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FOLIAR NEMATODES IN HOSTA AND OTHER ORNAMENTALS

By Carolyn Diamond, Region 3 Inspector
Pesticide and Plant Pest Management Division
Michigan Department of Agriculture and Rural Development

Nematodes are non-segmented roundworms.  Plant parasitic nematodes 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.

There are two species of foliar nematodes known to occur in Michigan, Aphelenchoides fragariae and A. ritzemabosi, which have a host range of herbaceous perennials and bedding plants.  A. fragariae primarily attacks strawberry, hosta, and fern.  A third species of foliar nematode, A. besseyi, is more commonly associated with tropical and sub-tropical plants.  A. ritzemabosi can feed both ectoparasitically and endoparasitically, with the latter causing the most significant damage.

Nematode Development
Foliar nematodes have a short life cycle and can grow from egg to adult in as little as 10 days. Optimum temperatures for foliar nematode development are 21-24C (70-75F).  The nematode reproduces by laying eggs which hatch to release larvae.  Each larva molts as it enlarges and develops into a mature worm.  Females lay 25-50 eggs in leaf axils or within leaves.  Each female can lay as many as 3,500 eggs in her life span.

Host Range
Many different plants are susceptible to Aphelenchoides, including African violet, Anthurium, Boston fern (Nephrolepis), bird's-nest fern (Asplenium), columbine, Begonia, Crossandra, Cyclamen, Gloxinia, Dahlia, Gerbera, Hibiscus, Lantana, Mimulus, Geranium, Cineraria, Primula, Ranunculus, Thanksgiving cactus, India rubber tree (Ficus elastica), Phlox, Zinnia, and Iris to name a few.  A. ritzemabosi has a host range of some 200 plant species.

Symptoms and Damage
Typical damage is characterized by necrotic zones between the veins of the leaves.  Leaf veins are a barrier to foliar nematodes.  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.  The discoloration will be between leaf veins.  Both the top and bottom of the leaf will show the symptoms, and eventually, the entire leaf will die.

Foliar nematode 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.
 
 
Foliar nematode damage on Hosta photos courtesy of Missouri Botanical Garden

Management
Foliar nematode 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.  Only nematode-free plants should be propagated in this way.

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.

Host plant resistance is not feasible, at this time because of the large host range.  However, there is a degree of resistance in some chrysanthemum cultivars.

The bulb mite, Rhizoglyphus echinopus, was found to feed on A. ritzemabosi; however, this method is in the research stages and is not readily available.

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


References:
1. Kohl, L. M. 2011. Astronauts of the Nematode World: An Aerial View of Foliar Nematode Biology, Epidemiology, and Host Range.
2. University of Illinois Extension Report on Plant Disease RPD No. 1102, July 2000
3. http://msue.anr.msu.edu/news/take_steps_to_avoid_foliar_nematodes   (2006 Fred Warner)
4. http://msue.anr.msu.edu/news/foliar_nematodes_are_enemies_of_greenhouse_growers  (2009 Fred Warner)
5. http://www.apsnet.org/publications/apsnetfeatures/Pages/foliarnematodes.aspx Kohl
6. http://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/pests-and-problems/diseases/nematodes/hosta-leaf-nematodes.aspx
7. Sturhan D, Hampel G, 1977. Plant-parasitic nematodes as prey of the bulb mite Rhizoglyphus echinopus (Acarina, Tyroglyphidae). Anzieger für Schadlingskunde, Pflanzenschutz, Umweltschutz, 50:115-118