How to Manage Pests

UC Pest Management Guidelines

SYMPTOMS

Scab first appears as velvety, dark olive-to-black spots on fruit, leaves, and stems. When infections occur early, fruit spots become scablike with age and the fruit may become misshapen. On leaves, infections cause leaf puckering and twisting and eventually tear with age. Secondary infections that occur later in the season appear as black, velvety pinpoint spots on fruit and leaves.

COMMENTS ON THE DISEASE

In California, the fungus overwinters in dead leaves on the ground. Primary spores are discharged from flask-shaped fruiting bodies in infected dead leaves during spring rains and infect young leaves and fruit during periods of prolonged moisture. These infections produce secondary spores, which may cause further spread of disease during wet periods. Overwintering twig lesions from secondary infections may also be an important source of innoculum in spring.

MANAGEMENT

The main objective in scab management is the reduction or prevention of primary infections in spring. Extensive primary infections result in poor fruit set and make scab control during the season more difficult. If primary infections are successfully controlled, secondary infections will not be serious.

Applying high rates (20 to 24 gal/acre) of lime sulfur in fall before leaf fall may help suppress maturation of primary spores the following spring, but more research is necessary to confirm this. Similar rates of lime sulfur applied in the delayed dormant period can significantly reduce the viability of overwintering twig lesions, which are usually only a significant source of disease inoculum during severe scab outbreaks. Applying a 5% solution of biuret urea spray at 50 pounds per 100 gallons dilute spray before leaf fall hastens leaf decomposition and may reduce spore production the following spring. Combining the two practices of applying lime sulfur in fall and urea before leaf fall may offer complementary control. Cultivating leaves into the soil, however, has not significantly reduced infection potential.

Biological Control
No biological controls have been found.

Monitoring
Pear scab outbreaks can be predicted based on temperature and moisture conditions. The table listed here, derived from research by Mills and LaPlante, gives hours needed at various temperature levels under constantly wet conditions for primary spores to cause infection. In recent years automated weather stations have been programmed to provide this information. If wet periods are intermittent, total their durations until there is a period of at least 8 hours of continuous dryness. If the dry period is sunny and drying is quick and thorough, it is assumed that 6 hours after the trees have dried the danger is passed. If drying is slow and humidity remains high, then the 8-hour dry period is extended by a safety margin of 3 to 4 hours.

Harvest fruit sample. At harvest, assess your IPM program by monitoring fruit in the bins for pear scab damage. Sample 200 fruit per bin from 5 bins per orchard (or 20-acre block in large orchards). (For more information, see HARVEST FRUIT SAMPLE.)

Postharvest shoot sample. Following harvest, sample 20 top shoots for scab lesions on the leaves. If lesions are found, make plans to treat in spring. (For more information about sampling, see POSTHARVEST SURVEY and DORMANT/DELAYED DORMANT SAMPLING.)

Treatment Decisions
Preventive program. For areas of heavy rainfall, treat rows every 7 to 10 days starting at green tip until primary spore release is complete (5 to 10 weeks). Thereafter, maintain vigilance until hot, dry weather produces conditions unfavorable for infection.

Eradicant program. For low rainfall areas, treat as soon as possible after an infection period with a material that has kickback control of established infections. See the table below ("Period of Effectiveness") for a list of fungicides with kickback activity. Monitor infection periods daily throughout the spring and use the Mills and La Plante table to ensure coverage of newly exposed growth as long as rains are occurring frequently. Where infections are severe, consider applying fall and/or delayed dormant treatments of lime sulfur to reduce overwintering inoculum.

PUBLICATION

UC IPM Pest Management Guidelines: Pear
UC ANR Publication 3455
Diseases
R. B. Elkins, UC Cooperative Extension, Lake Co.
W. D. Gubler, Plant Pathology, UC Davis
L.G. Varela, UC IPM Program, UC Cooperative Extension, Sonoma Co.

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