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Foliar Nematodes

While most nematodes are soil-dwelling, the foliar nematode Aphelenchoides lives only briefly in soil. More importantly, it lives on the above-ground portions of plants, often without causing any obvious symptoms.
Piggy-back Plant

Piggy-back Plant

Aphelenchoides like all other nematodes is a non-segmented roundworm. Compared to other plant-infecting species, it is rather large, often 0.5 to 1.2 mm (0.02 to 0.05 in.) long. It has a fine, needle-like, hollow spear mouth part (stylet; a structure characteristic of plant parasitic nematodes and lacking in free-living and animal parasitic nematodes) that is pushed into the plant cell. The worm forces enzymes through the stylet into the cell where cell components are digested and then drawn back into the nematode's digestive system through the stylet. Of the more than 220 known species in this genus, the most important species on ornamentals are Aphelenchoides besseyi, A. fragariae, and A. ritzemabosi.

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) and iris.

The nematodes can remain on the outside of the plant or can force its way into leaf and stem tissue. Wetness on the stems and leaves provides an excellent environment for their movement. Splashing water during irrigation readily moves the nematodes from leaf to leaf and plant to plant. In some cases such as certain types begonias (Elatior), it can move into the water conducting tissue and develop large populations without causing any symptoms. Optimum temperatures for foliar nematode development are between 21 and 24C (70-75F). The nematode reproduces by laying eggs which hatch to release a larva. The larva molts as it enlarges and develops into a mature worm.

Symptoms

  • Yellow, brown to purple to black wet-looking areas on leaves
  • Angular, yellow areas on the leaf bounded by the veins of the leaf
  • General yellowing, reddening, or bronzing of leaves, not limited in shape by veins (begonia)
  • Death of leaves that remain attached to the plant
  • Cupping and distortion of leaves (African violets especially)
  • Small, sunken areas on the undersides of leaves
  • Stunting of the entire plant
  • Chlorosis similar to iron deficiency

Plants with symptoms characteristically occur in scattered clumps because of the splashing of nematodes from one or two plants to neighboring plants.

Detection

Remove tissue showing symptoms or symptomless tissue you desire to examine.

Using tweezers, needles, or some other clean instrument, tear the tissue into small pieces in a clean drop of water in a clear glass container or on a microscope slide. Use as little water as possible but enough so that some of the pieces are submerged. Allow the pieces to soak for a few minutes.

Using at least 10X magnification, look for small worms moving in rapid, snake-like motions. It is important to have the lighting correct and a dark background color to see the worms. The nematodes will sink to the bottom of the container over time.

Management

  • Plant nematode-free material.
  • Discard infected plants.
  • Do not use overhead watering.
  • Space plants well so that any splashing during watering is minimized and plant surfaces dry quickly.
  • If foliar nematodes are encountered, examine symptomless begonias for the presence of the worms.

While some insecticides greatly suppress the activity of foliar nematodes and can reduce the nematode population, none are known to totally eliminate foliar nematode from an infected plant. In some cases, the rate of insecticide necessary to suppress this nematode is phytotoxic to the plant.

foliar nematodes

Prepared by Gary W. Moorman, Professor of Plant Pathology

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Foliar Nematodes

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