Nematodes: Getting to Know the Enemy Better
Bill Meyer, Woodbury, Connecticut
   Many consider foliar nematodes to be the #1 hosta pest and there is good reason for thinking so. These microscopic worms have proven to be the most difficult pest to eradicate in any environment from nurseries to home gardens. The heavy restrictions placed on pesticides in the latter part of the 20th century unleashed a plague of pests that were under control previously to the point that most had forgotten them. 
   From the tiny nematodes to the watermelon-seed-sized bedbugs, old insect enemies are back and flourishing and the weapons that once vanquished them are nowhere to be found. Most of us are used to thinking that we can eliminate a particularly bad pest from our garden, but to date the foliar nematode has won every battle. While the pesticides of old offered a chance of eliminating them, nothing available to the home gardener these days has offered more than a reasonable chance of keeping their numbers and the resulting damage to hostas down to an acceptable minimum.
   It must be understood that controlling them does not mean getting rid of them. It is theoretically possible to get rid of them with the pesticides we are no longer allowed to use, but so far it seems totally impossible to eliminate them with what remains available even to the nursery owner in most states. New pesticides have not appeared to replace the old effective ones and for now it seems foliar nematodes have scored a clear victory.
   The key to effective control lies in understanding them better, because knowing how they survive from year to year and how and when they move back into the hostas gives us our best chance of hitting them when they are most vulnerable. That's where the great 2005 study by Drs. Ganpati B. Jagdale and Parwinder S. Grewal comes in. Prior to this study, we really had little more than guesswork for how they overwintered, when and how they moved into the plants in spring, and how they moved from one hosta to another. This study is a bit easier to read than most scientific papers and most OLJ readers should get the gist of it without too much effort. What they learned in the study falls into two areas - how they overwinter and how they spread from plant to plant in the summer. In the sections below we'll look at what they learned and what it means to both gardeners and nurseries.

Hosta Nematode Facts

  Name   Aphelenchoides fragariae  
  Size   Under .8 millimeter long  
  First Discovered   1890 by Ritzema Bos  
  Reproduction   Sexually reproducing; males required  
  Life cycle   10-13 days from egg to reproduction  
  Egg Production   Female produces about 30 eggs  
  Time to Hatch   4 days  
  Survives in soil   About 3 months  
  Overwinters   Soil, Dead Leaves, Dormant Buds  
  Number of Host Species   Over 700 from 85 plant families  


   In their study, titled "Infection Behavior and Overwintering Survival of Foliar Nematodes, Aphelenchoides fragariae, on Hosta" Drs. Jagdale and Grewal found that nematodes overwinter in soil, dead leaves, and the dormant buds of hostas, but not in the roots. This indicates that fall cleanup of dead leaves will reduce the number of overwintering nematodes, as will disposing of old potting soil and replacing with new soil in the spring while it is still cold. Because some are also overwintering between the nascent leaves inside the dormant buds, however, it is impossible to prevent them from remaining in the plants and setting up shop again in the spring. Even a systemic pesticide probably cannot reach into the spaces between the tiny leaf initials on the dormant buds, and hiding there even offers good protection from fumigant-type pesticides. The nematodes really couldn't have found a better place to hide.
   While fall cleanup does not offer any hope of elimination, it should be the first step in efforts to control them. Removing all dead leaves before a spring warm-up will reduce the numbers that overwinter significantly, leaving fewer to try to kill by other means later. Pesticides are unlikely to work during the cold months because the nematodes have a dormant stage in which they are toughened and more resistant.
   The presence of foliar nematodes in the soil also makes it clear that control schemes like heating the plants, which can successfully kill all nematodes in the plants, are pointless in the garden because they will be re-infected when they are returned to the ground. Optimal times and temperatures for heat treatment probably vary significantly with different size plants and probably with different cultivars as well. To date no study has been done to determine for example the difference in heat treatment between a huge plant and a mini - one would expect that temperatures and times that would kill the mini might not even warm up a massive plant. 
   Other interesting results from this and other studies are that nematode eggs do not overwinter - only adults and juveniles - and that cold, even at levels found only at the Poles, has no real effect on them.


   The nematodes overwintering in the dormant buds were found to be hiding between the nascent leaves of the buds and not internally in the rhizome, roots, or other parts of a dormant hosta. As the soil warms in spring and the hostas emerge from dormancy the nematodes climb the outside of the petioles to enter the leaves. To climb and enter the leaves they require high humidity or wet conditions, which indicates that plants in controlled conditions such as greenhouses could be bottom-watered with ventilation keeping the humidity down as a step in controlling them. Applications of somewhat-effective modern pesticides made regularly by bottom watering could successfully kill nematodes in the pots before they can climb up into the plants.

Typical nematode damage in a hosta leaf

   Future studies hopefully can pin down the times and temperatures when they become active again. Outdoors, this information is still somewhat helpful in understanding when the nematodes may be ascending the outside of the plant and re-infecting it.
   Foliar nematodes do not seem to be terribly difficult to kill when they are on the exterior of the leaves and petioles, and even weak (and inexpensive) control measures like insecticidal soap and hydrogen peroxide can have good effect in reducing the numbers significantly. We still don't know how long the period of migration is, which is a piece of the puzzle that would be greatly helpful in moving towards actual elimination of the nematodes. The timing and length of the migration from soil level up into the leaves is likely very dependent on temperatures and how wet the exterior of the plant is, but a better understanding of these conditions should help define when pesticide applications will have the most effect. Killing them on the outside of the plant before they can get up to the leaves is the best shot we have today with our reduced chemical arsenal.
   Hopefully further research in that area will reveal more, as control measures during the migration stage hold promise to be the most effective. After all, it is at that time that their population is at its lowest, so killing them then will have the most impact on their numbers. Once the nematodes have entered the leaves by mid-spring or so, control becomes much more difficult as systemic or translaminar agents like Pylon are then necessary and the best of these are now restricted primarily to farming in most states, and are priced beyond what most gardeners are accustomed to paying.
Ganpati B. Jagdale and Parwinder S. Grewal
Astronauts of the Nematode World: An Aerial View of
Foliar Nematode Biology, Epidemiology, and Host Range
an excellent overview of the foliar nematode from Lisa M. Kohl of the USDA

Nematode photo above courtesy of Jason D. Stanley

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