When fungi or bacteria enter the roots and stems of a plant and cause stunting, wilt and death by plugging the vascular system, this is called a "vascular wilt." Infected plants wilt and die. If you cut into the stem, the vascular tissues show discoloration as tan, reddish, or dark streaking. Several fungi and bacteria may cause vascular wilts in herbaceous plants. The primary means of introducing these pathogens is by purchasing infected plants that are not showing symptoms.
Fungal wilts
Fungal wilts are caused by species of Fusarium, Verticillium and Phytophthora. The fungus grows into the roots of a susceptible plant, and eventually grows up into the stem. If you cut into the stem, you can see a brown, olive green or sometimes reddish brown streaking in the vascular tissues just under the stem epidermis. Fusarium wilts tend to be very host specific, that is the Fusarium that attacks one species of plant is unlikely to cause disease on other species in that genus, or other plants not closely related. In contrast, Verticillium and Phytophthora tend to have broader host ranges. Fusarium and Verticillium are favored by droughty conditions while Phytophthora is a "water mold' and it is favored by frequent irrigation and wet, slow draining soils. When the soil remains saturated, it stimulates the Phytophthora fungus to produce many infective swimming spores (zoospores), causing more severe disease.
Fusarium wilt
Often attacks Astilbe, Dendranthema (garden chrysanthemum), Dicentra (bleeding-heart), Echinacea (cone flower), Lupinus (lupine), Nepeta (catnip), Ocimum (basil), and Sedum.
Verticillium wilt
Verticillium wilt often attacks Aconitum (monkshood), Dahlia, Liatris (gayfeather), Paeonia (peony), Papaver (poppy), Phlox, Rudbeckia (black-eyed-Susan), and Salvia. Phytophthora often attacks Euphorbia (spurge), Lavendula (lavender), Sedum, and Vinca.
Management
These fungi remain in the soil for many years. Once one of these diseases appears in a particular growing area, you must not plant the susceptible plant or take other cultural measures to reduce future losses. For Fusarium, which is usually highly host specific, the control can be a simple rotation away from the plant that was damaged. Verticillium has a broader host range and so presents a more difficult problem in selecting "non-susceptible" plants for rotation. Phytophthora is so strongly influenced by drainage that often construction of raised beds and incorporation of organic soil amendments to improve soil drainage will reduce disease and permit replanting with a susceptible crop.
Bacterial wilts
Bacterial wilts are much less common on herbaceous perennials than fungal wilts. Several genera of bacteria including Erwinia and Pseudomonas cause bacterial wilt in herbaceous perennials. Dendranthema (garden Chrysanthemum) and other composites, and Begonia are sometimes infected. They are usually introduced in infected plants and can be carried in/on infected seed. They seldom persist for longer than one growing season in soils in temperate regions. Bacterial wilts can be diagnosed by examination of the vascular tissues for bacterial streaming, and by isolation in culture. These procedures are done in a diagnostic laboratory. Very thin slices of the vascular tissue are examined with a compound microscope at a magnification of 400X, and the bacteria can be seen oozing or streaming from the vascular tissues. Specific identification of the bacteria may require multiple cultures, analysis of the bacterial fatty acids with gas chromatography, or serological tests.
Management
The best solution for bacterial wilts is to remove infected plants.