MSU Extension

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.

Daggers are typically defined as short, pointed weapons. However, the daggers referred to in this article are nematodes, Xiphinema sp. These nematodes should be of concern to tree and small fruit producers because they are potential vectors of plant viruses known as nepo (nematode transmitted polyhedral-shaped) viruses. Nepoviruses commonly vectored by X. americanum (the most common species of dagger nematode in Michigan) affecting small and tree fruit are tomato ringspot virus, tobacco ringspot virus and peach rosette mosaic virus.

Dagger nematodes are large plant-parasitic nematodes that feed as ectoparasites (outside the host) on plant roots. High population densities of these nematodes can injure small and tree fruits. Their feeding often results in swollen root tips preventing the roots from functioning properly. Roots often die as a result of this feeding. Therefore, dagger nematodes can affect a plant’s growth and yield even if they are not harboring viruses.

Tomato ringspot virus

This virus affects peaches, plums, cherries, apples, grapes, raspberries, blueberries and strawberries. It also infects a wide range of common orchard weeds, the most notable being dandelion. It produces different symptoms in different hosts. In peaches and cherries, the virus causes stem pitting. Infected trees have abnormally thickened, spongy bark and pits and grooves in the wood beneath. Symptoms are more severe in the trunk area adjacent to the soil line. Affected trees are unthrifty and often unfolding of leaf buds is delayed. Leaves are often droopy, turn color prematurely and drop early.

In plums and apples, tomato ringspot virus causes brown ring necrosis. In this situation, the area below the graft union grows slower than the scion portion of the tree, producing an overgrowth appearance. Removal of the bark reveals tissue death (a brown line) at the scion-rootstock junction. Trees often die quickly after the onset of brown line.

In grapes, symptoms due to tomato ringspot virus are indistinguishable from those of tobacco ringspot virus (and will be covered below).

In strawberries, symptoms typically are dwarfing, reduced runner production, leaf mottling or plant death. However, due to the shorter life of strawberry plantings, this virus is not considered a major problem. Tomato ringspot virus is not considered a problem in blueberries grown in Michigan.

Tobacco ringspot virus (TRSV)

This virus affects grapes and blueberries, primarily. Tobacco ringspot virus also infects a wide range of weeds, such as dandelion and common chickweed. In grapes, during the first year of infection with the virus, plants may grow normally. However, as the plants grow older, symptoms become very evident. Shoots are short with distinctly shortened internodes. Leaves are distorted often with leaf areas about a third of the normal size. Plants are typically stunted. Berries on fruit clusters are sparse and develop unevenly. Yield is significantly reduced. In general, tobacco ringspot virus infections have not been observed in interspecific hybrids. This disease has been confined primarily to Vitis vinifera cultivars. V. labrusca is resistant to both tomato ringspot virus and tobacco ringspot virus.

In high-bush blueberries, tobacco ringspot virus causes necrotic ringspot. Symptoms vary with cultivars, but the virus causes a slow, steady decline in productivity. Affected leaves are misshapen and crinkled and have very small necrotic spots (0.1 inches in diameter), which may fall out. Some cultivars may have very short internodes, but no dead spots on leaves. Small twigs are often necrotic.

Peach rosette mosaic virus

This virus affects peaches, grapes and blueberries. Peach rosette mosaic virus can also infect many weeds, most notably dandelion and curly dock. In peaches, a severe shortening of internodes (rosette) is evident in late spring and summer. Infected trees often appear darker green, are stunted, and produce little or no fruit. Leaf emergence of infected trees is delayed.

In grapes, infected vines exhibit an umbrella-like growth habit due to shortened internodes. Leaves are often deformed. Clusters are uneven and berries often shell off. Vines are typically killed. Peach rosette mosaic virus affects V. labrusca. Infections are quite rare in blueberries in Michigan.

Symptoms caused by viral infections often resemble those caused by other pathogens or nutrient deficiencies. One technique that can be utilized to identify plant viruses is ELISA (Enzyme-Linked Immunosorbent Assay). If you suspect you have plants infected with viruses, you should collect leaf samples and submit them to MSU Diagnostic Services. Diagnostic Services has ELISA test kits for some viruses. Often samples have to be sent to another lab for viral screens due to the expense of the test kits and the lack of significant volume to justify their costs. Feel free to call Diagnostic Services for proper sampling protocols.

Nepoviruses spread very slowly. Circular or elliptical patterns will appear in orchards, vineyards, etcetera, infected with nepoviruses. Because dagger, needle and stubby-root nematodes can also vector plant viruses, nematodes move very little. The spread will be slow. It is important to recognize the role of nematodes in the transmission of these viruses. In fields, where nepoviruses have been positively identified, a nematode sampling program should be initiated to map the distribution of these organisms. A nematode control program should be implemented. It only requires one dagger nematode to transmit a nepovirus to a new host. Therefore, detecting both the virus and the vector is a critical part of any control program.

For more information on these viruses, nematodes or any other plant problems, don’t hesitate to contact the personnel at the MSU Diagnostics Services, 517-355-4536.