July 15, 2021
"Plants get sick, too," said Timothy Widmer, USDA-ARS national program leader, during the 53rd American Peanut Research and Education Society (APRES) Annual Meeting.
Widmer compared lessons learned from COVID-19 to plant disease epidemics. "I don't in any way want to minimize the impact COVID has had on us, but we have to point out that agricultural commodities are in just as much danger," Widmer said. "Over the years, we've done a good job maintaining the food supply that these dangers are often not on the minds of the average consumer."
The food shortage throughout the pandemic pushed this issue to the forefront, he said. "Now, we need to help the public make that connection between plant diseases and pests and food security.
"The reality is, we lose a lot of food and agricultural productivity."
Timothy Widmer, USDA-ARS national program leader (Photo Alexandra Widmer)
The Food Agriculture Organization estimates up to 40% of global production is lost to plant pests and diseases with an economic impact of $220 billion," he said.
Peanuts have more than 70 known pathogens and more than 15 known viruses, with more being discovered. For example, yield reductions are associated with foliar diseases such as leaf spot. "The greatest threat of all diseases to the southern United States is early and late leaf spot. It can potentially cause annual losses of around $600 million," Widmer said. "In Florida alone, annual yield losses attributed to peanut leaf spot vary from 5% to 40%."
Yield losses to white mold are most significant where peanuts are grown year after year or every other year, he said. "Estimated losses may reach 20% or more."
Widmer also listed tomato spotted wilt and pests such as nematodes as yield reducer culprits. "Each year in Alabama, 5% to 10% of the potential peanut yield is lost to nematodes."
Pandemic terminology used over the last year is similar to terms used to discuss plant diseases and pests. Widmer, who's worked for ARS for more than 20 years, listed comparable terms along with USDA-ARS research examples as they relate to plant diseases.
Widmer compared masks to row covers. "I realize they are impractical for peanut production, but the concept is the same, to prevent or limit plant-to-plant spread. It's a physical barrier. For many viruses, the key is limiting or controlling the insect vector from infecting new plants from an initial source."
Planting density, crop rotation and weed control also can be used to reduce virus spread. "I'm sure everyone is familiar with the role weeds play in disease cycles as they can be reservoirs for viruses and other pathogens."
But identifying which weed hosts are reservoirs is difficult as they are often asymptomatic, Widmer said. "They must be laboriously tested, but which ones to test may be difficult to determine. Since many plants infected with these viruses do not die, the inoculant can continually spread."
"Quarantines are an important part of keeping plant pathogens out of the U.S., and further spread within the U.S. USDA-APHIS has specific regulatory policies for certain pathogens. Although at times, it may seem restrictive to us researchers, these regulations are put in place to protect U.S. agriculture from invasive pathogens," Widmer said.
Voluntary U.S. quarantines are sometimes necessary to avoid working on a pathogen for fear it might escape, Widmer added. "There are still ways we can do research on these pathogens, and one very effective way is through international collaborations."
For example, peanut smut, native to Argentina and not yet found in the U.S., is a serious yield threat, with reductions reported up to 70%. "ARS set up field trials in Argentina to screen U.S. germplasm and found some resistance," Widmer said.
ARS also has a biosecurity Level 3 facility in Fort Dietrich, Maryland. "This is an APHIS-approved facility that can safely work on any pathogen or isolate regardless of its origin," he said.
Congress recently awarded ARS new money to build a facility with a lab and greenhouse, which should break ground soon. Widmer encouraged researchers listening to his presentation to seek collaborations with the new facility, "if you're working on expanding your research to include exotic pathogens."
Diagnostic testing has been used throughout the COVID pandemic. But it's also a tool the plant world has utilized for years. "Accurate, reliable, easy to use, and quick diagnostics are critical for determining the pathogen that is impacting the crop. Without it, you may not be applying the proper management strategy, or it may delay management enough that the pathogen spreads further," Widmer explained.
Available plant tests are similar to those used for COVID, such as dipsticks, PCR (polymerase chain reaction), and more recently, the digital droplet PCR, which analyzes individual micro drops of DNA. "It's more sensitive, and it's reported in some cases to be a hundred times more sensitive in detecting viral genetic material than the standard PCR."
Dipsticks are simple to use with no special equipment required. "You get results in a matter of minutes," Widmer said. "However, sometimes they are for more general identification and may not get the specificity that is desired. But as a quick diagnostic, they're a great first approach."
Dogs sniff out diseases
Canine detection is another tool used to expose COVID-19 and plant pathogens. "ARS has been successfully using canines to eradicate plum pox virus from the U.S. and also help detect the citrus canker and citrus greening. Canines are perfect as an early infield detection system," Widmer said.
When detecting citrus greening, the canines can locate the pathogen in the tree quicker than typical molecular methods, such as PCR, and quicker than visual symptoms appear on the tree due to detected volatiles released by the pathogens.
"To put this sensitivity into perspective, dogs are capable of detecting one drop of blood in the volume of three Olympic-sized swimming pools. I like to say, it's old technology being used in a new way."
Just like COVID has variants, plant pathogens do as well. "Resistance breaking tomato spotted wilt virus isolates are now known," he said.
Variants highlight the importance of diagnostics. "Variants happen because of rapid replication and natural selection. Resistant plants with resistant genes are the most effective way to stay ahead of this arms race."
Resistance is touted to be the most effective and durable strategy against losses caused by plant pathogens. But, Widmer admitted, it is also time-consuming, taking years to acquire material. "This is one reason why it's critical to assess and predict which plant pathogens might be of importance in the future."
Traditional breeding techniques are most commonly used, but molecular genomics also are utilized to determine threatening pathogens. "At some point, genetic engineering may also be considered," Widmer said. "The technologies are quite amazing."
While plants aren't injected with a vaccine like humans, the concept of protecting plants is similar. "Plants do not produce antibodies toward the pathogen per se, but an immune response by the plants is put in motion. We normally call this cross-protection."
Widmer discussed labor shortages and how USDA-ARS is investigating robotic or automated solutions for the farm.
While COVID and plant terminology are similar, Widmer said the last year has revealed that the agriculture sector is vulnerable. "We already knew about food deserts but during the pandemic we saw disruptions in the food supply chain and harvesting. This was reflected all the way to the store shelves," Widmer said.
Peanut butter was in high demand. "It's considered a staple, and we saw shortages. It highlights the importance of this industry to the consumer," Widmer added.
"So, this brings me back to plants get sick, too. We are one plant disease epidemic away from a catastrophe. I don't want to be an alarmist. I just want others to make that connection."
Read more about:Pathogens
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