Last spring, I started noticing odd oblong spots on my Solomon’s seal (Polygonatum odoratum). At first, I wasn’t worried, merely curious. Then some of the spots morphed into slits in the leaves.
In June, similar oblong spots appeared on a lily of the valley, followed by dark spots on ferns and brownish-purple spots on my beloved geraniums (Geranium macrorrhizum and Geranium x cantabrigiense ‘Biokovo’). Before long, I was spending much time studying not only my plants, but also hunched over my computer trying to get a handle on this mystery.
My research suggested that the Solomon’s seal and lily of the valley were infected with something called foliar nematodes. Foliar nematodes — what were they? I wanted confirmation, so I enlisted the Help Desk at the Extension Office, which called in the cavalry in the form of the Plant Disease Clinic at Virginia Tech. The diagnosis: foliar nematodes. The official report had an apologetic tone, something I now fully understand, because, as one authority puts it, “little can be done to salvage a plant that is already infested” with foliar nematodes. Univ.of Florida Dept.of Entomology & Nematology. This was devastating news.
The upshot was that most of the perennials in my front garden were infected, including my mainstay geraniums, my huge swaths of stinking hellebores, and ostrich ferns. The only advice was to pull up and destroy infected plants, which would denude the garden I had created and tended for years. This felt like a tragedy, but in the midst of the pandemic, any diagnosis other than Covid-19 seemed like a lesser matter.
As time went on, I learned a lot about foliar nematodes, and I’m eager to share what I’ve learned with other gardeners. As I’ve discovered, if you’re armed with the best information, you can spot foliar nematodes early on and prevent widespread damage. Let’s start with the basics.
Foliar nematodes are one kind of tiny parasitic roundworm, of which there are many:
You may think humans own the planet. You’d be wrong. Worms like this one are Earth’s animal overlords; nematodes are the most numerically abundant animals on Earth. They’re not just a slim majority. Four out of every five animals on Earth is a nematode.
— “Nematode Roundworms Own This Place,” Scientific American
So the earth teems with nematodes. There are also many types of nematodes — including the root-knot nematodes with which you may be familiar; they attack the roots of plants. But foliar nematodes attack the foliage and stems, basically “eating” the cell tissue using a needle-like mouth part called a stylet. There are several nematode genera that feed on foliage and stems, but it is the genus Aphelenchoides that impacts some food crops and many garden plants. The primary threats are Aphelenchoides fragariae, Aphelenchoides ritzemabosi, and Aphelenchoides besseyi, with the A. besseyi species more common in tropical regions, and the A. fragariae and A. ritzemabosi more common in temperate regions.
These three species of Aphelenchoides feed on the host plant’s surface or enter leaves through open stomata (natural openings, like pores in the leaf surface). To migrate outside the leaf, they are dependent upon water; any time a film of water is present on a leaf from irrigation, rain, or even dew, the nematodes can move to another location on the plant, or, more worryingly, to a nearby plant. The role of water in nematode migration is a key fact for gardeners, and it’s the source of the primary advice for gardeners with foliar nematodes: stop overhead irrigation. Research indicates that nematode populations increase during late spring and late fall when the air temperatures are moderate and relative humidity is high, and also during rainy periods. Since their reproductive cycle is very short, populations can increase rapidly.
Foliar nematodes have a huge host range; they have been found on over 700 species of plants, including 126 plant families, from monocots to dicots, gymnosperms to angiosperms, and to ferns, liverworts, and clubmosses. For a seemingly endless list of affected plants, see Nematodes/Host Species. Worse yet, one plant group may be a suitable host for more than one species of foliar nematode. Although they have a broad range of host plants, foliar nematodes are most commonly found on many herbaceous perennials, as well as on strawberries, rice, and other edible crops. For more about their destructive impact on strawberries and damage potential for other food crops, see Foliar or Bud Nematodes in Florida Strawberries/Univ.Fla.Ext.
In ornamental gardens and among house plants, the more common host plants are hosta, chrysanthemum, ferns, peonies, begonias (often symptomless), anemones, baptisia, Hepatica, Heuchera, Hypericum, iris, lilies, Ligularia, orchids, Oriental poppy, phlox, Polygonatum, Rogersia, salvias, verbena, zinnia, African violet, and toad lily. Woody plants seem to be infected less often, although infections have been reported on rhododendron, azalea, privet, Ficus and abelia, as well as on wild forest plants. The plant families with the highest number of reported diagnoses are: Asteraceae, Ranunculaceae, Scrophulariaceae, Primulaceae, Lamiaceae, and Liliaceae.
