- Author: Mark Bolda
By Mark Bolda and Steven Koike, UC Cooperative Extension
Strawberry plants in Santa Cruz County have been identified as being infected with Macrophomina phaseolina. This is the first confirmation of this disease on strawberry from the Watsonville-Salinas production district. Beginning at least as early as 2005 and continuing through 2010, the UCCE diagnostic lab in Salinas has documented that limited outbreaks of collapsing strawberry plants from other coastal and inland strawberry producing counties (Orange, Ventura, Santa Barbara) in California have been caused by this fungus. It is noteworthy that in these cases we have never isolated other important pathogens such as Colletotrichum, Phytophthora, or Verticillium.
Symptoms of Macrophomina disease, known as charcoal rot or crown rot, in strawberry consist of wilting of foliage, plant stunting, and drying and death of older leaves, though the central youngest leaves often remain green and alive (Photo 1). Plants can eventually collapse and die. When plant crowns are cut open, internal vascular and cortex tissues are dark brown to orange brown (Photo 2). We do not see fungus fruiting bodies or other structures directly on plant tissues. Disease is often most severe if the infected plant is subject to stresses such as weather extremes, water stress, poor soil conditions, or heavy fruit loads.
Affected fields generally have small, limited patches with this problem. However, for some locations where the disease has developed for more than one season, the patches can become quite large and appear to have spread from the initial problem area (Photo 3). Such patterns are consistent with the spread of a soilborne pathogen.
Macrophomina produces numerous tiny, black, irregularly shaped sclerotia. These sclerotia are survival structures that allow the fungus to persist for extended periods in the soil (Photo 4).
Growers with Macrophomina infested fields need to first concern themselves with limiting the spread of the fungus and then eliminating it from the infested field. Since the fungus is spread within and between fields mostly by the movement of contaminated soil during soil tillage and preparation operations, care should be taken to not move soil from field to field by machinery or physical transport. To eliminate Macrophomina from strawberry fields, pre-plant flat fumigation with a high rate of methyl bromide/ chloropicrin provides the best control. There is currently no recommendation for in-season treatment of this disease and resistant cultivars have not yet been identified.