How to Manage Pests
UC Pest Management Guidelines
Symptoms include yellowing and stunting of older plants and yellowing, stunting, and death of seedlings. 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. Thus, the fungus that causes wilt of carnations is Fusarium oxysporum f. sp. dianthi, specific only to 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 use of plant breeding in developing resistant cultivars.
There are many saprophytic forms of F. oxysporum and recovery of the fungus from diseased plant material does not guarantee that a wilt fungus is present. 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.
The fungus produces two kinds of spores. Chlamydospores are resistant to drying and adverse conditions, and enable the fungus to survive extended periods in soil. Conidia are produced in a sporodochium, which is a mass of conidiophores (conidia-bearing stalks) placed tightly together. Sporodochia are sometimes visible as small, pink-to-orange cushions on dead tissue as well as along hyphae. Conidia are spread by splashing water and can contaminate tools and hands. There are two types of conidia: macroconidia (large, multi-celled spores) and microconidia (small, one-celled spores). Conidia generally are not airborne, but the fungus can become airborne in bits of infected plant debris, in dust, or in splashing water.
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) and disease may not occur at low soil temperatures (below 68°F). An infected plant may remain symptomless at lower temperatures. The fungus can be spread through the use of 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. It is always prudent to treat seed with a fungicide or heat to destroy the fungus on the seed and to protect the emerging seedlings from infection.
Fungus populations can be reduced from soil by heat treatments and by chemical fumigation. These treatments, while effective in controlling the fungus in annual plantings, leave much to be desired in perennial plantings. In field crops, Fusarium wilt diseases are controlled by selection and plant breeding. In perennial ornamental crops such as carnations, the best way to deal with the disease is through the use of resistant cultivars; however, the most popular cultivars are not resistant. Liming soils and using nitrate nitrogen fertilizer have been effective for management of Fusarium oxysporum on chrysanthemum, aster, gladiolus, cucumber, tomato, and watermelon.
UC IPM Pest Management Guidelines: Floriculture and Ornamental Nurseries