Looking at host plants from a local perspective, it is useful to note that between 2005 and 2020, the Plant Disease Clinic at Virginia Tech diagnosed foliar nematodes on the following types of plants (the number in parentheses indicates the number of times diagnosed during those years):
Coral bells (3)
Lily of the Valley (1)
Solomon’s Seal (1)
Butterfly Bush (3)
Commercial nurseries often have foliar nematode outbreaks, and it is newly-purchased plants that are often the source of foliar nematodes in home gardens. Another source is nearby weeds — yet another reason to eradicate weeds as soon as they appear.
Foliar nematodes are tough little characters. They survive harsh winters, hot summers, and can survive for several years in dead or decaying plant material, including leaves dropped by infected plants in the autumn.
How to Spot Foliar Nematode Damage in your Garden
The typical symptoms of foliar nematodes are spots on the leaves, but it’s important to note that a plant can be infected without showing any symptoms, especially in the early stages of infestation. Because most foliar nematodes cannot cross through a major vein, the lesions or spots made by munching nematodes are usually separated by leaf veins. Since they can’t chew through a leaf vein, the nematodes have to slither out onto the surface to enter into another area of the leaf. If a spot is isolated between veins, you may very well have foliar nematodes. The “vein-delimited” nature of the lesions governs their shape, so the lesions may be either angular leaf spots, long thin streaks, or V-shaped wedges. Angular and oblong leaf spots — which are the signs I first observed in my garden — merit close attention.
But remember that not all nematode spots look alike. The lesions on my perennial geraniums looked very different; they were a dark purplish-brown and not particularly oblong. And the lesions on my ferns were different from all the others. The one thing they all had in common, though, was the vein-delimited pattern.
On monocots (which usually have parallel leaf veins) such as lilies, hostas, and iris, the injuries look like stripes or rectangular spots. On dicots (which usually have net leaf venation) such as anemone, ligularia, and peony, the discolored areas look like a patchwork of purple, yellow, or tan areas.
Gardeners should also be aware that the spots can progress from light colors to darker colors and even to holes or slits as the infestation progresses. I noticed this development in my ostrich ferns.
The following list details the variety of symptoms that might be found, depending upon the plant involved:
Possible Symptoms of Foliar Nematodes: (depending upon the plant)
–Yellow, brown to purple to black wet-looking areas on leaves
–Angular, yellow areas on the leaf bounded by the veins of the leaf
–General yellowing, reddening, or bronzing of leaves, not limited in shape by veins (begonia)
–Death of leaves that remain attached to the plant
–Cupping and distortion of leaves (African violets especially)
–Small, sunken areas on the undersides of leaves
–Stunting of the entire plant
–Chlorosis similar to iron deficiency
If you suspect foliar nematodes, you can confirm the diagnosis with a 10X hand lens. Tear off a leaf and submerge it in a small dish with water. Use a clean tweezer, tear the plant tissue into small pieces, and use only a small amount of water. After approximately 24 hours, examine the water with the hand lens and look for tiny roundworms moving about rapidly. Of course, you can also confirm a diagnosis through the Plant Disease Clinic at Virginia Tech by contacting the Help Desk at the Extension Office.
How to Deal with Foliar Nematodes
There’s a reason why many authorities recommend destroying any plant infected by foliar nematodes: there is no way to eradicate them, and they can easily spread to other nearby plants. The effective pesticides — fairly effective, that is — are available only to licensed commercial applicators, and if you’ve got a major infestation, you may want to consider consulting one of these professionals. Even commercial nurseries are limited in the available weapons because some pesticides that were effective are no longer approved for use.
One effective measure is soaking the infected, dormant plant in hot water, but this treatment requires very particular temperatures for particular time periods, yet still risks death or injury to the plant. For these reasons, it’s rarely recommended except for so-called “high value” plants. Most of us would say that all of our garden plants are “high value,” but I suspect that the experts have another definition. To learn more about the hot water treatment, start with Purdue landscapereport.org.
Interestingly, research suggests that the bio-pesticide clove oil can reduce foliar nematode numbers, but there are few EPA-approved pesticides capable of significantly reducing foliar nematode populations, and except for insecticidal soap, most are not available to homeowners. To read more about this, check out Rutgers.ed/Research Summary (2018).
