Pear Scab

Scab first appears as velvety, dark olive-to-black spots on fruit, leaves, and stems. The olive velvety spots are the sporulating fungus and growth of the pathogen on the lesions. 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 tearing with age. Secondary infections that occur later in the season appear as black pinpoint spots on fruit and leaves.

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.

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, such as by applying lime sulfur during the delayed-dormant season, or preventative fungicide treatments starting at green tip based on the Mills and La Plante Table output (Table 1), secondary infections should not be serious.

Many of the fungicides, especially the new products used for controlling scab in pears, are single-site mode of action materials, meaning they stop or change only one metabolic step within the fungus. As these products are used over and over again in a season or in multiple seasons, the pathogen has the ability to develop resistance to them. Resistance to fungicides can occur in several ways. While the fungus can mutate on its own to develop resistance, selection pressure caused by the repeated use of one class of fungicides is usually the major cause. This has been documented with several fungicide modes of action—demethylation inhibitors (DMIs), benzimidazoles and the quinone outside inhibitors (QoIs)—in California's North Coast pear production areas. Other practices that often lead to resistance are the practices of using reduced concentrations of product and poor coverage; the latter is usually the result of tractor speeds that are too fast, reduced volumes of water, or making applications only to every other row.

No biological controls have been found.

Monitoring weather for potential pear scab outbreaks is the most important component of a control program. Pear scab outbreaks can be predicted based on temperature and moisture conditions. The table listed here, derived from research by Mills and La Plante, 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, add 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 has 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.

Preventive program

Apply high rates (6 gal/100 gal water with up to 24 gal/acre) of lime sulfur in the delayed-dormant period to significantly reduce the viability of overwintering twig lesions, which can be a significant source of inoculum during and in years following severe scab outbreaks.

Use a preventive program in conjunction with the Mills and La Plante table, particularly:

• During high-rainfall years
• If scab was a problem last year
• For areas with heavy rainfall

If moisture and temperature predict conditions conducive to pear scab, treat 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

In most cases, an eradicant program uses single-site-of-action fungicides, and so resistance development is an issue. If needed, switch to an eradicant program after bloom to reduce the risk of resistance development. For low-rainfall areas that only require occasional treatments, only eradicant treatments may be necessary.

Treat as soon as possible after an infection period with a material that has kickback control of established infections. Thorough coverage is very important; apply eradicant fungicides to every row. See the table below ("Period of Effectiveness") for a list of fungicides with kickback activity. Treatments will be effective only for the stated period of time after the beginning of the infection period, not after symptoms of the disease have appeared.

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.