Apple scab, caused by the Venturia inaequalis, is the most common and economically important fungal disease for apples grown in non-arid climates. The pathogen over-winters in fallen leaves and releases airborne ascospores during rains in spring. Spore discharge usually starts when trees are at bud break and continues until trees have completed flowering (i.e., petal fall).
Apple scab ascospores can infect newly unfolding leaves, flower parts, and young fruits. Primary infections initiated by ascospores appear as dark, sometimes coalescing, lesions on leaves and fruit, and those lesions produce an abundance of conidia that are disseminated by rainfall, infect additional leaves and fruit, and produce secondary infections. Apple scab is usually controlled via regular applications of fungicides, although scab-resistant cultivars with excellent fruit quality have recently become available.
If apple scab is not controlled on scab-susceptible cultivars, 100% of the fruit will often be infected by apple scab before harvest and trees may defoliate prematurely, thereby limiting the ability of the trees to form fruit buds for the next season. Thus, a severe outbreak of apple scab can destroy the apple crop for two years, first by infecting fruit in the current year, and then by eliminating the ability of trees to produce flower buds for the next season. Fruit with scab lesions cannot be sold as fresh fruit, will not store well, and generally have little if any commercial value.
Extension articles about controlling apple scab:
New York Fruit Quarterly
2010 – Apple Scab Management Options for High-Inoculum Orchards
2003 Susceptibility of New Apple Cultivars to Common Apple Diseases
Scaffolds Fruit Journal
2005 Jump Starting Apple Scab Control Programs in High-Inoculum Orchards
2004 Revising Scab Control Strategies for 2004
2003 Apple Scab Management for 2003 Preventive Programs Are Essential
2003 Further Comments on Scab Management Program
2002 A Review of Strategies for Controlling Apple Scab and Mildew in 2002
2002 Determining the End of the Primary Apple Scab Season
2001 Assessing Ascospore Maturity of Venturia inaequalis
A useful article on how to use sulfur sprays to manage apple scab
Controlling Scab with Sulfur Sprays: a 1945 article by Dr. Art Burrell
Page last updated 27 April 2016