The primary advice offered by most experts is to dig up, bag, and remove any affected plant. I had a difficult time mustering the courage to do this. Instead, I emailed one of the researchers whose article I had read — Lisa M. Kohl — and received a reply that gave me an alternative approach. Here is Lisa Kohl’s advice:
“As far as control options go for homeowners, foliar nematodes can be tricky because there aren’t a lot of pesticides labeled for use. However, if a gardener doesn’t mind putting a little time in, they can make a big dent in the population and get to a point where the damage is barely noticeable if they spend time removing infected plant material. Gardeners would want to target two areas: the symptomatic (spotted) leaves on the plants and the dead/dried leaves on the ground. The foliar nematodes live in green leaves during the summer. The leaves then dry up and fall off the plant. The nematodes also dry up and remain inside those dried leaves to overwinter. In the spring, the nematodes emerge from the dried leaves on the ground and migrate back up plants to infect healthy leaves.
If a gardener makes an effort to remove the infected/spotted leaves from the plants and the dried leaves on the ground they should eventually reduce the nematode population. If they remove the infected leaves on a regular basis (maybe at least once a month or so) the nematode population should eventually get to a point where there won’t be many leaf spots any more. Since foliar nematodes are mainly an aesthetic problem, the gardener can decide how many leaf spots they’re willing to tolerate in their plantings. They may decide they don’t want any spotted leaves at all, or they may decide it’s okay to have a couple infected leaves here and there.”
Following Lisa Kohl’s advice — which was endorsed by Elizabeth Bush of the Plant Disease Clinic at VA Tech — I could soon be found snipping and pulling infected leaves from the geraniums, ferns, stinking hellebores and Solomon’s seals in my garden. I maintained a special garbage bag just for these pickings, and I was careful to sanitize tools and my hands afterwards. Within a few weeks, I was already seeing results; the spotted leaves were rapidly replaced with fresh, new, lesion-free leaves. Before I started, almost all my geraniums were spotted. By the end of the summer, almost all of my geranium leaves were free of spots!
Would I recommend this approach in all circumstances? No, I would not. In many situations, especially if the infected plants are isolated, it would make sense to remove all infected plants. Gardeners have to assess their situation with the migration potential of foliar nematodes firmly in mind. But since almost all the plants in my front garden were infested, it made sense to try to save them and focus on reducing the populations. By the way, it’s possible to propagate nematode-free plants from the roots of infested plants. These roots should then be planted into sterile media.
Here’s the sanitation drill for infected leaf picking: After working with infected plants, wash your hands with soap and water and decontaminate anything that has come into contact with the plants (tools, pots, work surfaces, etc.) for 30 seconds with either 70% alcohol (rubbing alcohol) or 10% bleach. Spray disinfectants that contain approximately 70% alcohol can also be used. It is essential that all plant material you pick off be bagged and destroyed; otherwise you’re asking for new infestations elsewhere.
Unless and until more effective treatments are developed for homeowners, prevention will continue to be a gardener’s best hope. Here’s how you can help protect your garden from foliar nematode attacks:
- Monitor newly-acquired plants for two to four weeks before planting them in your garden, and watch for vein-limited spots
- Avoid using overhead sprinklers and spraying; use soaker or drip hoses instead
- Avoid working with plants when they are wet
- Do not take cuttings from nematode-infected plants
If all your preventive measures fail, do not despair. Keep in mind that, at least with ornamental plants, the foliar nematode lesions are mostly an aesthetic problem, and carefully consider the best approach for your situation.
Featured Photo: Foliar Nematode spots on anemone by Elizabeth Bush, Plant Disease Clinic, VA Tech.
“Foliar Nematodes,” Univ. of Florida Dept.Entomology & Nematology/ufl.edu (Feb. 2020) and Univ.Fla.Ext.
“Astronauts of the Nematode World: An Aerial View of Foliar Nematode Biology, Epidemiology, and Host Range,” APSnet.org/The American Phytopathological Society (Lisa M. Kohl 2011)
“Foliar Nematodes,” Purdue University Landscape Report (May 2020)
“Foliar Nematodes: A Summary of Biology and Control with a Compilation of Host Range,” American Phytopathological Society, APS Journals/apsnet.org (Lisa M. Kohl, 2018)
Foliar Nematodes,”Univ.of Wisconsin Garden Fact Sheets (2015)
“Foliar Nematodes: Flowers,” Univ.Maryland Extension/HGIC
“Take steps to avoid foliar nematodes,” MSU Extension, Michigan State Univsity Diagnostic Services (2006)
“‘Patchwork Plants’ Created by Foliar Nematodes,” Univ.Conn. Home & Garden Education Center (2013)