P&PDL Picture of the Week for
February 21, 2005

Foliar Nematodes

Karen Rane, Plant Disease Diagnostician, Department of Botany & Plant Pathology, Purdue University

Plant parasitic nematodes are microscopic worms that are usually found in the soil attacking plant roots.  There are some nematode species, however, that can cause damage on aboveground plant parts.  Foliar nematodes (Aphelenchoides spp.) have become a more common pest in herbaceous perennials over the past few years.  Foliar nematodes move in films of water on plant surfaces and enter leaf tissues through natural openings in leaves called stomates.  Like root-attacking nematodes, foliar nematodes have a needle-like structure called a stylet that they use to pierce plant cells and feed on the cell contents, resulting in cell death.  Lesions caused by foliar nematodes are first chlorotic, then necrotic.  Movement of the nematodes within leaf tissues is restricted by larger leaf veins, which gives the lesions their typical angular shape (see figures).

Foliar nematodes overwinter in plant debris. They survive for long periods of time in leaf tissues, and are spread by propagating infested plants and by splashing water (rainfall, overhead irrigation).   The list of plants susceptible to foliar nematodes is quite large, and includes woody plants like azaleas as well as numerous herbaceous perennials (such as hosta, hellebores, ferns, begonias, salvia, anemone).  The easiest way to manage foliar nematode problems is to avoid introducing the pest into the garden. Carefully inspect plants for foliar nematode symptoms before purchasing.  If symptoms develop, remove and destroy affected plants. Do not put affected plants in a compost pile, since the nematodes can survive in plant debris and may be re-introduced to the garden from this source.  Commercial growers can use chemical treatments for controlling foliar nematode, but there are no effective pesticides available to homeowners.  For more information on foliar nematodes, check out the following links:

Foliar Nematodes (University of Maryland)

Foliar Nematodes as Pests of Ornamentals (University of Florida)

Foliar Nematodes (University of Minnesota)

Foliar Nematodes (University of Georgia)

 

Click on image to enlarge

Figure 1:  Leaves of Japanese anemone (Anemone x hybrida) showing angular necrotic lesions due to foliar nematodes

Figure 2:  Leaves of Lenten rose (Helleborus sp.) with chlorotic and necrotic lesions caused by foliar nematodes

Purdue Plant & Pest Diagnostic Lab Purdue Cooperative Extension Service