Foliar Nematodes on Ornamental Plants

The genus Aphelenchoides contains over 220 species, but A. besseyi, A. fragariae, and A. ritzemabosi are the most impactive species for ornamentals. These non-segmented roundworms insert their stylet into a plant cell to feed on the cell contents, and in the process, force enzymes into the plant tissue. The foliar damage that is a result of this feeding can be the stunting of the entire plant, angular areas on leaves of yellow, brown, or black color, distortion of leaves, death of leaves, sunken areas on lower leaf surfaces, general yellowing or blackening of leaves, and even chlorosis resembling iron deficiency.

Figure 2. Siberian Bugloss 'Jack Frost' (Brunnera macrophylla) is stunted with numerous brown angular areas as a result of nematode feeding. Photo: Sinclair Adam, Penn State

In many cases, the damage is bounded by the plant veins, and therefore the pattern of damage will relate to the leaf vein structure. On monocots such as Hosta, Iris, or Solomon seal the damage will be linear while with dicots such as Heuchera, Tiarella, or Brunnera the pattern is scattered throughout the leaf. Foliar nematodes can be found on perennials, greenhouse flowering plants, tropical plants, and fruits and vegetables.

Surface moisture on the foliage and splashing water from rainfall or irrigation will spread foliar nematodes and provide the ideal habitat for their success. The best temperatures for foliar nematodes are 70-75°F, but they can tolerate a wide range of temperatures. They overwinter in dead plant tissue or living buds on plants, where brown scars will be evident around those buds. Foliar nematodes lay eggs that become larvae which molt to become worms. In spring they emerge and climb plant stems to infect the leaf and stem tissue. With indoor plants, the life cycle continues without an overwintering phase. At optimum temperatures, the life cycle will be about ten days.

To ascertain if foliar nematodes are present in plant tissue cut some small sections where the damaged tissue and the healthy tissue meet (1/4 inch sections or smaller) and place the tissue in a clear glass container and add some water to get the tissue saturated with some bits submerged. After a few minutes using a good hand lens or microscope look for worms moving in the liquid. If the tissue stays in the liquid for 12-24 hours the worms will be motionless and straight because they drowned. It is also possible to use an empty water bottle by cutting the neck off the bottle and inverting the severed neck with the cap on in the bottle (Figure 3). Then add some tissue paper and the leaf segments that you prepared and add some water to cover the leaf segments.

Samples can also be sent to The Penn State Plant Disease clinic or you can contact your Extension office for assistance.

Figure 3. Severed water bottle neck with cap, water with tissue paper, and leaf segments used as a diagnosis tool. Photo: Sinclair Adam, Penn State

There are not many chemicals licensed for foliar nematodes on the market, and the recommended control is to discard infected plants. Inspect incoming plants and cuttings and plant only clean material. Avoid overhead irrigation. Practice good sanitation and if handling infected plants be sure to clean all surfaces where the plants were. Wash your hands thoroughly or wear disposable gloves when handling these plants.

Successful management of foliar nematodes requires an integrated approach using exclusion, discarding infected plants, weed control, and frequent inspections.