Fusarium wilt—Fusarium oxysporum
Various host-specific forms of Fusarium oxysporum can kill infected plants. Susceptible woody ornamentals include albizia (mimosa), certain cacti (e.g., prickly pear and saguaro), date palm, hebe, and pyracantha. Herbaceous ornamental hosts include aster, carnation, chrysanthemum, cyclamen, daffodil, dahlia, freesia, and gladiolus. Other hosts include asparagus, bean, cabbage, cantaloupe, pea, tomato, and watermelon.
Identification
Symptoms commonly appear first on one side of a plant. Fusarium wilt causes foliage to yellow, wilt, then turn brown and die. Older leaves generally discolor, wilt, and die first, followed by death of the entire plant. Cutting into infected vascular tissue (e.g., wood xylem) can reveal that conducting tissue has turned brown, commonly all the way from the terminal shoot to the soil line. Cross-sections of basal stems may reveal brown rings. Masses of spore-bearing stalks are sometimes visible on dead tissue and may look like small pink cushions.
Symptoms of Fusarium wilt and Verticillium wilt can be indistinguishable. Determining which is the cause of damage may require submitting samples to a plant diagnostic laboratory that cultures the fungus for positive identification.
Life cycle
Fusarium hyphae (vegetative growth structures) germinate from long-lasting survival structures (chlamydospores) in the soil. The fungus infects plants through rootlets. It invades the xylem and eventually spreads throughout the plant's vascular system. Pathogen spread within and between plants is favored by warm temperatures.
Fusarium has many special forms (called forma specialis, or f. sp.) or subspecies, each specific to certain plant hosts. Fusarium does not spread from one host to infect plants in other genera. However, those other genera may be susceptible to different Fusarium f. sp. Most forms of F. oxysporum attack only herbaceous ornamentals and vegetable crops.
Solutions
There is no effective fungicide or other cure for Fusarium wilt. The pathogen nearly always kills infected hosts. Prevention and exclusion are the only effective management strategies.
Avoid this problem by replanting at that site using species from different genera than plants previously infected there by Fusarium. Fusarium propagules persist in soil and will cause disease if the same plant genus is replanted at that site.
Grow cultivars resistant to Fusarium if available for that plant species. For example "FNV" on a plant label means the plant is resistant to Fusarium (F), nematodes (N), and Verticillium (V). Obtain new plants from a reliable, high-quality source.
Avoid planting in poorly drained soil; planting on mounds or raised beds is one remedy for this. Provide plants a good growing environment and proper cultural care. Especially do not overirrigate as this encourages shallow, surface rooting and shallow roots are more easily wounded and infected.
Avoid using undecayed organic amendments around Fusarium hosts. Avoid excessive fertilization especially with urea, which may promote development of Fusarium wilt. Keep mulch, other debris, and other plants away from the base of hosts, especially palms, because nearby plants and their management can wound palm roots. Promptly remove and dispose of Fusarium-infected plants to reduce pathogen spread to nearby hosts. Soil solarization before planting may be effective in reducing or delaying infection of subsequently planted hosts.
Adapted from Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM). |
Yellowed fronds of date palm with Fusarium wilt.
Browning of vascular tissue of freesia bulbs with Fusarium wilt.
Stems cut into revealing vascular discoloration due to a Fusarium sp. (right) compared with a healthy stem.
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