Symptoms and Signs
Symptoms of Macrophomina crown rot 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 and after plants begin bearing fruit or are subjected to stress. 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 may also be discolored and show the same dark brown coloration.
Comments on the DiseaseMacrophomina crown rot, also known as charcoal rot, is most severe when the infected plant is subject to stresses due to weather extremes, water stress, poor soil conditions, or heavy fruit loads. Note that foliar dieback and internal crown discoloration symptoms are identical to those caused by Fusarium wilt. Therefore, confirmation of Macrophomina crown rot requires diagnostic procedures in a pathology lab. This is a soilborne fungus and persists in the soil in the form of small, black survival structures called microsclerotia. In California, research suggests that most of the isolates of M. phaseolina that infect strawberry have a narrow host range and only infect strawberry.
Select locations that do not have a history of Macrophomina crown rot. Preplant fumigation, which historically has been an important component of managing Verticillium wilt in strawberry fields, will also help control Macrophomina crown rot. Fumigation will be most effective when crop residues are fully decomposed or removed. 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 as Macrophomina.
Drip application 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 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 reduce Macrophomina levels; it has not yet been thoroughly researched but is under investigation.
The following cultivars are listed in order of decreasing susceptibility to Macrophomina crown rot: Monterey, San Andreas, Albion, Fronteras, Portola, and Petaluma. Additional research is being conducted to develop new resistant cultivars.
Manage the strawberry crop so as to reduce stress on the plants. Managing plant stress is more important for managing Macrophomina crown rot than for any of the other soilborne diseases. Irrigate the crop as appropriate for the stage of development, current evapotranspiration requirement, and soil moisture levels. Plants at the edge of the bed may be more prone to collapse due to drying or other factors associated with this location in the bed. 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 Macrophomina, broccoli rotations may also be useful.
Organically Acceptable Methods
Select fields that do not have a history of Macrophomina crown rot. Plant tolerant cultivars. Rotate with nonsusceptible crops or with crops, such as broccoli, which have the capacity to suppress the pathogen. Avoid stressing plants.
Monitoring and Treatment Decisions
If drip fumigation is planned, good results have been obtained with a sequential application of chloropicrin or 1,3-dichloropropene plus chloropicrin followed 7 days later with metam sodium or metam potassium.
|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.|
|A.||METHYL BROMIDE*§/CHLOROPICRIN*§||Label rates||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 II)||100–265 lb (shank)||See label||0|
|COMMENTS: Effective for control of nematodes, soilborne fungal pathogens, and insects. One gallon of product weighs 10.1 lb.|
|. . . or . . .|
|(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: 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 60EC)||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.|
|(Basamid)||200 lb||See label||0|
|COMMENTS: Powder incorporated into the soil, followed by irrigation or tarping. It decomposes to a gaseous fumigant (methyl isothiocyanate).|
|**||Rates are per treated acre; for bed applications, the rate per acre may be lower.|
|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.|
|*||Permit required from county agricultural commissioner for purchase or use.|
|§||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.|