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PPV symptoms in stone
fruits may vary considerably with the age and nutrient status of the plant,
and especially with the cultivar. Symptomless cultivars are a great danger
to other more susceptible cultivars. Also, different strains of PPV may
vary in the symptoms they exhibit and in the severity of the disease they
cause. The virus often can be detected at the bottom of a branch but not
the tip; however, once a branch shows symptoms it will continue to display
them in subsequent years. Not every leaf or fruit on an infected tree
will show symptoms and some tolerant varieties may show no symptoms at
all. In many cases, only a few leaves on an entire tree may show any symptoms
even though the tree was infected. Symptomless trees act as a reservoir
for virus that can spread throughout the orchard and to other orchards.
The photographs in this guide show a range of symptoms, from mild to very
severe.
It is also important
to note that PPV is economically important on cultivars that become infected
but fail to produce severe or obvious symptoms. The only way to determine
the presence of PPV is by serological laboratory tests (see References
for more information). PPV infection of fruit trees not only causes symptoms
on leaves and fruits, but also reduces total quantities of even symptomless
fruits. In addition, it reduces fruit quality, resulting in reductions
in grade, and eventually debilitates the tree, reducing its useful life.
Every infected tree,
even if not showing symptoms, is a potential source of PPV allowing transmission
of PPV to other neighboring stone fruit orchard crops. Therefore every
infected tree must be removed as soon as detected in order to stop disease
spread.
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