Over 300 species of flowering plants belong to the Dianthus genus. Commonly called dianthus, Sweet William and carnations, these plants appear in gardens throughout the United States and are known for their colourful ornamental flowers. Dianthus suffers a handful of problems, among them pest insects such as aphids and bacterial plant diseases caused by pathogens such as Alternaria and Fusarium. Expect resources offer advice with regards to managing these problems and maintaining health dianthus specimens.
Common diseases in Dianthus species include leaf spot, bacterial blight, bacterial spot, stem rot, branch blight and Fusarium wilt. Leaf spot caused by Alternaria pathogens result in brown spots on leaves. These spots eventually kill affected leaves. Fusarium wilt stunts growth and causes yellowing of new growth. Stem rot, often caused by the Rhizoctonia fungus, rots the plant stem just below the soil line and may also affect the roots of a plant. This disease spreads through soil and is impossible to eradicate once it affects a plant. Branch blight causes ashy spots on leaves and rots the base of leaves.
Managing Dianthus Diseases
Dianthus diseases caused by Fusarium and Rhizoctonia can be managed biologically and chemically. Biological management entails introducing a biological agent known to kill the disease-causing pathogen but not the plant. Fusarium and Rhizoctonia are susceptible to Steptomyces lydicus and Streptomyces griseoviridis and Trichoderma harzianum and Trichoderma virens. Chemical management entails the use of fungicides. Always use fungicides in exact accordance with instructions so as to not risk damage to plants. Stem rot may prove impossible to combat; remove severely affected plants from the soil, destroy them and test the soil for remnants of the disease before planting new specimens. Michigan State University Extension recommends removing and burning all plants affected by bacterial spot as well.
Aphids and spider mites are the most common dianthus problem pests. Small, pear-shaped insect, aphids appear in shades of green, yellow, blue-green, white, black, red and grey. These insects pierce leaves so that they can suck the sap out of them. Some species of aphids inject poison into plants while feeding. Aphids stunt plant growth and cause deformities on leaves and fruit and abnormal growths on stems, roots and leaves. Spider mites remove cells from plants for feeding; these leaves eventually fall. Plants heavily infested with spider mites become discoloured, stunted and sometimes die. These small insects resemble spiders and are generally red.
Managing Dianthus Pests
Ideal spider mite management occurs before the insects inflict damage on a plant. Regular monitoring helps gardeners spot spider mites before they do damage. To rid a plant of spider mites, wash it thoroughly with hard hose spray. To prevent aphid infestation, plant dianthus plants far enough way from one another than air circulates between specimens without the plants touching and by building screens around plants. Also avoid excessive use of nitrogen. If aphid infestation occurs, biologically manage insects by introducing predators or parasites such as ladybirds, parasitic wasps of the Aphidiidae genus, green lacewing and minute pirate bugs. Aphids are largely impervious to chemical management.
Rabbits may eat dianthus species found in the garden. Building a small mesh screen around the base of a plant and a few inches into the soil around the roots helps prevent rabbit damage. Deer don't often eat dianthus species though in periods of food scarcity eat any plant material they can find. Gardeners experiencing trouble with deer in the yard should consider erecting a large fence around the garden -- at least 6 to 8 feet tall -- to prevent deer from eating and destroying plants.