- Author: Steven T. 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.
Figure 1. Charcoal rot results in the collapse and death of strawberry plants.
Figure 2. Internal crown tissue of strawberry infected with Macrophomina will show a dark to orange brown discoloration.
Figure 3. Charcoal rot can affect large portions of a field and cause significant dieback.
Figure 4. Tiny, black microsclerotia enable the Macrophomina pathogen to survive in the soil.
- Author: Steven T. Koike
Along with researchers in Europe, Jim Correll (Univ. Arkansas) and Steven Koike (UC Cooperative Extension, Monterey County) report here another new race, the 14th, of the downy mildew pathogen (Peronospora farinosa f. sp. spinaciae) of spinach. First identified in November 2010 from spinach in Ventura County, California, this race breaks the resistance of several important cultivars. The isolate was initially designated as UA4410 and was characterized with a standard set of differential varieties. Isolates with the same disease reaction as UA4410 were subsequently found in locations throughout California and Arizona in 2011 and 2012. This race has not been reported in Europe. After careful evaluation of the significance of this development to the spinach industry, the International Working Group on Peronospora (IWGP) has designated this isolate as race Pfs: 14. Isolate UA4410 will be the type isolate (or official isolate) of Pfs: 14. The IWGP is located in The Netherlands and is administered by Plantum NL.
Race Pfs: 14 poses a threat to the spinach industry because it is particularly well-adapted to modern hybrids with resistance to races 1-13. Similar developments have taken place when races Pfs: 5 (1996), Pfs: 6 (1998), Pfs: 7 (1999), Pfs: 8 and 10 (2004), Pfs: 11 (2008), Pfs: 12 (2009), and Pfs: 13 (2011) were identified and named. The occurrence of Pfs: 14 will encourage development and eventual use of Pfs: 1-14 resistant spinach cultivars.
A collaboration of researchers with the IWGP, University of Arkansas (Correll), and University of California (Koike) is monitoring the development of new races of spinach downy mildew on a global scale by continuously collecting and testing suspected new isolates. Collected field samples are tested for race identification using a fixed, standardized host differential set of varieties that contains the full range of available resistances. New race designations will be mutually agreed upon by this collaboration based on persistence of the race over several years, occurrence in a wide area, and significant economic impact. In this way it is hoped that research findings and conclusions will be agreed upon and better communicated between the researchers, seed industry, spinach growers, and other interested parties.
For California and Arizona, the Correll-Koike team will continue to receive and test spinach downy mildew samples for growers, pest control advisors, and seed companies. Industry is encouraged to continue to submit downy mildew outbreak samples to Correll-Koike, as such samples facilitate the discovery of additional new races. The Correll-Koike research is made possible by support from the California Leafy Greens Research Board and by active participation from the agricultural industries in California and Arizona.
The IWGP consists of spinach seed companies (Pop Vriend, Monsanto, Rijk Zwaan, Nunhems, Takii, Sakata, Bejo, Enza, Syngenta, Vilmorin, and Advanseed) and Naktuinbouw (the Inspection Service for Horticulture in The Netherlands), and is supported by researchers at the University of Arkansas and the University of California Cooperative Extension (Monterey County) in the USA. Researchers all over the world are invited to join the IWGP initiative and use the common host differential set to identify new isolates.
For more information on this subject you can contact Steven Koike (stkoike@ucdavis.edu), Jim Correll (jcorrell@uark.edu), Diederik Smilde (d.smilde@naktuinbouw.nl), or IWGP chairperson Jan de Visser (JandeVisser@popvriendseeds.nl).
- Author: Jim Correll, University of Arkansas Steven Koike, University of California Cooperative Extension
Another new race, the 13th, of the downy mildew pathogen (Peronospora farinosa f. sp. spinaciae) of spinach has been found and documented. First identified in January 2010 from spinach in Holtville, California, this race breaks the resistance of several important cultivars. The isolate was initially designated as UA0510C and was characterized with a standard set of differential varieties. Isolates apparently identical to UA0510C have been found in an increasing number of locations throughout California in 2010 and 2011. After careful evaluation of the significance of this development to the spinach industry, the International Working Group on Peronospora (IWGP) has designated this isolate as race Pfs 13. The IWGP is located in The Netherlands and is administered by Plantum NL.
Race Pfs 13 poses a threat to the spinach industry because it is particularly well-adapted to modern hybrids with resistance to races 1-12. The appearance of a new race is not unexpected because hybrids with resistance to races 1-12 have been widely planted over the past few years. Similar developments have taken place when races Pfs 5 (1996), Pfs 6 (1998), Pfs 7 (1999), Pfs 8 and 10 (2004), Pfs 11 (2009), and Pfs 12 (2009) were identified and named. The occurrence of Pfs 13 will clearly encourage the industry to develop and use new spinach cultivars having resistance to races 1-13. A history of the detection of the various spinach downy mildew races is presented in Table 1.
A collaboration of researchers with the IWGP, University of Arkansas (Correll), and University of California (Koike) is monitoring the development of new races of spinach downy mildew on a global scale by collecting and testing suspected new isolates. In this way it is hoped that research findings and conclusions will be agreed upon and better communicated between the seed industry, spinach growers, and other interested parties. For California and Arizona, the Correll-Koike team will continue to receive and test spinach downy mildew samples for growers, pest control advisors, and seed companies. Industry is encouraged to continue to submit downy mildew outbreak samples to Correll-Koike, as such samples facilitate the discovery of additional new races. The Correll-Koike research is made possible by support from the California Leafy Greens Research Board and by active participation by the agricultural industries in California and Arizona.
For more information on this subject you can contact Steven Koike (stkoike@ucdavis.edu), Jim Correll (jcorrell@uark.edu), Diederik Smilde (d.smilde@naktuinbouw.nl), or IWGP chairperson Jan de Visser (JandeVisser@popvriendseeds.nl).
Table 1. Races of spinach downy mildew and year of detection
Year | Race |
1824 | 1 |
1958 | 2 |
1976 | 3 |
1990 | 4 |
1996 | 5 |
1998 | 6 |
1999 | 7 |
2004 | 8 |
... | (9)* |
2004 | 10 |
2008 | 11 |
2009 | 12 |
2010 | 13 |
*One time detection only
Downy mildew of spinach is the most important disease on this crop and results in quality and yield losses.