Apple Scab

Apple scab produces olive-green to brownish-black, velvety spots on leaves that can enlarge and cause premature leaf drop in severe cases. On fruit, lesions start as similar dark spots and can become corky, cracked, and scabby as the fruit develops, reducing marketability even when the fruit remains edible. Severe early-season infection can also distort or stunt young fruit.

Venturia inaequalis survives winter mainly in fallen, infected leaves on the orchard floor, where sexual fruiting structures mature over winter and release ascospores during rain events in spring, providing the primary source of new-season infection around bud break and early leaf emergence. Later in the season, secondary infections spread from lesions on infected leaves via asexual spores splashed or blown to healthy tissue during further wet periods.

• Primary infection depends on ascospores released from overwintered leaf litter during spring rain
• Secondary infection cycles continue through the season whenever wet periods coincide with susceptible tissue
• Practices that speed the breakdown of fallen leaf litter can reduce the following spring’s inoculum load

Apple is the principal host of Venturia inaequalis, and susceptibility varies considerably between cultivars, with some carrying genetic resistance that greatly reduces disease pressure. Ornamental and wild crabapples can also be affected and may serve as a nearby source of inoculum in some settings.

• Extended leaf wetness from spring rain, especially around bud break and early leaf growth
• Mild-to-moderate temperatures that coincide with wet periods
• Heavy leaf litter left on the orchard floor over winter
• Susceptible varieties without scab-resistance genetics
• Dense canopies that slow drying after rain

Because primary infection is closely tied to overwintered leaf litter and spring rain, management combines reducing overwintering inoculum with monitoring wet-weather infection periods during the susceptible early season.

• Plant resistant or tolerant cultivars where available
• Remove, mow, or shred fallen leaves in autumn to speed their breakdown and reduce spring inoculum
• Prune to improve airflow and light penetration, helping foliage dry more quickly after rain
• Use local infection-period forecasting and extension guidance to time scouting around bud break
• Scout regularly through the wet-weather period from bud break through early fruit development