- Author: Steven Koike
- Author: Mark Bolda
Beginning at least as early as 2005 and continuing through 2013, collapsing strawberry plants from various parts of California have been associated with the soilborne fungus Macrophomina phaseolina. The disease, called charcoal rot, appears to be the most important current concern for the industry due to its steady increase over this period of time. Each year finds additional new fields infested, and the disease has now been found in all of the major strawberry producing counties in the state. In 2005-2006, charcoal rot was restricted to southern California in Orange and Ventura counties. Most recently this disease has been confirmed in Santa Barbara, Monterey, Santa Cruz, and Santa Clara counties. The spread of Macrophomina to new fields and counties portends that charcoal rot may be a long term threat to the industry which at present does not have satisfactory plant resistance with which to combat the pathogen.
Symptoms of Macrophomina infection in strawberry consist of wilting of foliage, plant stunting, and drying and death of older leaves, with the central youngest leaves often remaining green and alive. Plants can eventually collapse and die (Figure 1). When plant crowns are cut open, internal vascular and cortex tissues are dark to orange brown (Figure 2). Disease is often most severe if the infected plant is subject to stresses such as weather extremes, water stress (shortage of water), poor soil conditions, or heavy fruit loads. In locations where the disease has occurred for more than one season, the patches can be quite large and appear to have spread from the initial problem area (Figure 3). Such patterns are consistent with the spread of a soilborne pathogen. It is noteworthy that in these cases we have never isolated other important, well known pathogens such as Colletotrichum, Phytophthora, or Verticillium. However, it is important to note that another recently described disease, Fusarium wilt, is also occurring in the same regions; symptoms of Fusarium wilt are identical to those caused by charcoal rot.
Macrophomina produces numerous tiny, black, irregularly shaped microsclerotia (Figure 4). These microsclerotia are survival structures that allow the fungus to persist for extended periods in the soil. The fungus is spread within and between fields mostly by the transport of contaminated soil during soil tillage and preparation operations. Spread of Macrophomina in strawberry fields deals with the same issue of field sanitation that concerns growers of many other commodities. Verticillium wilt (lettuce, strawberry, pepper), clubroot (broccoli, cauliflower), Fusarium wilt (lettuce), Fusarium yellows (celery), and lettuce dieback disease (lettuce) are all problems caused by soilborne pathogens that are spread in infested soil.
Current management strategies involve the following: (1) Crop rotation. Do not plant strawberry in fields having a known history of the problem and avoid back-to-back strawberry plantings in infested locations. (2) Pre-plant fumigation. This remains a useful tool for managing Macrophomina and the other soilborne pests, even though bed-applied fumigants may not provide complete control. (3) Avoid stressing the plants. Stress will hasten the development and increase the severity of symptoms, so use appropriate growing and irrigation practices to reduce stress. Note, however, that even in the absence of stress, infected plants will eventually develop the disease. (4) Sanitation. Growers with Macrophomina infested fields need to be concerned with limiting the spread of the fungus from infested to clean fields.
- Author: Mark Bolda
- Author: Steven Koike
A big field of strawberries in Santa Cruz County has been confirmed as being infected with Macrophomina phaseolina. This is the third confirmation of this disease in the Watsonville- Salinas production district. Long time readers of this space will know that the first confirmed find was two years ago which interestingly is in close proximity to this latest situation.
Symptoms of this disease, known as charcoal rot or crown rot, are marked by plant stunting, initial wilting of older foliage, and the drying and death of outer leaves (Photo 1 below). Similar to a plant infected with Verticillium, the central leaves will remain green and alive for a period of time. However, in contrast to most Verticillium infections which show little to no discoloration of the crown when cut open, a plant infected with Macrophomina will show a distinctive dark brown to orange brown discoloration of the crown (Photo 2 below). Eventually, plants infected with Macrophomina will collapse and die.
What is notable about this particular field is the astonishing distribution and severity of the infection (Photo 3 and 4 below) even though the field was flat fumed with methyl bromide- chloropicrin every other year for more than a decade. It is presently a mystery to researchers how this infection reached this magnitude so quickly.
A grower with a Macrophomina infested field should take action to limit the spread of the fungus. Any sort of tillage of an infected field should be followed by a thorough washing with water to remove soil clods and trash which are all potential carriers of the pathogen. Finally, to attempt to clean up Macrophomina from strawberry fields, pre-plant flat fumigation with methyl bromide/ chloropicrin provides the best control.
What about rotating to a non-host crop? This is actually a difficult question to answer. Macrophomina has a broad host range and is reported as a pathogen of many vegetable, fruit, and field crops. Rubus, the genus of caneberries, has not been reported as a host, but that does not necessarily mean such plants would never be infected. Still, it is too early to know for certain whether Macrophomina from strawberry can infect other hosts. Some researchers have successfully infected strawberry with Macrophomina from other hosts, but not the other way around. Our UCCE team is conducting research to answer these and other questions about Macrophomina in California fields.