Foliar Nematodes
Click for larger image Foliar nematode on hosta (Hosta)

Foliar nematodes, Aphelenchoides spp., are considered disease-causing organisms that attack plant parts above the ground in over 200 host plants. They are an emerging problem for gardeners because of this broad host range, ability to spread quickly and lack of symptom recognition. Additionally, there are few products registered for use in the home landscape that can limit their infection and spread. Some of the most common hosts include anemone, strawberry, hosta, phlox, verbena, zinnia, carnation, impatiens, begonia, fern and African violet.

Symptoms and Diagnosis

The two most prominent foliar nematodes are A. ritzemabosi called the chrysanthemum foliar nematode and A. fragariae called the strawberry crimp or fern nematode. Both are serious pathogens of ornamental plants both in the greenhouse and outside in the garden. However, their host ranges rarely overlap except on African violet, begonia, gloxinia, Siberian bugloss, violet and verbena. The most prominent symptoms occur in the leaves of infected plants. Two interveinal symptom types are commonly found. The first is a development of linear lesions between the leaf veins causing the leaf to become striped in appearance. Plants that have parallel venation like hosta are likely to display this leaf pattern. The second leaf symptom which is more common is the development of angular, water-soaked lesions occurring between the veins of netted-veined plants. These lesions will become brown and eventually turn black and perhaps drop out of the leaf leaving a ragged, wind-tattered appearance. Other symptoms may appear as stunting, leaf proliferation or bunching of leaves around the crown, multicolored leaves, lack of flowering and plant death. It is not uncommon to find new leaves emerging without symptoms while older leaves turn brown and collapse or fall. This occurs because the spread of nematodes requires free moisture. Lack of water on the leaves causes them to become localized so that they cannot move to new plant parts. Thus, under dry conditions, new leaves may appear unaffected.

Life Cycle

Unique to this group of pathogens, foliar nematodes live in and feed upon the aerial portion of the plant, mostly leaves, and are rarely associated with the soil unless it is within infested plant debris. A common over wintering site is within the crown of the plant where new leaves will emerge in spring. They commonly spread from plant to plant by splashing water and leaf contact. Nematodes can swim in a thin film of water to move upward towards leaves where they enter stomata. Another way they spread is by vegetative cuttings of leaves, stems and crowns. Gardeners who are active in propagation are likely to move the pathogen with every division. Nematodes remain active, feeding on leaf tissue and repeating their life cycle multiple times in the growing season. Once it becomes cooler, they will migrate towards the plant crown, live in dormant buds or leaf debris.

Integrated Pest Management Strategies

1. Sanitation: remove all infested debris from the plant and ground. This should be buried or burned. Floors and benches of propagation areas should be thoroughly cleaned of debris. Tools and containers should be heat treated in an oven or steamed for 30 minutes at 180 F.

2. Destroy infested plants: it is almost impossible to rid the plant of foliar nematodes. Therefore, it is best to destroy infested plants. Plants that are suspected of being infected should be isolated from healthy plants.

3. Select disease-free plants: Only healthy, nematode-free plants should be purchased and used for propagation stock. Contact between plants and extended periods of free moisture from overhead irrigation should be avoided. Dormant plant material can be treated with warm water (120 F) for 15 minutes to eliminate the nematode infestation on valuable planting stock.

4. Resistance: Some resistant cultivars of chrysanthemum are available. At this time, no resistant varieties are known for other garden plants.

5. Pesticides: Commercial growers who are certified for using nematicides can control foliar nematodes on greenhouse and nursery crops. There are no nematicides labeled for residential users.

Organic Strategies

Strategies 1 through 4 are strictly organic approaches.

More images

Click for larger image
Symptoms of foliar nematodes on peony (Paeonia)

Pests and Problems

Click a link in the site map below to see other "Pests and Problems" pages