Disease Management - Growing Organic Orchard Fruits - page 510

By far the most effective means of controlling plum leaf scald is to plant
resistant varieties in the southeastern United States and other areas where
the insect vector and disease occur. The Auburn University AU series of
plum varieties are resistant to this disease.
Shothole Blight
Shothole or Coryneum blight is a fungal disease that is especially serious on
apricot, nectarine, and peach. It also infects cherries and plums during long
wet periods in spring and early summer. The pathogen Wilsonomyces carpo-
philus was formerly called Coryneum beijerinckii . Varietal resistance is rare,
although peach varieties 'Lovell' and 'Muir' appear to tolerate the disease.
Symptoms. This fungus attacks the buds, twigs, stems, leaves, and fruits. The
first symptoms are reddish, slightly sunken spots that appear on current-
season and 1-year-old wood. The buds may be killed and the cankers can
extend into and girdle 2- to 4-year-old wood. Gummy sap around wounds is
common. Leaf symptoms begin as small tan or purplish spots that become
scabby and fall out, producing the characteristic shothole effect. On infected
fruits, similar spots develop and can become sunken with gummy exudates.
On cherry and plum, usually only the leaves and fruit are infected, and then
only during long wet periods.
Management. Maintain open canopies and good air movement in your
orchard. Avoid overhead irrigation, and follow practices that minimize the
humidity in the canopy. Prune out cankered wood and twigs, disinfecting
tools between cuts and burning or burying the prunings.
Apply Bordeaux mix when half of the leaves have dropped in fall, and
repeat when the trees are dormant. Follow up with a Bordeaux mix or lime
sulfur spray as buds are swelling in spring. Do not apply Bordeaux or other
sulfur products to apricots. Fixed coppers have also proven effective against
the disease.