08/15/2002

Stop Hosting Foliar Nematodes on Your Hostas

Writer:

Mauricio Espinoza
espinoza.15@osu.edu
(330)202-6109

Source:

Parwinder Grewal
grewal.4@osu.edu
(330)263-3963

WOOSTER, Ohio — The time has come to put up a fight against those microscopic intruders that are getting a free lunch at the expense of your hostas.

Parwinder Grewal —an entomologist with the Agricultural Research and Development Center (OARDC) in Wooster— has found an effective way to combat foliar nematodes, which have been preying on the country’s most popular herbaceous perennials during the past six years.

Foliar nematodes (Aphelenchoides fragariae and Ditylenchus dipsaci) are microscopic, non-segmented roundworms that cause damage to the foliage of a large number of flowering ornamental plants in nurseries, greenhouses and landscape plantings.

And yes, they love a succulent hosta meal. Foliar nematodes live and feed on tender tissues of the plants' broad leaves, which first turn yellow, then chocolate brown, and finally die.

"The exact cause of this nematode outbreak is unknown," said Grewal, an internationally renown nematologist. "One likely explanation is the unavailability and reduced use of broad spectrum pesticides that may have been suppressing the nematodes in the past. In fact, the outbreak coincides with the withdrawal of oxamyl [Vydate], the most effective pesticide against foliar nematodes, back in 1995."

With the assistance of postdoctoral researcher Ganpati Jagdale, Grewal studied 50 varieties of hosta donated by growers from the Midwestern states, all of which had been attacked by A. fragariae.

"Our research shows that most standard nematicides that are still available to licensed users can substantially reduce nematode multiplication in leaves and soil," Grewal said. "Several insecticides commonly used in the ornamentals industry for insect and mite management are also effective."

Dylox (80S) and Mocap (10G) helped reduce nematode multiplication in the leaves by as much as 90 percent, 45 days after the treatment. Other chemicals such as oxamyl (10G), Diazinon, Dylox (6.2G), ZeroTol (hydrogen dioxide) and insecticidal soap killed 60-75 percent of the roundworms.

"There are no chemical nematicides that fully cure the nematode problem after plants get infected," Grewal pointed out. "However, commercial growers and homeowners can significantly control the nematode population by using these insecticides."

According to Grewal, the war on foliar nematodes has to be fought in the soil, too.

"Since A. fragariae is capable of feeding and reproducing on soil fungi, survival in the soil is the nematodes’ primary means of overwintering," he said. "In one experiment, we found that when all infected leaves and shoots were removed before the winter, the plant again developed nematode symptoms the next fall. What happens is that these nematodes can escape freezing by moving deep into the soil."

Fortunately, Grewal’s experiments show that Diazinon, Mocap, oxamyl (10G), ZeroTol and Dylox (80S) can cut the nematode population in the soil by as much as 75 percent.

ZeroTol and insecticidal soap are the only two nematode-control agents available for home gardeners, since the rest of the chemicals Grewal tested can only be used by individuals holding a valid Pesticide Applicator’s License.

Since none of these pesticides has been registered specifically for the treatment of foliar nematodes, Grewal advises users to follow label instructions for insect or mite control.

Grewal also experimented with "heat treatment," a method that has been successfully employed in Europe to eliminate foliar nematodes from the bulbs and rhizomes of many ornamental plants.

"Our research showed that submerging infected hosta leaves in water at 124 degrees Fahrenheit for 10 minutes killed 100 percent of the nematodes," he indicated.

Heat treatment can be used to treat isolated plants prior to breaking dormancy in the nurseries. However, Grewal said, the utility of this technique for home gardeners is questionable, as the nematodes can survive and reproduce on saprophytic fungi in the soil.

Regardless of the weapon you choose to fight foliar nematodes, their presence should be first confirmed. To do this, remove small pieces of the leaf tissue showing symptoms, place them in a clear glass dish and add sufficient water to submerge the tissues. After 24 hours, carefully examine the water with a 10X hand lens and a strong light.

"You will see the nematodes as actively moving, glistening white threads," Grewal explained. "Further confirmation and complete diagnosis can be made by sending suspected plant tissues in a plastic bag to a laboratory that analyzes nematode samples."

The following tips will also help you keep your hostas from hosting those unwelcome guests:

*Do not wet foliage while watering. The nematodes move in films of water outside of leaves and stems.

*Cut and burn infected leaves. Nematodes can become dormant and survive for over a year in fallen leaves; when the dried leaves are exposed to water, the nematodes revive and infect new plants.

*Use mulch. Nematodes are unable to move on dry surfaces.

*Clean your plants during the fall, removing all dried leaves and stems.

*Apply insecticidal soap or ZeroTol as soon as the nematode symptoms become evident in July or August.

Grewal's research was funded by OARDC, the American Hosta Society and the Horticultural Research Institute.

OARDC is the research arm of Ohio State University's College of Food, Agricultural, and Environmental Sciences.