P&PDL Picture of the Week for
November 2, 2009

Dianthus Anthracnose

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 bed.

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.

Purdue Plant & Pest Diagnostic Lab Purdue Cooperative Extension Service