Foliar nematodes

About

Microscopic roundworms that live in leaves and stems instead of roots; they can infect more than 450 kinds of plants and are most common on hostas (Wisconsin). The damage is usually cosmetic, but it makes plants look ragged.

Spotting it

Damage

On mature plants, discolored blotches with straight edges form between the leaf veins, turn brown and dry, and may fall out, leaving holes, often not until late summer, around August (Wisconsin). New growth on young plants may twist and stay stunted (Wisconsin, Penn State).

How to find it early

In late summer, look for vein-bounded brown streaks or patches, often on several plants in a clump. To check, tear a piece of leaf into a drop of water, wait a few minutes, and look at it with a 10x lens against a dark background for wriggling worms, or send a sample to your extension plant clinic (Penn State).

When

Symptoms mostly mid to late summer.

Life cycle

They swim in films of water on wet leaves and stems and get into the leaf tissue, where females lay eggs; they do best around 70 to 75°F (Penn State). They survive winter in dry leaves, dormant buds and soil, tolerate drying and freezing, and spread by splashing water, infested plants and cuttings (Wisconsin).

What to do, gentlest first

Start at the top. Sprays are a last step; always read and follow the product label.

1. Start clean and keep leaves dry Prevent

   Inspect plants before you buy, though early-season plants may not show symptoms yet. Use drip or soaker hoses instead of sprinklers, space plants apart, stay out of wet beds, and never take divisions or cuttings from suspect plants (Wisconsin, Penn State).

2. Remove infected plants By hand

   Dig up, bag and throw out infected plants as soon as you see symptoms. Wash your hands, disinfect tools and pots for 30 seconds with 70% alcohol or 10% bleach, and don't replant susceptible plants in that spot, since the soil is nearly impossible to clean (Wisconsin).

   There are no chemical controls for home gardens, and hot water dips are not recommended at home because they easily kill the plant or fail (Wisconsin).

Plants it goes after

Anemone (windflower), Azalea, Bird's nest fern, Chrysanthemum (garden mum), Columbine, Dahlia, Garden ferns (ostrich, Christmas, lady, Japanese painted), Hosta (plantain lily), Iris, Strawberry, Veronica (speedwell)

Pressure by region

Ratings are estimates; problems follow infested plants and wet foliage more than climate.

Ratings come from extension sources where available; some are estimates, as noted above.