Late Summer Pest: Foliar Nematodes

As disturbing as it may sound, there are microscopic roundworms called nematodes all around our natural environment. They live in the soil, on roots, in leaves and in many other unexpected places. The ones that I always think about as unbelievable and icky are the nematodes that can be found in bottles of unpasteurized pepper vinegar. They are benign to you and me, but just the idea that they could be there makes me cringe. (Are y'all now going to reconsider before you sprinkle your collards with pepper vinegar this fall? I'm still going to use it.....it's like thinking about sharks when you are swimming in the ocean. You just can't think about it.)

My favorite nematodes (as we've established, I am a geek) are the Foliar Nematodes. Each nematode swims into a stomate in a film of surface water and feeds on the leaves within a leaf cell. Once they have eaten all of the carbohydrates from within an area, they swim out of the stomate and into another area of the leaf (They can't chew through the tough leaf veins).

I found nematodes last week on Hostas while I was scouting a nursery. Here is what the damage looks like:The darker areas are show where the nematodes have already been. The lighter yellow-green areas are where they are now.

Hostas are monocots, so the leaf veins run parallel to each other. This explains the pattern of damage that they cause on Hostas.
On dicots the damage is different, as seen on this Viburnum leaf. The injury has more of a stained glass effect because of the netted vein pattern. To diagnose foliar nematodes, you simply cut out a tiny square from an area that is discolored (not brown) and put it in a glass dish with some water. I use a watch glass.

When you look through a dissecting scope, you will see clear nematodes swimming out of the leaf edges and into the water.So what's the big deal about nematodes? Well, foliar nematodes cause leaf damage, stunting and impact the overall health and vigor of the plant. And they are hard to treat.

If you have a nursery or greenhouse, throw away badly infested plants. Spray the rest of the crop with Pylon at the high rate (2 applications, one week apart).

If you are a homeowner, do not buy plants with this type of damage. These nematodes have the ability to live for several years in a state called anhydrobiosis where they dry down completely and go dormant. As soon as moisture returns and conditions are right, they revive themselves and infest plants. You could put an infested leaf in an envelope, file it on a shelf and rehydrate the nematodes years later! Not exactly fair, yet very interesting.