Symptoms and Signs
Symptoms of Fusarium wilt in strawberries consist of wilting of foliage, plant stunting, and drying and death of older leaves, while the youngest leaves in the center of the plant often remain green and alive. Symptoms usually first appear well after plants are established. Plants bearing heavy fruit loads or subjected to stress often show the most severe symptoms. Plants can eventually collapse and die completely. When internal tissues of plant crowns are examined, vascular and cortical tissues are dark to orange-brown. Internal tissues of the main roots are typically not discolored.
Comments on the Disease
Fusarium wilt is often most severe if the infected plant is subject to stresses due to weather extremes, deficiency or excess of water, poor soil conditions, or heavy fruit loads. Note that foliar dieback and internal crown discoloration symptoms are identical to those caused by Macrophomina crown rot. Therefore, confirmation of Fusarium wilt requires diagnostic procedures in a pathology lab. This soilborne fungus only attacks strawberry and will not cause disease on other crops. The fungus persists in the soil in the form of small survival structures called chlamydospores, which can last years in the soil in the absence of a host.
Select locations that do not have a history of Fusarium wilt. Preplant fumigation, which historically has been an important component of managing Verticillium wilt in strawberry fields, will also help control Fusarium. Fumigation will be most effective when crop residues have fully decomposed. Consequently, it is advisable to allow some time after incorporation before the fumigant is applied. Shank application of fumigants such as chloropicrin or chloropicrin plus 1,3 dichloropropene (Pic-Clor 60) applied at high rates under retentive film can control fungal pathogens, such Fusarium and Macrophomina (though only chloropicrin affects the fungus). Bed fumigation will not control pathogens in the untreated furrows.
Crop rotation with broccoli has been shown to help reduce Verticillium levels in the soil; the practice of crop rotation may also help manage Fusarium; it has not yet been thoroughly researched but is under current investigation.
Field tests have shown that cultivars such as Fronteras, Portola, and San Andreas are resistant to Fusarium wilt, whereas Albion and Monterey are susceptible. Additional research is being conducted to develop new resistant cultivars.
Manage the strawberry crop so as to reduce stress on the plants. Irrigate the crop as appropriate for the stage of development, current evapotranspiration requirement, and soil moisture levels. Control pests, especially mites, which can exert significant stress on strawberry plants.
Rotating strawberries with broccoli can significantly reduce levels of the Verticillium pathogen in the soil. While not yet tested with Fusarium, crop rotations may also be useful.
Organically Acceptable Methods
Select fields that do not have a history of Fusarium wilt. Plant resistant cultivars. Rotate with any non-strawberry crop or with crops, which have the capacity to suppress the pathogen. Avoid stressing the plants.
Drip fumigation of pre-plant fumigants may not effectively control soilborne pathogens in the entire bed; field research has demonstrated pathogen survival at the bed shoulders and in soil profiles deeper than 12 inches. Use of this application method should be considered carefully. Growers may improve fumigant distribution by using more than two drip lines per bed and by applying larger volumes of water to deliver the fumigants. Bed fumigation will not control these pathogens in the untreated furrows.
|Common name||Amount per acre**||REI‡||PHI‡|
|(Example trade name)||(hours)||(days)|
|Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least likely to cause resistance are at the top of the table. When choosing a pesticide, consider information relating to the pesticide's properties and application timing, honey bees, and environmental impact. Always read the label of the product being used.|
|Note: Fumigants such as 1,3-dichloropropene and metam products are a source of volatile organic compounds (VOCs) but minimally reactive with other air contaminants that form ozone.|
|(Tri-Con 50/50)||See label||See label||0|
|COMMENTS: Methyl bromide use is allowed only in strawberry nurseries through the quarantine and pre-shipment (QPS) exemption. 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.|
|First, apply one of the following|
|(Telone C35)||Label rates||See label||0|
|(InLine)||Label rates (drip)||See label||0|
|COMMENTS: Effective for control of nematodes, soilborne fungal pathogens, and insects. InLine requires a plastic tarp. Use higher rates or impermeable films to improve weed and nematode control. According to state permit conditions, the maximum application rate of 1,3-dichloropropene is 332 pounds active ingredient per acre. One gallon of product weighs 11.2 lb.|
|. . . or . . .|
|(Tri-Clor)||150–350 lb (shank)||See label||0|
|(Tri-Clor EC)||200–300 lb (drip)||See label||0|
|COMMENTS: A liquid that diffuses as a gas through soil. Very effective for control of soilborne fungal pathogens and insects. Drip irrigation requires an emulsifier. For shank fumigation, use higher rates or impermeable films to improve weed and nematode control. For drip fumigation the use of TIF will improve both nematode and weed control. Tri-Clor: One gallon of product weighs 13.88 lb; Tri-Clor EC: One gallon of product weighs 13.46 lb.|
|. . . or . . .|
|(Pic-Clor 60)||300–332 lb (shank)||See label||0|
|(Pic-Clor 60 EC)||200–300 lb||See label||0|
|COMMENTS: Very effective for control of soilborne fungal pathogens and insects. Drip irrigation requires an emulsifier. For shank fumigation, use higher rates or impermeable films to improve weed and nematode control. For drip fumigation the use of TIF will improve both nematode and weed control. According to state permit conditions, the maximum application rate of 1,3-dichloropropene is 332 pounds active ingredient per acre. Pic-Clor 60: One gallon of weighs 12.1 lb; Pic-Clor 60 EC: One gallon of weighs 11.8 lb..|
|Then, 5-7 days after fumigation apply one of the following|
|(Vapam HL, Sectagon 42)||37.5–75 gal||See label||0|
|COMMENTS: Water-soluble liquid that decomposes to a gaseous fumigant (methyl isothiocyanate). Efficacy affected by soil texture, moisture, temperature, and percent organic matter. One gallon of product contains 4.26 lb of metam sodium.|
|. . . or . . .|
|(K-Pam HL, Sectagon–K54)||30–62 gal||See label||0|
|COMMENTS: Water-soluble liquid that decomposes to a gaseous fumigant (methyl isothiocyanate). Efficacy affected by soil texture, moisture, temperature, and percent organic matter. One gallon of K-Pam HL contains 5.8 lb of metam potassium; one gallon of Sectagon-K54 contains 5.63 lb of metam potassium.|
|**||Rates are per treated acre; for bed applications, the rate per acre may be lower.|
|*||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 action. 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 a fungicide with a different mode-of-action group number.|
|§||Do not exceed the maximum rates allowed under the California Code of Regulations Restricted Materials Use Requirements, which may be lower than maximum label rates.|
|‡||Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases, the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.|