Symptoms
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).
Disease Cycle
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.
Disease Management
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.
