Beginning at least as early as 2005 and continuing through 2008, our diagnostic lab in Salinas confirmed that collapsing strawberry plants from throughout California have been infected with the fungus Macrophomina phaseolina. Limited outbreaks have been observed in a number of coastal and inland strawberry producing counties. Symptoms consist of wilting of foliage, plant stunting, and drying and death of older leaves, though the central youngest leaves often remain green and alive. Plants can eventually collapse and die. When plant crowns are cut open, internal vascular and cortex tissues are dark brown to orange brown. We do not see fungus fruiting bodies or other structures directly on plant tissues.
Thus far, 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 be quite large and appear to have spread from the initial problem area. Such patterns are consistent with the spread of a soilborne pathogen.
The soilborne fungus Macrophomina phaseolina has consistently been isolated from these symptomatic crown tissues. It is noteworthy that in these cases we have never isolated other important pathogens such as Colletotrichum, Phytophthora, or Verticillium. In culture, all Macrophomina isolates produce numerous tiny, black, irregularly shaped sclerotia. These sclerotia are survival structures that allow the fungus to persist for extended periods in the soil. The spore producing stage (pycnidia) of the fungus can be induced in culture, though we have yet to find these fruiting bodies on field grown plants. This disease is referred to as charcoal rot or crown rot.
The particular biology and ecology of M. phaseolina in the strawberry production environment is not yet defined for California. However, the fungus has been extensively studied in other systems.
We know that Macrophomina, due to the production of microsclerotia, can survive for extended periods in the soil. The fungus probably is spread within and between fields mostly by the movement of soil during soil tillage and preparation operations.
Disease is often most severe if the infected plant is subject to stresses such as weather extremes, water stress, poor soil conditions, and other factors. If fumigation treatments fail to significantly reduce soil inoculum, possible disease management steps could include applying post-plant fungicides (if effective, registered materials are available), rotating to non-host crops, and planting resistant or tolerant strawberry cultivars.
In preliminary tests, it appears that some strawberry cultivars differ in their susceptibility to this pathogen. In replicated shadehouse experiments, cultivars such as Albion, Camarosa, Diamante, and Ventana were quite susceptible; inoculated plants showed wilting symptoms two weeks after exposure to the fungus, and by four weeks the plants had collapsed. In contrast, cv. Seascape was very tolerant and showed only minor dieback of the oldest leaves by the end of the experiment.
While in years past M. phaseolina has been periodically associated with strawberry in California, it appears that only recently has charcoal rot disease become a production concern for the commercially grown crop. Charcoal rot of strawberry also has been reported in Egypt, France, India, Israel, and other parts of the US (Florida and Illinois).
In California there is circumstantial evidence that the fields most seriously affected by Macrophomina have all been treated with pre-plant alternatives to the methyl bromide + chloropicrin fumigation standard.
This crown rot problem may therefore be triggered by this change in production practices. Our UC Cooperative Extension group will be continuing our examination of this disease and plans to study disease epidemiology, strawberry cultivar resistance, fungicide treatments, and other aspects.