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Pear Scab

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Venturia pirina

Overview

  • Pear scab is caused by the fungus Venturia pirina. It is similar to apple scab, however, occurs more sporadically. Where present, however, it can cause serious economic loss unless controlled.
  • Infection occurs when overwintering ascospores on leaves and shoots are released during periods of wetting in the spring.
  • Sanitation by flail mowing leaves and brush may help reduce overwintering inoculum (similar to apple scab), however, chemical control is still typically necessary when pear scab is a persistent problem.
  • Chemical control using contact and systemic fungicides beginning in the spring and targeting primary infection are recommended to control pear scab.

Symptoms & Signs

Pear scab symptoms on leaves and fruit are similar to those of apple scab, however, pear scab lesions can also occur on shoots. Scab lesions on leaves are more visible on the bottom of the leaf, but can be kind of small and inconspicuous. Nevertheless, they are olive-colored circular spots. On fruit, pear scab infections are larger and more obvious, again being olive-colored, velvety, circular spots that become more corky and black later in the season. Badly infected fruit results in skin cracking and deformity, and if lesions infect the fruit pedicels, fruit will drop prematurely. If fruit is infected late in the summer, fruit can develop rough, black lesions during storage. Pear scab lesions on shoots are similar to those on foliage and fruit depending on the age of the shoot and length of time infected, ranging from smaller olive-colored lesions to larger, brown-black lesions which typically are walled-off and slough off the shoots.

Disease Cycle

Primary infection results from overwintering ascospores on leaves on the orchard floor and conidia from twig lesions, but the presence of ascospores is usually necessary for significant infection to occur. Spores are mature with bud swell and can be released for up to three months. Pear scab ascospore maturation begins at the same time as green tissue emerges at budbreak, releases peak during pear bloom, then declines. With adequate moisture, degree-day models predict that nearly 100% of pear scab ascospores will have reached maturity by about 1400 degree-days base 32F. As with apple scab, ascospore density can vary by a factor of 100 times or more between a low-scab and high-scab pear orchard. Unlike apple scab, pear scab ascospores can release at night, so all infection period durations should be timed from beginning of the rain, whether is the rain started during the day or at night. Periods of wetting with rainfall more than 0.01 inch are required to release spores and initiate infection. Like apple scab, more spores are released during daytime wetting vs. night.  Wetness duration for infection to occur can range from 10 hours to 25 hours at 75 degrees F. and 45 degrees F. respectively. Symptoms appear in 10 to 19 days from infection, and pear fruit become more resistant to infection with age. The severity of pear scab infection is highly correlated with amount of inoculum present, just like apple scab.

Chemical Control

Many of the same fungicides used to control apple scab also control pear scab. Protectant fungicides are the first course of action, and must be applied before wetting periods long enough to initiate infection occur, and beginning when green tissue emerges. They must be repeated at 7-10 day intervals until the supply of inoculum is gone. Systemic “kickback” fungicides may also be used, as long as they are applied in time to provide post-infection suppression of scab.

Fungicide Resistance in the Eastern U.S.

Where protectant fungicides are used, resistance should not be an issue. If, however, systemic fungicides in the same mode-of-action class are used repeatedly, resistance could develop. Thus, fungicide rotation is always advised.

Non-Chemical Control

Biological Control

There is no significant biological control of pear scab.

Cultural Control

Flail mowing/chopping overwintering leaves and/or applying urea to foliage in late fall will help leaves break-down and decrease the amount of inoculum present for infection in the spring.

Resistant Varieties

Pear cultivars do vary somewhat in their resistance to pear scab, however, it is not well documented and resistance to one strain of pear scab may not work on another strain. Differences in strains and variety susceptibility means resistance cannot be counted on to provide adequate protection from infection.

Revised May 2, 2020. J. Clements and G. Koehler

References

Diseases of Tree Fruit in the East, Alan Jones and Turner Sutton

Factors Affecting Maturation and Realease of Ascospores of Venturia pirina in Oregon. R. A. Spotts and L. A. Cervantes. APS.

Pear Scab: Components of Potential Ascospore Dose and Validation of an Ascopsore Maturity Model. R. A. Spotts, L. A. Cervantes and F. Niderholzer. APS.