An interview with Lisa Shepherd
This month’s column completes the four-part series on seed health with an interview with Lisa Shepherd, seed health testing coordinator for Iowa State University.
What type of seeds do you test?
A wide, wide variety. Being located in Iowa, many people assume we are all about corn and soybeans, but that’s only part of it. We also test a large amount of vegetable seed: onion, tomato, melons, peppers, carrot, cucumber, squash, watermelon, spinach and many others. We also test small grains (wheat, etc.), flower seeds and grasses, although with less frequency than the field and vegetable crops. I’ve also tested peanuts, tree seeds and even organic natural cat litter made of corn parts and pieces.
My main focus of testing is for phytosanitary export, so these are seeds originating in the U.S. and shipped to other counties. For instance, peanuts grown in Georgia [for export to] Nicaragua needed to be free of three different viruses.
What guidelines do you follow?
We are certified by the USDA-APHIS to perform testing for all states for phytosanitary export. Currently, we are testing over 300 pathogens for phytosanitary export.
Officials at USDA-APHIS test seeds for import into the U.S. In most cases, you would need a phytosanitary permit to bring seeds or other plant material into the U.S. More information can be found at www.aphis.usda.gov/plant_health/permits/plantproducts.shtml or by calling 877-770-5990.
Let’s talk about seed testing methods. Do your methods differ by pathogen, or by type of pathogen (bacteria, fungus, virus, etc.)?
Our main methods include blotter and agar plate testing for fungi [agar is a gelatinous
substance derived from seaweed]. This helps the fungus express itself on the media or seed, allowing for microscopic examination after an incubation period (usually one to two weeks). For bacterial pathogens, we often use selective agar media designed to promote growth of the target pathogen while keeping other organisms [that feed on dead organic matter] suppressed. For virus testing, we use ELISA (enzyme-linked immunosorbent assay) test kits. These are available commercially from many companies and are pathogen-specific. We also do some PCR (polymerase chain reaction) testing.
PCR is a method used to multiply one small section of DNA [repeatedly] until there are enough fragments to be detectable. The segment being multiplied is specific and unique to the target. For example, if the goal is to identify a specific bacteria like E. coli, you find a small region of DNA that is specific to E. coli and nothing else. The PCR reaction uses specific primers (starter strands of DNA) that recognize each end of the specific region, like a pair of parentheses. The section between the two ends is multiplied by the millions in a thermocycler. (Thermocyclers are machines that alternate temperature up and down, which separates and reattaches the double-stranded DNA, aiding in the replication of the DNA strand). The millions of copies of DNA are now detectable, whereas the original strand is much too tiny to notice. If the specific region of DNA you are trying to find is not there, there will be no amplification of the DNA, and the test will show a negative.
Do testing methods differ by seed type also?
The testing methods depend more on the pathogen you are looking for than necessarily the seed. For instance, is a certain bacteria located on the outside of the seed or the inside? If it’s on the outside, you’ll want to soak it in buffer to release the bacteria from the seed surface. If the bacteria is located on the inside of the seed, you will probably need to grind the seed to release it so it is able to be detected.
Who develops your seed testing methods?
Methods come from many different sources. Some are publicly developed; some developed by seed companies to ensure quality assurance on their seed lots. Many publicly available and internationally recognized methods are available at the Web sites of the National Seed Health System (www.seedhealth.org), the International Seed Federation (ISHI) (www.worldseed.org) and the International Seed Testing Association (www.seedtest.org).
Many methods have also been developed here at the Iowa State University Seed Science Center, both from my testing laboratory and as projects by some of our graduate students and faculty. One student is currently working on PCR-based methods for detection of bean bacterial pathogens; she just presented her research at the annual American Phytopathological Society meetings.
How does one determine the most effective testing method?
Organizations such as ISTA or ISHI will bring infected samples to various laboratories to run “blind” tests and see how methods compare. This work might also be done by a graduate student, comparing different methods or steps in a protocol. For instance, when I was developing a testing method for Goss’ wilt of corn as a graduate student, I compared a selective media I had developed that I thought would be useful in isolation of the Goss’ bacteria in corn seed to other media that had been developed for isolation from soil or other areas. Then, I compared my developed method to other methods available.
How does the work you are doing affect U.S. growers of vegetables, fruits and tree nuts?
In two ways:
1. For quality assurance testing, where seeds are being examined for disease. This helps ensure home growers and transplant greenhouses are not germinating infected seed. While no test can ever be 100 percent sure of being free of a disease (unless you test 100 percent of the seed), this type of testing should prevent field epidemics and issues for growers if done properly.
2. [The intent of] phytosanitary testing is to move seed into other countries. A lot of this work is done to move seed to “winter nurseries” in order to increase the amount of seed available for customers, many of whom are in the U.S.
The author is a freelance writer based in Massachusetts and a monthly contributor to Growing.