiotic influences made the disease problems worse. Although root rot fungi can be carried from the greenhouse to the landscape, some already living in the bed soil can attack healthy transplants.

Root rot is a general or localized root decay, whether from infectious or non-infectious causes or from natural aging. Root rot begins when cortical cells (outer tissue of the root) become non-functional or die.


As cell death continues, the root becomes discolored brown to black and appears decayed.

Warm, humid weather and warm soils favor Rhizoctonia root and stem rot, which causes affected plants to turn yellow, wilt, and die in the landscape. Plant roots and crowns are infected and decayed by the fungus Rhizoctonis, a common soil-inhabiting pathogen.

A combination of cultural practices and avoidance of susceptible plants will be needed to reduce root and stem rot diseases in annual flower beds.

* Establish annual flowers in beds where good growing conditions can be provided.

* Avoid flower bed sites that are poorly drained, contain excess salts, or were subject to devastating disease in prior years.

* Replace or leach out contaminated soil, if necessary (soils with a prior history of severe black root rot or with excess de-icing salts or other toxic contaminants).

* Use only disease and pathogen-free plants.

* Rotate crops in planting beds in the flower garden.

* Where black root rot or Phytophthora root rot have been involved, use tolerant or resistant plants, if possible.

* Remove and destroy diseased plants as they appear in flower beds.

* Avoid unnecessary stresses in growing the plants.

* Although there are many chemicals that are effective in producing healthy bedding plants in the greenhouse, chemical drenches in outdoor beds are mostly ineffective and impractical.

* At the end of the growing season, remove and destroy all plants for purposes of sanitation.

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