Foliar nematodes are enemies of greenhouse growers

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Simply put, growers of greenhouse plants should take whatever steps necessary to avoid foliar nematodes. This is imperative because if plants become infested, there are virtually no effective control options. The best tactic is removal of plants from the greenhouse and their immediate disposal. This can result in thousands of dollars in losses of material.

Foliar nematodes are plant-parasitic nematodes. Similar to other nematodes of this type, they are microscopic. However, where 95 percent of all plant-parasitic nematodes feed within or on roots, foliar nematodes are found within leaf tissue. In Michigan, two species of foliar (leaf) nematodes are pretty common,
Aphelenchoides fragariae and A. ritzemabosi. Both species have relatively wide host ranges but A. fragariae is often associated with Begonia, ferns, lilies, strawberry and violets whereas, A. ritzemabosi is more common on other plants. Attack of herbaceous species is common where it is rare with woody plants.

Leaf and bud infesting species of
Aphelenchoides do not readily survive long periods in the absence of host foliage and weed hosts. On outdoor plants, they spend the winter in the soil in contaminated leaf tissue or in dormant buds and growing points of their hosts. In the spring, they migrate onto stems and leaves in films of water. They often congregate in plant crowns living within the buds and leaflets. They have short life cycles and can go from egg to adult in as few as 10 days at the optimal temperature of 65°F. Females lay 25 to 50 eggs in leaf axils or within the leaves of their hosts. Therefore, population densities of foliar nematodes can build very rapidly particularly indoors. In greenhouses, there are no interruptions in the disease cycle except when infected plants are removed.

Feeding by foliar nematodes results in necrosis (death) of damaged tissue forming leaf blotches. However, the pattern of the blotches varies from plant species to species and is closely correlated to leaf anatomy and venation. With most dicots, the leaves are rather thin so the main veins subdivide them into areas with little or no continuity of intercellular spaces between them. Therefore, the major veins act as barriers to nematodes within leaf tissue. To reach other sections of a leaf, foliar nematodes emerge from stomata and migrate over the surface when moisture is present. The result is a leaf with discrete areas showing different stages of discoloration from light green, to yellow to brown or black. In plants with thicker, fleshy leaves, such as Begonia and
Cyclamen, the veins do not act as barriers resulting in irregular patches of necrosis with poorly defined margins.

Lower leaves are often the first to exhibit symptoms. Upper leaves are often distorted due to infestations of buds. Dead leaves typically cling to stems and foliar nematodes can remain viable in these dried leaves up to several months.

Avoidance of foliar nematodes is the principle management strategy. Be sure to use nematode-free propagating material and sterile growing media. All new plant material should be inspected and screened for nematodes. Suspect material should be culled. Plants should initially be placed under quarantine for 30 to 60 days and monitored closely. It is advised all nurseries have at least one section of greenhouse devoted to the isolation of new plant material. To minimize or eliminate spread of foliar nematodes, plants should not make leaf-to-leaf contact and avoid splashing water. Reduce moisture on leaf surfaces as much as possible. After their isolation followed by close inspection, these quarantined plants can be moved into other sections of the greenhouse(s) if they are free from pathogenic nematodes.

If foliar nematodes become established in a commercial planting, their eradication is laborious, time-consuming and costly. Sanitation is imperative in all situations. It is important to note, there are no post-plant nematicides labeled for use in greenhouses, so there are really no effective chemical controls.

All infected plants and fallen leaves should be removed from the greenhouse and burned.

Be sure to clean all floors, benches and storage areas. Containers and tools should be steamed and/or sterilized. Dormant plant material can be treated with hot water but always should be done on an experimental basis, at least initially, in order to assess the viability of the treated plants. Since foliar nematodes are found in leaf tissue, if plants can be propagated by using roots only, it is possible to produce nematode-free material.

If infested by foliar nematodes at low population densities, plants may exhibit no symptoms and some plants may even be symptomless carriers of these nematodes. Symptoms will not appear until the nematodes build in numbers to damage threshold levels. The only way to properly diagnose a foliar nematode problem is to send leaf tissue to a nematode lab for analysis. At MSU, nematode samples are handled by
Diagnostic Services. The cost is $25.00 for a nematode test which really is a cheap price to pay in an attempt to avoid the headache of a foliar nematode infestation.

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