Foliar Nematodes In Nursery Crops
By: Robert A. Dunn, Department of Entomology and Nematology, University of Florida - IFAS
Foliar nematodes (Aphelenchoides spp.) have arisen again as serious nursery pests in 1995, on crops including azaleas, mums, and others in Florida, the Northeast, and the Pacific Northwest. These are among the few plant- parasitic nematodes which live in and damage leaves, buds, and other soft above ground plant parts rather than roots. They are small, very active animals that live in and feed on the tender tissues in the center of young leaves. The areas in which they feed usually turn dark in color, often becoming chocolate brown, then dying and drying out.
These nematodes can not penetrate, feed on, or pass through the tougher tissues around major veins; they move to new areas of the same leaf or to new foliage by exiting to the surface when it is wet and moving on the outside of the plant in the film of free water that is there. There can be little re-distribution of foliar nematodes without free surface moisture on leaves and stems. Their limited spread within a leaf means that the areas of evident symptoms (darkened and dead tissue) tend to be defined by the vein pattern. Thus, in birds-nest fern which has pinnate venation (parallel veins like the vanes or rays of a feather), the affected areas are almost rectangular, with strongly parallel sides. In a chrysanthemum leaf with a net-like vein pattern, affected areas are irregularly-shaped patches as outlined by the veins. Confirmation of the diagnosis should be readily available from any laboratory that does nematode sample analysis.
Bird's-nest fern is one of the plants on which foliar nematodes have most commonly been found, and serious problems have arisen in 1995 on azalea, chrysanthemum, and Phillipine violet. However, foliar nematodes can infect and cause serious problems on a wide range of hosts, including other ferns, begonias, dahlias, strawberries, many lilies, and African violets.
The diseased and dead tissues on leaves are cosmetic blemishes that make ornamental plants less salable. Even worse, foliar nematodes sometimes cause serious defoliation. Many plants (e.g., azaleas) shed an infected leaf when a significant part of the leaf blade has been affected by foliar nematodes. The fallen leaves contain thousands of nematodes which can survive being dried out in the leaf for a very long time. If infested leaves are moved about the nursery and re-exposed to water, the nematodes can revive and infect new plants. Foliar nematodes may also be introduced into a nursery from natural infestations of native plants growing around the property, and probably are introduced into nurseries fairly often. So, why have outbreaks of foliar nematodes been seen so rarely in recent years? One likely explanation is that pesticides which have been used to control other pests such as mites and insects also may have been suppressing foliar nematodes.
In at least some cases, the new emergence of foliar nematodes seems to coincide with changes in pest management programs. At least one product known to be one of the most effective against foliar nematodes, Vydate, has been withdrawn recently from the ornamentals market. Other powerful and often systemic insecticides/acaracides are being replaced by biological control systems that are very specifically targeted against one or a few mites or insects. The nursery thus no longer has the significant but unrecognized secondary effect of the displaced pesticides against foliar nematodes.
Long-term management efforts must be made to minimize survival and redistribution of these nematodes within the operation. Sanitation, keeping foliar wetting to a minimum, isolation of plants that may be infested, and care to use nematode-free propagating material all are important. Since foliar nematodes survive so readily in dead and even dried foliage, all fallen leaves should be cleaned up and destroyed. If symptoms suggest that a block of plants might be infested or if laboratory analysis has proven it, that block should be clearly separated from other plants, to reduce risk of further spread. Mother plants and other sources of propagating material must be kept free of foliar nematodes, for introduction of even a few nematodes into the closely-packed cuttings in a mist system is the best possible way to encourage the survival and spread of a foliar nematode population.
Cuttings should never be taken from plants suspected or known to be infested with foliar nematodes. Benches, mist chambers, floors, plastic ground- sheets, and other surfaces that have been exposed to foliar nematodes should be thoroughly cleaned with chlorine bleach or another strong surface disinfestant before re-use.
Short-term control still depends heavily on pesticides, although few remain. Many products identified as most effective against foliar nematodes in trials conducted in the 1980's in both the U.S. and Europe are no longer available for use in nursery situations. Products proven effective against foliar nematodes which are still available include Oxamyl 10% Granular (can be used in most sites: consult label) and Nemacur 10% granular formulations (label does not include plants being grown inside). It is possible that some other pesticides that may be used in nurseries to control mites and insects can also suppress foliar nematodes, but specific recommendations for control of foliar nematodes are lacking on the labels. Older products that were found to have some activity against foliar nematodes include dimethoate and disulfoton. Avermectins might be effective (they were among the most toxic materials to nematodes in lab tests, but seem less active in soil and plant systems) but no real-world test results on control of foliar nematodes in plants have been published.
In view of the limited chemical control options, preventive measures to avoid introduction and limit spread in the nursery must remain the primary means of foliar nematode management. Clients who have questions about diagnosis of possible foliar nematode problems can contact the Senior Biologist in charge of the Nematode Assay Laboratory, Frank Woods: 352/392-1994.