Fusarium Wilt

Symptoms include yellowing, stunting, and death of seedlings and yellowing and stunting of older plants. Infected plants wilt readily, lower leaves yellow and dry, the xylem tissues turn brown, and the plant may die. In the early stages of disease, the roots are not rotted. In many plants such as carnation and gladiolus, the symptoms may be one-sided at first.

The fungi that cause Fusarium wilt diseases are composed of a group of host-specific forms (forma specialis) abbreviated f. sp. For example, the fungus that causes wilt of carnations is Fusarium oxysporum f. sp. dianthi and infects only carnations and closely related plants. Generally, the f. sp. relates to the host; e.g., callistephi (China aster), pisi (pea), cyclaminis (cyclamen), etc.

Within the specialized forms are races of the fungus that are characterized by specialization on different cultivars of a host species. Cultivar A may be susceptible to race 1 and resistant to race 2, while cultivar B may be susceptible to both race 1 and race 2. This complicates the grower's task of selecting which cultivars to grow, as well as making it more difficult to select a breeding strategy for developing resistant cultivars.

There are many saprophytic forms of F. oxysporum, and recovery of this fungus from diseased plant material does not mean that the isolate is a wilt pathogen. For example, it is quite common to recover a saprophytic F. oxysporum from the roots of chrysanthemum plants killed by Pythium spp. or other pathogens. There are also strains of F. oxysporum that cause root rots but are not wilt pathogens.

The fungus can produce several different kinds of spores. Chlamydospores have thick walls and are resistant to drying and adverse conditions, enabling the fungus to survive for extended periods (years) in the soil. Conidia are thin–walled spores that can be either long and multi-celled (macroconidia) or short and only one- or two-celled (microconidia). Conidia are produced in a sporodochium, which is a mass of conidiophores (conidia-bearing stalks) growing tightly together. However, sporodochia are rarely seen in Fusarium wilt diseases. Conidia are spread in contaminated soil, by splashing water, and on contaminated tools and hands. Conidia are generally not airborne, but the fungus can become airborne on bits of infected plant debris or in dust.

In the presence of roots, chlamydospores or conidia germinate and penetrate susceptible plants. The fungus enters the xylem and grows upward, plugging the tissue and reducing the movement of water. Toxins are produced that cause the foliage to turn yellow.

Fusarium wilts are favored by high air and soil temperatures (75° to 86°F). Disease may not occur at low soil temperatures (below 68°F), or an infected plant may remain symptomless at lower temperatures. The fungus can be spread with infected cuttings or other forms of vegetative propagation taken from healthy appearing but infected plants.

If seed is taken from infected plants, the seed itself is usually healthy, but the seed coat often becomes contaminated by microscopic pieces of infected tissue and by spores. Many important Fusarium wilt diseases are spread in this manner.

• Treat seed with a fungicide or heat to destroy the fungus on the seed and to protect the emerging seedlings from infection.
• Dip bulbs and corms in fungicide or hot water (or both) to reduce Fusarium.

Presence of pathogenic Fusarium in the soil can be reduced by heat treatments and chemical fumigation. These treatments are more effective in controlling the fungus in annual plantings than in perennial plantings. In general, however, Fusarium wilt diseases are best controlled by using resistant or tolerant cultivars, not by using soil applied fungicides. Liming soils and using nitrate nitrogen fertilizer have been effective for management of F. oxysporum on chrysanthemum, aster, gladiolus, cucumber, tomato, and watermelon. In greenhouse production use steam from boilers and apply to raised field beds or raised benches to control soil borne diseases such as Fusarium wilt. In field production use long rotations or try to follow a fumigated crop like strawberry.