Charcoal rot is a disease of strawberries caused by the pathogenic fungus Macrophomina phaseolina. Also referred to as strawberry crown rot, the disease tends to make its appearance after plants have begun to produce fruit. Older foliage turns grayish green and wilts. Growth slows, and eventually plants completely collapse and die.
Charcoal rot is usually noticed due to its impact on leaves and overall plant growth, but it also produces symptoms similar to Fusarium rot.
Strawberry plants which contract charcoal rot often initially don’t show any sign of infection. But symptoms begin to appear once plants are close to fully grown and begin to produce fruit, which puts additional strain on plant metabolic processes.
The first sign of infection is that older leaves will turn grayish, wilt, and dry up, while new growth is left unharmed. Close observation may also reveal that fruit production slows in plants that are infected with charcoal rot. It soon becomes evident that the growth of infected plants is stunted relative to healthy plants. Gradually, many plants collapse and succumb to the disease.
When conditions are warm, and in fields where plants are subjected to water stress, the progress of the disease is expedited.
When dead plants are uprooted and examined, the vascular tissues of the plant are found to be orange or brown in color. Roots are sometimes similarly discolored, and in some cases are found to have rotted away entirely.
While the discoloration of the vascular tissues rules out Verticillium wilt, the overall symptomology is identical to Fusarium wilt. Thus, laboratory testing of infected plants is necessary in order to make a positive identification of charcoal rot.
Infection tends to occur in patches, where up to 75% of plants may be infected, with these infected patches surrounded by healthy plants.
Charcoal rot is caused by Macrophomina phaseolina, a fungus which causes damping off, collar rot, and root rot in many plant species.
For many decades, methyl bromide was widely used in strawberry fields and in other agricultural applications as a fumigant and pesticide. However, once the scientific community recognized the profound toxicity of the chemical, much of the world—including the United States—mandated that it be phased out of use in the 1990s and early 2000s.
During this phaseout process, in 2001, strawberry growers in Florida observed that plants were rapidly collapsing and dying in a way they hadn’t previously seen. Laboratories identified Macrophomina phaseolina as the cause of the disease, which was dubbed ‘charcoal rot.’ The disease quickly crossed the United States, first appearing in California in Orange County in 2005, and subsequently spreading to much of the rest of the state.
The means by which initial infection occurs is not yet understood. While M. phaseolina infects many hundreds of species of plants, the strain that infects strawberries appears to not infect other species, while other strains appear to have an affinity for strawberries.
Regardless of how the fungus is introduced to fields, once infection occurs, it’s difficult to eliminate. M. phaseolina produces structures called microsclerotia, which can easily overwinter in soils, and infect the next year’s crop.
There is no cure for charcoal rot, and thus treatment requires the use of preventative measures, which can be backstopped with chemical interventions.
It is known that charcoal rot tends to be most pervasive in warm conditions, with temperatures in the 80s, as well as low moisture conditions and sandy soils. It is critical that proper irrigation measures are implemented to ensure that plants do not experience water stress, as this makes them much more vulnerable to infection.
Treating the soil with preplant fumigants prior to planting has been found to be effective against M. phaseolina—though not to the degree that methyl bromide once was.
If a field has previously been infected with M. phaseolina, avoid planting strawberries in it the following year. Planting strawberries year after year will only maintain high levels of inoculum, which ensures the continued presence of the disease. Instead, plant other crops—broccoli in particular is known to naturally suppress soil-dwelling pathogens.
When disking under infected previously infected plants, treat the field to accelerate microbial activity. This will help to ensure complete decomposition of all infected plant matter, which can otherwise harbor the fungus for years.
Lastly, practice proper hygiene. If you know that a field is infected with M. phaseolina, avoid moving laborers and equipment directly between infected and uninfected fields to decrease the likelihood of cross-contamination. Wash boots, equipment, etc. immediately after working in infected fields.