Macrophomina Crown Rot

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.

Macrophomina 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.

Strawberry Cultivars

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 Stress

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.

Crop Rotation

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.

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.

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.