Late in the summer, watch the leaves of herbaceous perennials and other plants for symptoms that look like a patchwork of multicolored, angular-shaped leaf spots. This could be the work of foliar nematodes. Read on to find out all about this interesting plant pathogen.
Nematodes are non-segmented roundworms. They range in size from less than a half millimeter (microscopic) to nearly eight meters long and live in moist or aquatic environments. While most of the approximately 20,000 named species feed on microbes, fungi or other tiny organisms (even other nematodes), some are parasitic on plants or larger animals, including humans. Most plant parasitic nematodes are soil inhabitants that damage plants by feeding in or on the roots. Foliar nematodes, in contrast, feed on above-ground plant parts, causing injury to leaves, buds and young stems. The most common species is the strawberry leaf nematode, Aphelenchoides fragariae. It is found in temperature regions worldwide and is a common inhabitant of wild woodland plants. Other species in the northeast include A. besseyi and A. ritzemabosi, the chrysanthemum foliar nematode.
As a group, foliar nematodes have a very large host range that includes many herbaceous (over 200) and some woody plants. Susceptible herbaceous plants include African violet, anemone, Anthurium, columbine, Begonia, Brunnera, Chrysanthemum, Cyclamen, Gloxinia, Dahlia, Gerbera, Hibiscus, iris, Lantana, lily, Geranium, Hosta, Primula, Heuchera, peony, Ranunculus, Salvia, strawberry, Thanksgiving cactus, orchids, ferns and others. Woody hosts include privet, azalea, rhododendron and Ficus.
Symptoms include stunting, twisting and curling of new growth and leaf spots delimited by leaf veins. Early season feeding on young shoots, buds and developing leaves can result in deformity and poor growth. Following this period of external feeding, foliar nematodes enter the leaves through the stomates (pores) and begin feeding on mesophyll cells. Their movement within the leaf is restricted by the veins resulting in a distinctive pattern of yellowing and browning. In monocots such as Hosta, iris and lily, the veins are parallel to the leaf edges and foliar nematode injury appears as longitudinal areas of yellowing and browning. In dicots, the veins form a lace-like network and the spots have an angular, patchwork pattern. Symptoms in the landscape tend to appear during mid to late summer as the population builds up over the season. As plant damage increases, leaves may brown, wither and fall from the plant. The host plant is not usually killed. In some begonias, foliar nematodes colonize the plant’s vascular tissue and the plant may remain symptomless.
Foliar nematodes have a life cycle typical of other plant parasitic nematodes that includes the egg, four juvenile stages, and adults. Adults (Figure 3) and juveniles overwinter below ground in the soil, plant debris and living plant tissue such as below-ground buds. Overwintering populations are generally low and numbers increase during the growing season. As new growth begins in the spring, nematodes migrate up onto plant surfaces when a film of water is present and begin feeding. Early feeding sites include stems, buds and young leaves. As the plant matures, the nematodes enter the leaves and feed there for the remainder of the season.
Reproduction occurs within the leaves and a complete life cycle can be completed within 2-4 weeks depending on temperature. When the leaf surfaces are wet, nematodes can move from one feeding location to another by exiting through a stomate and re-entering the leaf through another. Some studies indicate that they are able to enter the leaf directly in addition to through stomates. They spread from leaf to leaf or plant to plant when they are in contact or via splashing water. Spread can occur during vegetative propagation or when infested plant material is introduced into greenhouses, nurseries or gardens. Foliar nematodes can survive in a dormant state for several years in dried plant material.
Prevention and sanitation are the key elements of foliar nematode management. Once they are established in a landscape, it is difficult to completely eradicate them. Always inspect new plant material for foliar nematode symptoms. Segregate new plants before planting them near susceptible hosts and monitor them for symptom development.
Spread of foliar nematodes can be reduced by minimizing periods of leaf wetness. Avoid overhead irrigation. Space plants generously to allow for plenty of drying airflow between them. Do not work among plants when they are wet.
Monitor garden plants to detect an infestation early. Remove infected leaves when they are dry and dispose of them in the trash. Severely affected plants should be removed completely and discarded. Do not compost foliar nematode-infested plant material. If you do decide to use a product in combination with other strategies, an option is insecticidal soap. Effectiveness will be limited because nematodes within the leaf tissue will be protected from exposure.
Photos copyright Joan Allen 2013.