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Foliar Nematodes

January 13, 2014

In recent years there has been an increase in the occurrence of foliar nematodes on annuals, perennials, potted crops, and woody ornamentals.  Foliar nematodes are plant-parasitic nematodes that are highly specialized, microscopic (adults are about 1/15th of an inch long) roundworms which feed on plant leaves and stems.  

Growers should note that foliar nematodes are very different from the beneficial nematodes (Steinernema feltiae and S. carpocapsae) used to control fungus gnat larvae, western flower thrips, and shore fly larvae. Beneficial nematodes do not feed on or cause injury to plants.

There are numerous species of foliar nematodes that commonly attack ornamental crops including: Strawberry leaf nematode (Aphelenchoides fragariae), Chrysanthemum foliar nematode (Aphelenchoides ritzemabosi) and the stem and bulb nematode (Ditylenchus dipsaci).  

There are over 300 plant species which are hosts to foliar nematode feeding; this includes a wide range of perennials including, but not limited to, Anemone, ferns, hostas, and phlox) and woody ornamentals including Hydrangea, Rhododendron and Viburnum to name a few).  Many growers are unaware that foliar nematodes can also feed on a number of annuals, bedding plants, and potted crops such as African violet, Ageratum, basil, Begonia, Cyclamen, Gerbera, Kalanchoe, Viola, and Zinnia.  There are many more plants not listed that are documented as host plants for foliar nematodes.

There must be a thin film of water present for foliar nematodes to move across the leaf and plant surfaces. They are frequently splashed from plant to plant in the splashing water that occurs with rainfall or overhead irrigation.  Nematodes can feed on the outside of the plant which causes the new growth to appear curled, twisted, stunted, or there may not be any visible symptoms, particularly when low populations of nematodes are present.  

Foliar nematodes enter plant leaves through the stomata; once inside the leaf they feed on the spongy mesophyll cells. Inside the plant, they can complete their lifecycle in as little as two weeks.  Nematodes commonly overwinter in an inactive state on plant debris that has fallen to the ground and often become active again as the temperatures warm up and wet conditions are present.

Nematode populations have to be relatively high for visible symptoms to be present.  As the nematodes feed, visible symptoms may develop. Affected leaves may change colorations starting pale green, then yellow and possibly appearing brown later on.  The most common symptom is a brown to necrotic coloration that is confined to the areas between the veins of the leaf (since it is difficult for them to move across the veins) creating a patch-like appearance on dicots and a stripe-like pattern with monocots.  

The symptoms of angular spots are often mistaken for bacterial leaf spots or fungal pathogens such as downy mildew which are also commonly confined between the leaf veins.  

Many of the nematicides and insecticides that successfully controlled foliar nematodes in the past are no longer registered for use in the United States.  This is likely one reason there has been an increase in foliar nematode infections in recent years.  

With limited chemical control options, what are the best options for managing foliar nematodes?  Even though foliar nematodes are considered a minor pest, it is best to use an integrated approach to managing them.  Plants susceptible to infestations should be scouted regulatory to observe signs of infections early when they can be managed.  Severely infected plants should be removed from the production site and destroyed to prevent the distribution on foliar nematodes to non-infected plants.  If possible modify the irrigation practices to reduce splashing water from plant to plant and to limit the time to leaf surfaces remain wet after watering.  

Before applying any chemicals, be sure to confirm the presence of foliar nematodes by submitting samples to a diagnostic clinic.  Applying bactericides or fungicides to control leaf spots may not be an effective strategy when foliar nematodes are the culprits of the leaf injury. Multiple applications of products containing abamectin and chlorfenapyr, combined with good cultural practices have been effective at suppressing nematode populations. Of these, chlorfenapyr is the only product labeled for controlling foliar nematodes.

There are no good management practices for managing heavy infestations. The key is to implement management strategies on susceptible crops preventatively or upon early detection of their presence.  

Paul Pilon
Perennial Solutions Consulting

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