Tom Creswell, Department of Botany & Plant
Pathology, Purdue University
Dianthus is a much-admired perennial but occasionally it develops
serious disease problems. The most dramatic of these is anthracnose,
caused by the fungus Colletotrichum.
The disease may spread rapidly on greenhouse
benches or in landscape plantings of susceptible dianthus varieties.
The old-fashioned Sweet William varieties plantings don’t
seem to be affected by anthracnose.
Symptoms begin as irregular spotting and blighting
of lower leaves, which later turn brown. All lower leaves may turn
brown and, if left unchecked, the infection may move into the main
stem, causing collapse of the entire plant. Infected leaves and stems
later show characteristic black spots as the fungus reproduces. Spores
of the fungus are spread by splashing water and by handling operations
in the greenhouse or by wind and splashing water in the landscape
Non-chemical controls include removing diseased material and avoiding
splashing water. Irrigate using a drip system in the landscape bed
to reduce leaf wetness. Fungicides may be necessary in some cases.
Click image to enlarge
Figure 1: Dianthus plants showing browning and spotting of lower
leaves caused by anthracnose.
Figure 2: Heavily infected plants have tan to brown discoloration of lower
leaves with black dots.
Figure 3: In advanced stages the fungus invades the main stem.
Figure 4: On closer examination the black “dots” show small black
needle like structures surrounded by a mass of spores. These needles are called
setae and are characteristic of many of most fungi in the genus Colletotrichum.
Figure 5: A microscopic view of a single spore mass shows the crecent moon-shaped
spores which spread to other plants to cause new infections.