Scab

Grayish black, soft looking spots form on leaves, fruit, and twigs. Young lesions are indistinct small yellow specks, best seen by holding a leaf up to the light. Lesions usually are not visible until late spring or early summer.

The fungus survives in twig lesions, and spores are spread by wind or rain. The disease is favored by prolonged wet spring weather, and is most prevalent in the Sacramento Valley. Severe scab infections cause early defoliation; if left uncontrolled for several years, infected trees will become weakened. The disease often occurs in sprinkler-irrigated orchards where water reaches foliage.

To determine if a fungicide application is warranted, follow the sampling and treatment threshold guidelines in the section DORMANT SPUR SAMPLING. Record observations in a sampling form (PDF).

Scab may be controlled by shot hole fungicide sprays. However, a scab treatment may be required if rain occurs into mid- to late spring. One application as late as 5 weeks after petal fall can protect against scab, but an earlier application (2 weeks after petal fall) may improve control.

In severe outbreaks, dormant or delayed dormant applications of copper with oil or liquid lime sulfur can be used effectively to reduce and delay sporulation of twig infections in spring. Copper and oil has been found to be the most effective delayed dormant treatment, and when used in combination with petal fall applications of protective fungicides, the disease can be effectively managed.

Scab resistance to quinone outside inhibitor fungicides (also known as strobilurins) has been documented; do not use FRAC mode-of-action group number 11 fungicides in these orchards.