A novel strain of downy mildew that attacks spinach appeared in Salinas Valley fields in July of 2008, and if officially designated as a new race, it will bring the total to 11, seven of which have sprung up in California, Arizona, and elsewhere since 1992.
The downy mildew, also known as Peronospora farinosa f. sp. spinaciae, continued to be a problem in the Valley during September and October on varieties previously resistant to it.
The fungus-like pathogen causes light green to yellow spots on leaves with purple formations of spores mostly on the undersides, making the leaves unmarketable. It's at home in all corners of the globe, but spinach is its only known host. It is managed with resistant varieties and fungicides.
The disease was discussed at a plant disease seminar in Salinas by James C. Correll, plant pathologist at the University of Arkansas.
He has been collaborating with Steve Koike, Monterey County plant pathology farm advisor, in spinach industry supported studies in California and Arizona.
Correll, a specialist in vegetables, fruits, and rice, said the disease has a long history, the first race having been identified in 1824. It has shown its ability to evolve into the 10 races now in the U.S. and Europe.
“There's been a rapid increase in the number of races in recent years,” he said, recalling that he and Koike started working together in 1992 on race 4 and have since described the subsequent six.
“We don't know exactly why this proliferation of races has occurred,” Correll said, “but it most likely has much to do with production practices. Many changes have come in Salinas Valley lettuce, and they've been paralleled in spinach.”
Practices leading to “opportunities for variation to occur” in the pathogen range from higher plant densities and much more favorable micro-climates to adoption of sprinkler irrigation that splashes its spores.
Seed of organic spinach does not receive the typical commercial treatment with metalaxyl fungicide, and that may have also helped the introduction of novel strains. Some speculate that overcast in the Valley caused by smoke from forest fires in the summer of 2008 may have also contributed to the development.
Peronospora can germinate in four to six hours to infect plants in 12 to 24 hours during cool, wet conditions, after which the disease can regenerate on infected plants in about a week.
Correll works closely with the International Working Group Peronospora Committee, growers, and seed companies around the world to collect samples from infected plants for screening and identification. He and Koike started collecting samples from this year's California crop in late June.
To avoid confusion with all the data collections by various nations, the committee serves as a clearinghouse to assign new race designations. “The purpose of the group is to balance the scientific accuracy of reporting new strains of the pathogen with the legal requirements for seed companies to screen new germplasm materials,” Correll explained.
In Europe, seed companies have to demonstrate their products are actually resistant to whichever races they claim to be. This prevents arbitrarily identifying as races those new strains that may be temporary or have little impact on the industry.
The committee will meet in January 2009, to consider whether the new California strain should be officially designated as race ll.
Tests to identify races are done on greenhouse flats of indicator spinach plants and typically take about two weeks, but they can extend to three to four months for further observation in cases where novel traits are discovered.
Mildew isolates, or samples from the field, are used to inoculate the ten international, differential cultivars. The collections this year severely infected four of the 10: Viroflay, Resistoflay, Bolero, and Lazio. The resulting ‘fingerprint’ or reaction of the four indicator cultivars is different from the 10 known races.
He noted that while newer spinach varieties, such as Lazio, are highly susceptible to the new strain, some older varieties such as Califlay and Lion are holding up to it.
Meanwhile, spinach breeders are working to incorporate as broad a resistance as possible in anticipation of additional new strains.
Since spinach is a year-round crop in California, Correll said, the downy mildew can easily find a new host after being carried by trucks, farm equipment, or airborne spores.
Spores can survive for as long as 10 years, and the disease has even been found in small spinach gardens in Nepal at the 14,000 foot elevation, thousands of miles from commercial plantings.
Koike reminded the spinach industry to continue to be watchful of any unusual developments with downy mildew. His laboratory at the Monterey County Cooperative Extension office in Salinas will be analyzing samples submitted by growers, PCAs, and others.
“This service acts as a gauge of the downy mildew situation and an early warning system should new races occur,” he said.
Research on the disease in California is a joint project of the University of California and the University of Arkansas.
In comments about various other plant diseases on the Central Coast during 2008, Koike said Macrophomina dieback is showing up in strawberry fields where methyl bromide with chloropicrin has been replaced by Telone 235 or Vapam in fumigation.
“It's become more severe and more common since 2006, primarily in Ventura County, Orange County, and a few fields in Santa Barbara County,” he said.
Macrophomina, a root pathogen, is associated with plant stress, such as occurs when mid-season strawberry plants have a fruit load, temperatures become warmer, and there is water stress. Infected plants collapse.
Laboratory diagnosis is key, and assumptions should not be made based on visual symptoms, which resemble Phytophthora.
Fusarium, until recently not thought to be a disease of strawberry, has also been found in the region in fields where the alternative fumigants were used.
“From the windshield it can look like Phytophthora, Verticillium, or Macrophomina. Interestingly, we have not found Fusarium in the same field with Macrophomina, and vice versa. We do not know why at this point,” he said.
Koike also noted these diseases are spreading, with patches of them becoming larger the following season.