Take steps to avoid foliar nematodes

This week and last, plants were submitted to MSU Diagnostic Services infected with foliar nematodes. Foliar nematodes are plant parasites found within leaves and buds. They differ from most plant-parasitic nematodes because they do not feed on roots.

Foliar nematodes primarily attack herbaceous perennials. Infection of woody plants is rare. There are two species of foliar nematodes known to occur in Michigan, one primarily attacks strawberry, Hosta and ferns whereas; the other species is more common on other plants.

Foliar nematodes typically do not survive winter well in the absence of host foliage or weeds. In the spring, they migrate onto stems from the soil or leaves adhering to hosts in films of water. They often congregate in crowns living within buds and leaflets. They have very short life cycles. Females deposit eggs in leaf axils or within leaves and the nematodes usually mature within 14 days. Foliar nematodes migrate, feed and reproduce in water films. They enter leaves via stomata or wounds to feed on spongy mesophyll cells. These cells help facilitate gaseous exchange.

Feeding by these nematodes usually results in necrosis (death) of damaged tissue forming leaf blotches. The form and pattern of these blotches varies from plant species to species and is very closely correlated to leaf anatomy and venation. Many plant species have very thin leaves so the main veins divide them into areas with little or no continuity of intercellular spaces between them. In other words, the veins act as barriers to the nematodes. The nematodes reach other sections of the leaves by crawling out of the stomata and migrating over the leaf surfaces when the plants are moist. The result is a leaf with discrete areas showing different stages of discoloration beginning with light green to yellow and then brown. Plants with thicker leaves, such as Begonia and Cyclamen, possess veins that do not act as barriers resulting in irregular shaped blotches with poorly defined margins. The progression of the symptoms typically is from the bottom of the plant to the top.

The primary management strategy is avoidance. Be sure to start with foliar nematode-free planting material. Problems usually arise from planting infected stock. All new planting material should be placed in quarantine for a period of 30-60 days following initial propagation and introduction to the greenhouse. Individuals associated with plant propagation should have an area of a greenhouse designated for quarantine. After close inspection of the growing plants and subsequent analyses of plant tissues, material can be moved into other sections of the greenhouse if free from nematodes.

Foliar nematodes, like all plant-parasitic nematodes, are virtually impossible to eradicate. If the presence of foliar nematodes is confirmed in plants via nematode testing, ideally those plants are destroyed. However, this isn’t always an economically viable approach, although 100% control is nearly impossible. Nemacur, a nematicide, will provide excellent control but is only labeled for nursery stock grown outdoors in Michigan. Reports from Ohio indicate ZeroTol, a fungicide, provided good to excellent control (70%) of these nematodes when applied as a 2% solution. Typically, use of these products will reduce foliar nematode population densities and improve plant health but when left unchecked, these nematodes will once again increase in number to damaging levels.

Many cultural tactics can be utilized to lessen the impact of foliar nematodes. Because these nematodes are typically found in leaves, nematode-free propagating material can be potentially generated from roots. These roots should be planted into sterile media. Hot water treatment of dormant plants can provide control. The duration of the treatment and temperature varies by plant species but using water at 46°C and treating for 5 to 15 minutes often is adequate. Attempts should be made to space plants in the field or greenhouse so they don’t make leaf to leaf contact. Overhead watering should also be avoided as nematodes can be spread in splashing water but regardless, surface water on leaf surfaces is necessary for movement of foliar nematodes. Plants exhibiting symptoms should be rogued out (or removed if in containers) and destroyed. Neighboring plants should also be eliminated. Good sanitation is also critical. Be sure to remove old plant material because the nematodes survive for months in old, desiccated leaves. Be diligent about surface sterilizing pruning tools.

New plantings of susceptible hosts should not be made on land documented with foliar nematodes. Since woody plants are very rarely attacked, these are good choices on infested sites. Soil fumigation will provide excellent control if necessary.

Management of foliar nematodes, as with all plant-parasitic nematodes, requires an integrated approach. Attempts should be made to determine the source of any new infestation. Nematodes are worms, so they do not fly or walk to new destinations. We, people, do an excellent job disseminating them. If you discover an infestation, it didn’t occur via spontaneous generation. Try to elucidate the cause so mistakes can be avoided in the future. Unfortunately, foliar nematode infestations can result in severe economic losses.

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