Foliar Nematodes are microscopic round worms, which are not visible to the human eye. Nematodes live on the topside of the leaf and in the crown. They can devastate the appearance of a hosta leaf in a short period of time.  They do not kill the plant, but they may weaken the plant by lowering its ability to generate energy for the next growing season.  You will not know you have them until the leaves of your hosta show brown streaks between the vein tissue of the leaf. They do not cross the veins.  Foliar nematodes can be found in over 250 other host plants. Check the plants around the suspect plant.

Foliar nematodes spread by contact from plant to plant via water as the method of transportation. Rain, and overhead watering, would be an example.  Experts suggest drip irrigation, but that does not stop the rain as the vehicle of transfer. While they love water, heat allows them to reproduce faster, and dry conditions allow them to live for several years in dying plant materials.

In Minnesota, the sign of nematodes start showing up in July, and by August you will know for sure. Their entire life cycle can be completed in 2-4 weeks, and faster if the temperatures are high. One hosta plant can have multiple generations of nematodes living within.

Remove all infected leaves to the garbage, do not the compost. Clean up all leaves in the fall, and minimize overhead watering. You can dispose of the entire plant or accept you have this problem knowing the nematodes live in the crown, and possibly the ground and are hosted by many other plants within the garden.

A few years ago The Minnesota Hosta Society purchased an organic product called ‘Nemakill® as a group. It is sold by ExcelAg, Corp. We did not collect data on the results of the usage. This product has not been proven to eradicate the foliar nematodes, but at this time it may be our only hope.

For more information:

Foliar Nematodes

MHS Established 1991