Upper left - early scab infection on bottom surface of leaf; upper right - late season scab with top surface lesions; lower left - early scab on fruit; lower right - late season scab.
If not controlled, the disease can cause extensive losses (70% or greater) where humid, cool weather occurs during the spring months. Losses result directly from fruit or pedicel infections, or indirectly from repeated defoliation which can reduce tree growth and yield.
Apple scab can be observed on leaves, petioles, blossoms, sepals, fruit, pedicels, and less frequently, on young shoots and bud scales. The first lesions are often found on the lower surfaces of leaves as they emerge and are exposed to infection in the spring. Later, as the leaves unfold, both surfaces are exposed and can become infected. Young lesions are velvety brown to olive green and have feathery indistinct margins (top left). With time, the margins become distinct, but they may be obscured if several lesions coalesce. Young leaves may become curled, dwarfed, and distorted when infections are numerous. The lesions may remain on the upper and lower surfaces for the entire growing season (top right). Lesions per leaf can vary and, if severe, cover the entire surface of the leaf causing "sheet scab."
Lesions on young fruit appear similar to those on leaves (Lower left), but as the infected fruit enlarge, the lesions become brown and corky. Infections early in the season can cause fruit to develop unevenly as uninfected portions continue to grow. Cracks can appear in the skin and flesh (lower right). The entire fruit surface is susceptible.
The pathogen generally overwinters in fallen leaves and fruit on the orchard floor. As a result, orchards are self-infecting. Primary spores develop during the winter and begin to mature early spring. Around bud break, the first mature spores will be released from the infected leaves and/or fruit. The length of time required for infection to occur depends on the number of hours of continuous wetness and the temperature during the wetting period (Table below). For example, if the average temperature is between 61 to 75 °F, a minimum of 6 hours of leaf wetness is required for spores to be dispersed. Once the primary spores have established infection on the plant tissue, in approximately 9 to 10 days symptoms can be observed. At that time, secondary spores are being produced and will do so the remainder of the season, being dispersed by rain or wind on susceptible tissue.
It is important to scout and control apple scab early in the season to prevent secondary infections from becoming established.
Calculating apple scab infection periods using the Revised Mills Table (adapted from the 2014 - 2015 Penn State Tree Fruit Production Guide).
|Average temperature (°F)a||Wetness (hours)b||Lesion appearance (days)c|
a - Add lowest and highest temperatures during wet period and divide sum by 2 to get average.
b - Calculate hours of wetting by either (1) beginning the count at the time leaves first become wet and ending the count when the relative humidity drops below 90%, or (2) adding consecutive wet periods (hours) if the leaves are again wetted with 8 hours from the time relative humidity dropped below 90%.
c - Number of days required for lesions to appear after infection has been initiated. If conditions are unfavorable for lesion development (prolonged periods above 80°F or very dry weather), additional days may be required.
- Monitor rainfall and duration of wetness closely beginning at green tip since mature spores begin to be released around this time. Peak mature spore release is around bloom time through petal fall. Continue to monitor rainfall and duration of wetness through mid-June as the final mature spores are released during this time.
- Start monitoring for lesions (spots) about 10 to 14 days after bud break, which is when the first symptoms can occur if disease conditions are favorable.
- For each orchard block, follow a "W shape" pattern within the block when scouting. Evaluate 10 trees by examining 20 leaves on each of 5 limbs per tree and record the number of leaves showing any scab lesions.
- Begin with the flower bud (spur) leaves where early infections are most likely to be noticed.
- Start with observing the undersurface of leaves since the undersurface of leaves may become spotted before the top surface. Take notice of early lesions which may be small light brown-black spots.
- As scouting continues during early spring, be sure to observe both the topside and underside of the leaf. Apple scab infection appears as brown velvety lesions, which will become darker as they age.
- After fruit has set, in addition to leaf observations, also examine 20 fruit on each tree and record the number showing any scab lesions. Use this information to better manage scab in the future.