Apple Scab

NEWA is a partnership of New York State Integrated Pest Management, Northeast Regional Climate Center, and Cornell University

Manage apple scab (Venturia inaequalis) fungicide inputs by tracking ascospore maturity, daily and cumulative ascospore discharge, and scab infection events.

Management Guide

Disease Cycle	Disease Management
The apple scab fungus overwinters in infected leaves that have fallen to the ground. In these leaves, from autumn to early spring, the fungus mates, resulting in the development of what are termed ascospores.	Low inoculum orchards - determining the potential ascospore dose (PAD) count. Before leaf drop in autumn, the number of apple scab infected leaves are counted on 600 shoots. If less than 53 leaves per 600 shoots have apple scab, the green tip spray can be omitted in most years. Never omit the green tip spray in orchards where PAD counts weren't done the previous autumn. High carry-over inoculum is a major contributor to scab epidemics. Orchard sanitation methods can reduce carry-over inoculum in orchards by 70% or more by using any one of the following methods. (a) Apply 40 lb/A of urea fertilizer (mixed in 100 gal of water/A) sometime after winter but before bud break. Urea softens senescent and fallen leaves and stimulates their microbial breakdown, promoting faster removal by earthworms that feed on them. It may also directly suppress ascospore formation. Treat the entire orchard, including the ground cover in the row middles. Apply the spray using either an air blast sprayer with the upper nozzles turned off or a boom sprayer set up to spray both under the trees and the row middles. Reduce subsequent nitrogen fertilizer rates by the amount of N applied under the drip line of the tree rows. Ignore the amount of N applied to the row middles, as the ground cover will use this. Repeat after harvest during leaf drop following seasons with severe scab outbreaks. (b) Shred overwintering leaves using a flail mower set low enough to contact the fallen leaves. Leaves must be raked or blown from under trees, or mower must be offset to reach them. Shredded leaves decay more quickly, and flail-mowing leaves in spring disorients many of the leaf pieces so they eject ascospores toward the soil instead of up into the air. Prunings can be chopped at the same time. However, the low mower settings required to effectively shred leaves may leave row middles so denuded as to be slippery or muddy at the time when early fungicide sprays are needed. (c) Apply 2½ ton/A of lime in early winter after leaves have fallen from trees. Lime raises the pH of fallen leaves enough to increase the rate of microbial breakdown of the fallen leaves.

Disease Cycle

Disease Management

The apple scab fungus overwinters in infected leaves that have fallen to the ground. In these leaves, from autumn to early spring, the fungus mates, resulting in the development of what are termed ascospores.

Low inoculum orchards - determining the potential ascospore dose (PAD) count. Before leaf drop in autumn, the number of apple scab infected leaves are counted on 600 shoots. If less than 53 leaves per 600 shoots have apple scab, the green tip spray can be omitted in most years. Never omit the green tip spray in orchards where PAD counts weren't done the previous autumn. High carry-over inoculum is a major contributor to scab epidemics. Orchard sanitation methods can reduce carry-over inoculum in orchards by 70% or more by using any one of the following methods.
(a) Apply 40 lb/A of urea fertilizer (mixed in 100 gal of water/A) sometime after winter but before bud break. Urea softens senescent and fallen leaves and stimulates their microbial breakdown, promoting faster removal by earthworms that feed on them. It may also directly suppress ascospore formation. Treat the entire orchard, including the ground cover in the row middles. Apply the spray using either an air blast sprayer with the upper nozzles turned off or a boom sprayer set up to spray both under the trees and the row middles. Reduce subsequent nitrogen fertilizer rates by the amount of N applied under the drip line of the tree rows. Ignore the amount of N applied to the row middles, as the ground cover will use this. Repeat after harvest during leaf drop following seasons with severe scab outbreaks.
(b) Shred overwintering leaves using a flail mower set low enough to contact the fallen leaves. Leaves must be raked or blown from under trees, or mower must be offset to reach them. Shredded leaves decay more quickly, and flail-mowing leaves in spring disorients many of the leaf pieces so they eject ascospores toward the soil instead of up into the air. Prunings can be chopped at the same time. However, the low mower settings required to effectively shred leaves may leave row middles so denuded as to be slippery or muddy at the time when early fungicide sprays are needed.
(c) Apply 2½ ton/A of lime in early winter after leaves have fallen from trees. Lime raises the pH of fallen leaves enough to increase the rate of microbial breakdown of the fallen leaves.