Back to testing basics
By Julienne Isaacs
There are other, more sophisticated methods of testing for the presence of late blight spores in growers’ fields, but that’s precisely the reason Eugenia Banks selected a very simple test for her 2016 project.
By Julienne Isaacs
“The main objective when you do research is that growers could use the results in the future. Growing potatoes is such a demanding job and growers don’t have the time to do complicated things,” Banks says.
Banks, who retired as potato specialist for the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) in 2015, is now the lead on a two-year Ontario Potato Board project evaluating one type of spore trapping technology in order to help growers improve late blight management. Funded in part through Growing Forward 2, the project involves the installation of “passive spore traps” in the Shelburne and Alliston potato growing areas to monitor the presence of late blight spores.
“I decided on these spore traps for late blight because they had never been researched in Ontario before,” Banks says. “I worked with two grower co-operators and they were very happy to have this project, especially because it didn’t disturb their management practices.”
Two passive spore traps were installed on each grower’s farm early in the 2016 growing season, close to potato fields where it was windy or where there had been late blight in the past.
Banks supplemented the spore traps with in-field scouting twice a week, as well as drone surveying once a week.
No late blight spores were found in the traps until July 7, when Banks found spores in both the Shelburne and Alliston traps. Growers in the area were immediately alerted by email, Banks says, and most of them opted to spray a late blight specific fungicide, even though scouting did not reveal any signs of infection in the fields. Spores were later trapped on Aug. 15 and Sept. 19.
But Banks says the traps’ effectiveness is proved in part because 2016 was such a dry year with conditions that aren’t normally favourable to late blight. In fact, it was the hottest, driest year the province has experienced in 100 years, and no late blight was reported in fields.
“The traps will be evaluated again in 2017,” she says. “That will be a good test because I don’t think that Mother Nature will send us the same weather.”
How passive spore traps work
Banks contrasts passive spore traps with their more complicated cousin, volumetric spore traps. The latter type of trap must be connected to a power source because it opens and shuts at set times, allowing researchers to measure the number of spores trapped in a particular volume of air.
Passive spore traps do not require a power source. They consist of funnels containing mesh filters called “cassettes” that trap spores carried by the wind. Passive traps are not moved and remain permanently open to the wind; researchers don’t know the volume of air passing through, but they change the cassettes twice a week, sending them to a lab for polymerase chain reaction (PCR) testing. The test measures the concentration of late blight spores as “low” “medium” or “high.”
“It’s easier for growers to use the passive type,” Banks explains. “We find that the passive traps are more adequate for our objectives: to detect the spores. We are not so interested in the number of spores, because late blight is such a devastating disease that if the spore traps get spores, it means that they’re in the air and growers should switch to a late blight specific fungicide immediately.”
Banks says researchers in the Netherlands have been able to show that infection in a single plant can cause an outbreak of late blight. “Just one viable spore landing in good conditions will produce lesions, and the lesions will expand and it will be explosive,” she says.
The traps are relatively inexpensive and lab tests cost just $25 each – a small price to pay compared to the economic losses wreaked by late blight in unprotected fields.
Growers have been extremely receptive to Banks’ research, she says, and once the research is completed in 2017, some of them are considering installing their own passive spore traps in high-risk areas.
First, though, Banks wants to gather more data. If 2017 is a rainy year with conditions favourable for late blight, she can measure the trap’s effectiveness in all conditions.
“Spore traps represent another tool to be added to the potato growers’ arsenal to combat late blight,” Banks says. “If late blight spores are not detected by the traps, growers should still follow a preventative fungicide program and apply a fungicide spray before rows close. Also, fields should be scouted regularly.”
Tracking late blight’s roots
New research from North Carolina State University (NC State) has tracked the evolution of differing strains of the major pathogen responsible for late blight disease in potatoes around the world.
NC State plant pathologists studied 12 key regions on the genomes of 183 Phytophthora infestans pathogen samples – including both historic and modern samples – from across the globe. They discovered that a lineage called FAM-1 caused outbreaks of potato late blight in the United States in 1843 and then two years later in Great Britain and Ireland. FAM-1 caused massive and debilitating late blight disease outbreaks in Europe, leaving starvation and migration in its wake.
According to study results published in PLOS ONE, FAM-1 was also found in historic samples from Colombia, suggesting the pathogen had its origins in South America.
Jean Ristaino, the William Neal Reynolds distinguished professor of plant pathology at NC State and the corresponding author of the study, theorizes that the pathogen either arrived in Europe via infected potatoes on South American ships or directly from infected potatoes from the United States.
But FAM-1 wasn’t just a one-hit wonder that made its mark and then quickly disappeared.
“FAM-1 was widespread and dominant in the United States in the mid-to-late 19th century and the early 20th century,” Ristaino said, in a press release. “It also was found in Costa Rica and Columbia in the early 20th century.”
FAM-1 survived for about 100 years in the United States but was then displaced by a different strain of the pathogen called US-1.
“US-1 is not a direct descendant of FAM-1, but rather a sister lineage,” Ristaino said.
US-1, in turn, has been elbowed out of its eponymous homeland by even more aggressive strains of the pathogen that have originated in Mexico. She explained that winter vegetable crops, grown in Mexico and imported into the U.S., can harbour the pathogen.
The pathogen’s effects aren’t limited to the decimation of Ireland’s potato crop 170 years ago. Billions are spent worldwide each year in attempts to control the pathogen, Ristaino added. Potatoes in the developing world are particularly vulnerable as fungicides are less available and expensive.
According to 2015 estimates from Agriculture and Agri-Food Canada (AAFC), producers in North America, Europe and developing countries spend approximately $1 billion on fungicides for late blight control every year. AAFC estimates the total global financial cost of late blight, including the price of fungicide applications and direct crop losses due to the disease, ranges from $3 to $5 billion every year.
Co-authors on the paper include Amanda Saville, a research technician in Ristaino’s lab, and Michael Martin, an associate professor with the Norwegian University of Science and Technology’s department of natural history.
The research was supported by a grant from the United States Department of Agriculture’s National Institute of Food and Agriculture.