by Courtney Llewellyn
While high tunnels are generally seen as boons for extended growing seasons, they’re not without their issues. Diagnosing the problems that arise is the first step in correcting them, and that was the subject of the second session in the three-part webinar series, “High Tunnels After Dark: 2020 High Tunnel Production Conference,” presented by Maine, New Hampshire and Vermont Extension Services.
Cheryl Smith, plant health state specialist, UNH Extension, led the webinar with a critical diagnostic tip: “Successful disease and insect management depends on correctly identifying the problem. Then we know how to manage it and the most effective time to manage it.”
Some tools are needed to help identify problems, including small hand or magnifying lenses or loupes. It’s also important to take notes about what you’re seeing. Details are critical – if you didn’t grow the plant affected, you need to know where the plant came from and if anything was sprayed on it.
You also need to know what a normal plant looks like before you can diagnose what’s wrong. Knowing the normal host characteristics assists you in looking for symptoms, like abnormal appearance, stunting, wilting or necrotic tissue. There are also some patterns to keep an eye out for. Is the issue seen on older or younger foliage or fruit? Is it on the upper or lower portion of the plant? Is it only on one side of a leaf, fruit or plant? Are lesions limited to veins? Is it on inner or outer leaves?
When it comes to an inner/outer leaf issue, Smith said the answer could be as simple as sunscald. “A lot of times if we something that’s one-sided, it’s an environmental problem, not a pathogen,” she said.
Also look for patterns in planting – localized groups of plants, a repeating pattern of affected plants, the gradient of the symptoms, whether or not they’re limited to certain varieties or sources of seeds, low or wet areas with drainage issues and overall exposure to sun or light. It’s all about being observant.
Smith said if the symptoms are uniform in a specific area, then the issue is likely environmental (or abiotic). If they’re non-uniform, then the issue is more likely to be pathogenic. The timing of symptoms can separate the source of symptoms as well. Something that appears “suddenly” (over a single night, days or a few weeks) is probably an environmental disorder. A pathogen-caused disease manifests more slowly.
“A significant bacterial disease will most likely not show up ‘overnight,’” Smith said. “It could take years – think of Dutch elm disease.”
Going through a process of elimination can help narrow down what the problem is. A root issue could result in stunting, nutritional issues or wilt. Environmental issues result in edema, an excess of ethylene or come from cold temperatures. Recent spray applications may result in leaf spots, necrosis or distortion.
If you need assistance figuring out what the problem is, taking high quality images to send to a professional is key. (They’re also good to keep for future reference.) Smith said digital images can be almost as good as a physical sample, if done correctly. The photos must be in focus, first and foremost. Placing your hand or a piece of paper behind the subject helps the camera focus. Take multiple images showing a progression of symptoms on a plant, but also take a whole tunnel view to illustrate “the big picture.” Photograph both surfaces of a leaf. If you suspect root issues, take photos of both the plant and its roots.
Insects are often the culprits behind plant issues, so correctly identifying them is also important. When photographing them, also get images of the host plant where you found the pests. Take photos of their life stages, if possible. Get good detail of the nature of the damage or injury on the plant. Also capture images of webbing, exit holes and frass (the powdery refuse produced by boring insects), if they exist.
Sometimes photos aren’t enough, though. Sometimes physical samples still need to be sent to labs. “The accuracy of the diagnosis depends on the sample quality,” Smith said, adding that it’s crucial to follow lab sampling instructions. Good packaging is also critical – labs don’t want “sample soup.”
What should you send? Enough material to be sampled – one leaf is not enough. Also provide a good range of symptoms, from a healthy sample to the worst. You can send whole plants if that’s practical. You can also ship insects. Do not send completely dead matter.
Make sure to include your photos and your sample submission form. You should also provide as much information as possible about your issue and your sample. You can find a list of labs that conduct testing via the National Plant Diagnostic Network at NPDN.org.