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Brown vascular discoloration due to Fusarium oxysporum.

Floriculture and Ornamental Nurseries

Fusarium Wilt

Pathogen: Fusarium oxysporum

(Reviewed 3/09, updated 3/09)

In this Guideline:


SYMPTOMS AND SIGNS

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.

COMMENTS ON THE DISEASE

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.

MANAGEMENT

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.

Common name Amount to Use R.E.I.+
(trade name)   (hours)

  Calculate impact of pesticide on air quality
When choosing a fungicide, consider the general properties of the fungicide as well as information relating to environmental impact.
 
SOIL FUMIGATION
(Note: The 2007 Critical Use Exemption List allows fumigation with methyl bromide for moderate-to-severe disease infestation.)
A. METHYL BROMIDE* Label rates 48
  COMMENTS: Allowed for use under a Critical Use Exemption only. Methyl bromide is a gas at temperatures over 40°F. It escapes rapidly from soil if not applied under a gas‑proof cover. Polyethylene sheeting is commonly used to confine methyl bromide, although the gas does slowly escape through polyethylene. Diffuses rapidly through the soil and kills many kinds of organisms, weeds, and many seeds. Soil generally can be planted a few days after removal of plastic covers, although there are exceptions. A few plants such as Allium spp., carnations, and snapdragons are sensitive to and may be damaged by inorganic bromide that remains in the soil following fumigation. Leaching the soil with water before planting is helpful in reducing the amount of bromide in the rooting area. Fumigants such as methyl bromide are a source of volatile organic compounds (VOCs) but are not reactive with other air contaminants that form ozone: methyl bromide depletes ozone. Fumigate only as a last resort when other management strategies have not been successful or are not available.
 
B. METHYL BROMIDE*/CHLOROPICRIN* Label rates 48
  COMMENTS: Inject into soil and cover immediately with plastic tarps. See Comments above regarding the use of methyl bromide, which is allowable for use under a Critical Use Exemption only. Fumigants such as methyl bromide are a source of volatile organic compounds (VOCs) but are not reactive with other air contaminants that form ozone: methyl bromide depletes ozone. Fumigate only as a last resort when other management strategies have not been successful or are not available.
 
SOIL FUNGICIDE
A. THIOPHANATE-METHYL
  (FungoFlo, etc.) 20 fl oz/100 gal water 12
  MODE OF ACTION GROUP NAME (NUMBER1): Methyl benzimidazole (1)
  COMMENTS: Apply as a drench at planting. Not very effective in controlling Fusarium.
 
B. STREPTOMYCES GRISEOVIRIDIS#
  (Mycostop) 0.08 oz/lb of seed or  
    5g/kg of seed 4
  COMMENTS: Good for Fusarium wilt on asters. May be used on both field-grown and greenhouse plants.
 
C. IPRODIONE
  (Chipco 26019) 0.4 lb/100 gal water 12
  MODE OF ACTION GROUP NAME (NUMBER1): Dicarboximide (2)
  COMMENTS: Apply as a drench (1–2 pt/sq ft) at seeding or transplanting.
 
* Permit required from county agricultural commissioner for purchase or use.
1 Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions (for more information, see http://www.frac.info/). Fungicides with a different group number are suitable to alternate in a resistance management program. In California, make no more than one application of fungicides with mode of action Group numbers 1, 4, 9, 11, or 17 before rotating to a fungicide with a different mode of action Group number; for fungicides with other Group numbers, make no more than two consecutive applications before rotating to fungicide with a different mode of action Group number.
# Acceptable for use on organically grown ornamentals.
+ Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing.

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Floriculture and Ornamental Nurseries
UC ANR Publication 3392
Diseases
S. T. Koike, UC Cooperative Extension Monterey County
C. A. Wilen, UC IPM Program, UC Cooperative Extension San Diego County
Acknowledgment for contributions to Diseases:
R. D. Raabe, (emeritus) Environmental Science, Policy, and Management (ESPM), UC Berkeley
A. H. McCain, (emeritus) Environmental Science, Policy, and Management (ESPM), UC Berkeley
M. E. Grebus, Plant Pathology, UC Riverside

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