Roundworms found in Hostas and Other Ornamental Plants

April 25, 2015

Nematodes are non-segmented, plant parasitic roundworms that are microscopic. Unlike most plant parasitic nematodes which feed on or within plant roots, foliar nematodes feed within or on plant leaves.  Foliar nematodes move by water film up plant stems and enter the plants through the leave’s stomata (small apertures in leaf surfaces that allow for gaseous exchange).  The nematodes feed within the mesophyll (interior cells of the leaf containing chlorophyll) and epidermis (the thin outer layer of cells) of the leaf by piercing cells with their stylets.  A nematode stylet functions in much the same manner as a hypodermic needle but pulls the plant’s liquid nutrients into the nematode’s digestive tract.  Foliar nematodes also infest stems and flower buds.

Nematodes are non-segmented roundworms that are parasitic and microscopic.   Unlike most parasitics, this feeds right on the leaves as opposed to the roots. There are two types currently present in Michigan, Aphelenchoides Fragariae and Aphelenchoides Ritzemasbosi. The Fragariae primarily attack strawberry, hosta and fern plants. The Ritzemasbosi attack a huge range of perennials and bedding plants, including but not limited to:

• African Violets
• Boston Ferns
• Bird’s Nest Ferns
• Begonias
• Dahlias
• Gerberas
• Hibiscus
• Geraniums
• Phlox
• Zinnia
• Iris

The development of these roundworms is quite quick, they grow from egg to adult in as littles as 10 days. They do have a short life cycle, but their production rate keeps them present, which is about 3500 eggs.

Typical damage is noticeable between the veins of the leaves. Yellow streaking, especially on hostas, is an early symptom, but later the yellow turns to brown.  On other species, blotchy and wedge-shaped yellow or brown areas appear. Both the top and bottom of the leaf will show the symptoms, and eventually, the entire leaf will die. Damage can also be stunting or dwarfing of basal stems, causing a bushy appearance on some species.  Expanding leaves may become crinkled and otherwise mis-shaped.  When flower buds are infested, the flowers will be smaller or the buds may fail to develop.
Presence and damage should be confirmed by laboratory analysis.  Control is more effective if the problem has been identified correctly.One problem is a large host range.  Many species of plants in a greenhouse setting allow easy spread by splashing water and leaf-to-leaf contact.  Foliar nematodes are easily spread in propagation material, like stem cuttings and leaf cuttings, from infested plants. Because these nematodes can be spread by splashing water, overhead irrigation should be avoided.

The most effective control is of course, prevention.  Acquire your stock from safe sources.  Good sanitation practices go a long way.  Always remove and destroy plant debris at the end of the growing season.  Do not allow dead tissue to overwinter in place.  Plants known to be infested should be removed and burned.  In greenhouses, benches, pots and other equipment should be steam cleaned before introducing other plants back into the area.

Hot water treatments are one method of controlling foliar nematodes when destroying the plant is not an option or is unwanted.  There are several species that can withstand temperatures high enough to kill the nematodes without killing the plants.

Cultural practices, hot water treatments, and clean stock remain the best methods of avoidance and control.