Shot Hole

Spots occur on leaves, fruit, twigs, and flowers; however, flower and twig lesions are relatively scarce or difficult to find. Leaf lesions begin as tiny reddish specks that enlarge by several millimeters into spots having tan centers and purplish margins. When the fungus sporulates, the fruiting structure appears as a small dark speck (the sporodochium and spores) in the center of the spot; this is a diagnostic characteristic of shot hole disease. Spots on young leaves usually fall out, leaving a hole (the shot hole); older leaves retain their lesions. Fruit spots are small with purplish margins, slightly corky, and raised. Spots are found on the upper surface of fruit with respect to the way it hangs on trees. Heavy infection of young fruit may cause fruit drop or distortion and gumming of fruit.

The fungus survives on infected twigs and as spores in healthy buds. Spores are moved by water to new sites; prolonged periods of wetness, either due to rain or sprinkler irrigation, are required for the disease to develop. Shot hole can cause losses in yield, defoliation, and weakened trees.

Monitor orchards in fall and spring for shot hole lesions and fruiting structures. Fruiting structures appear in the center of leaf lesions as small black spots and can be seen with a hand lens.

In the fall:

• If fruiting structures are present in leaf lesions, there is a high risk of shot hole development the following spring and a petal fall treatment should be applied
• If fruiting structures are not present on leaf lesions, the petal fall treatment is not needed for shot hole. (It may be necessary for control of other diseases such as scab or leaf blight, however.)

In the spring:

Whether or not a petal fall treatment is applied, monitor leaves for lesions with fruiting structures.

• As soon as fruiting structures are evident, apply a treatment and continue treatments at the recommended label interval as long as weather conditions are conducive to disease development.
• If fruiting structures are not present, a treatment is not required, but continue monitoring until weather conditions are no longer wet and conducive to shot hole development.

Contact fungicides serve as protectants, not eradicants, and provide control only if they are applied so foliage and fruit are well covered before a wet period. The minimum number of applications may vary each year, depending upon the rain pattern and use of sprinkler irrigation.

When zinc sulfate (20–40 lb/acre) is applied in late October to early November to hasten leaf fall, shot hole inoculum is prevented from increasing. Otherwise, high levels of inoculum may develop and overwinter on the trees, infecting leaves the following spring.