Prunus Stem Pitting
In general, affected trees are stunted and look unthrifty. Foliar symptoms develop in late summer after harvest but prior to normal defoliation. Leaves on affected trees cup upward along the midrib, turn various shades of yellow through red to purple and then begin to drop prematurely. Care must be taken in diagnosing stem pitting. These symptoms indicate that root problems exist and are not always diagnostic of stem pitting. Other factors can be involved such as root rot, nutrient deficiencies, herbicide damage, mouse girdling and various injuries.
The following season, the trees will appear weak, growth will be stunted and a girdling of the trunk will result. Diseased trees do not recover. Trees will die or they may break off near the groundline during heavy winds. The described foliar symptoms are, at least in part, similar to those of trees girdled from other causes. To further identify prunus stem pitting it is necessary to remove a strip of bark a few inches above the ground to 6 to 8 inches below the groundline on the trunks of suspect trees. On infected trees, the bark and sapwood will be much thicker than normal. The wood will have elongated indentations, pits or swellings. The degree of pitting depends on the kind of stone fruit tree, its scion and rootstock, and the stage of disease development.
A poorly developed root system results from severe virus infection of trees. When the trees are pulled, socket-like depressions remain in the crown where the roots break away. Trees broken over reveal a serious disorganization of the woody tissues. This disorganization results not only in a structurally weak tree, but also produces the girdling effect resulting in the above ground symptoms. For pictures of symptoms, see Apple Union Necrosis and Decline (apple fact sheet).
Prunus stem pitting is caused by the tomato ringspot virus (TmRSV). The virus is soil borne and is transmitted to healthy trees by a species of dagger nematode. TmRSV can also be introduced into orchard plantings by infected nursery stock or by infected weed seeds. Once the virus is established in the orchard it is spread by the dagger nematode. The virus can persist in the orchard for many years in common weeds such as dandelion and other weed species.
Purchase certified virus-free trees to avoid introducing the disease into new plantings. In sites already infested, the use of crop rotation for weed control and the use of green manure applications of selected rapeseed varieties to reduce nematode populations is suggested. Soil fumigation before planting can be used but it is not economical nor environmentally desirable.