This disease is caused by a bacterium (Xanthomonas)
that thrives in the rainy June and July weather we have
experienced throughout most of the state (apologies to those
of you who still have dry weather). Look for the disease on
peach, nectarine, almond, apricot, plum, prune, and cherry,
as well their ornamental equivalents.
Numerous spots (from as small as a pinprick up to 1/5
inch in diameter) form in the leaves. At first these spots
are circular and watersoaked, but soon enlarge to become
angular and deep purple to rusty brown or black. The centers
of the spots often dry out and tear away, so you may notice
only a shot-hole appearance or even a wind-tattered effect.
Infected leaves turn yellow and drop early. Although
symptoms resemble those of nitrogen deficiency, that
deficiency usually results in holes concentrated near the
midvein on the leaf. This bacterium also attacks twigs and
fruit, reducing fruit quality and yield or reducing
aesthetic appeal in the case ornamental species.
Some peach cultivars have resistance to bacterial
leafspot. Most apricot varieties are susceptible, as are
many nectarine varieties. Some resistant cultivars of peach
are listed in Report on Plant DiseasesNo. 810,
Bacterial Spot of Stone Fruits.
If you have a mature tree and do not wish to consider
replacement with a resistant variety, use balanced fertility
practices and open the trees through pruning so that air
circulation is improved. These steps will make conditions
less conducive to disease development.