Distorting leaves and damaging fruit, apple scab (caused by the fungus Venturia inacqualis) is one of the most common and destructive orchard diseases.  Apple scab emerges as dark brown to olive green velvety lesions on leaves, fruits, stems, and twigs. In the early spring, primary (ascospore) infections hit early fruit at blossom’s end with one or two distinct spots – causing leaves to pucker or yellow, and fruit to become cracked or fallen. In early autumn, secondary infections occur frequently as pin-point scab, and though relatively small in size, may appear more numerous or clumped together and develop furthermore after harvest.

Hosts

Apples and flowering crabapples (Malus spp.), hawthorn (Crataegus spp.), mountain ash (Sorbus spp.), firethorn (Pyracantha spp.), and loquat (Eriobotrya japonica). Pear (Pyrus spp.) is infected by a related fungus, Venturia pirina, which causes nearly identical symptoms.

Disease cycle

The scab fungus is one that overwinters in infected leaves that have fallen to the ground as leaf litter. During autumn, fungal fruiting bodies known as pseudothecia embed themselves in leaves’ surface, and by late winter or early spring, fungal sacs (ascospores) mature with the progression of spring. As spring weather warms and rainfall is aplenty, mature fungal spores thrive and become airborne after just 30 minutes of daytime rain. Infections that grow into splotches on leaves can produce new spores in as little as 9 to 17 days.

Reaching their spore population peak in the time trees blossom and the last petals fall, the season’s crop of spores become dispersed within 1 to 2 weeks. Wind and rain are the primary mechanisms by which the spores are spread, and infection occurs readily between infected trees. A wet spring with temperatures between 55 to 75 degrees Fahrenheit are the ideal conditions for the spread of apple scab.

Young tree tissues are most susceptible, as well as newly set fruit within the 3-4 weeks that the petals have fallen. While this disease can surely dismay orchard tenders, there are several controls one may enact to prevent apple scab’s spread.

Apple Scab Management

  • Remove, compost, or destroy dropped leaves in autumn or winter that may contain the spores so as to prevent their overwintering.

  • Because spores favor wet leaf conditions, allow adequate drying time for the foliage – irrigating between sunrise and noon, and providing adequate air flow to trees through regular pruning, spacing, and removing of tree suckers.

Cultivars

  • Choose a disease-resistant cultivar, as opposed to those that are more susceptible to the condition.

  • Susceptible apple varieties: Bellflower, Blushing Gold, Fuji, Gala, Golden Delicious, Granny Smith, Gravenstein, Grimes, Ida Red, Jonathan, Monroe, Mutsu, Paula Red, Red Delicious, Rome Beauty, Stayman Winesap, Winesap, Yellow Netwown, York Imperial.

  • Resistant apple varieties: Easy-gro, Enterprise, Florina, Freedom, Goldrush, Jon Grimes, Jonafree, Liberty, Macfree, Prima, Priscilla, Pristine, Redfree, Sir Prize, Spigold, Williams Pride

Chemical controls

If damp weather conditions are imminent or present with leaves likely to remain wet for more than 9 hours, application of fungicides including fixed copper, sulfur, mineral or neem oils, and Bordeaux mixtures (combination of copper sulfate and hydrated lime mixed before application), may be helpful. As soon as leaf tips emerge, apply a fungicide, with reapplication 10-14 days later if rain continues, and a third spray toward the end of the bloom period after petals have fallen. As always, read the instructions carefully for your designated spray to time your applications properly.

Here is a link to the sulfur spray POP tip:

https://groups.google.com/forum/#!topic/phog/nA5GoyDhOT8

Here is a more in depth pamphlet about apple scab:

https://www.extension.purdue.edu/extmedia/bp/bp-1-w.pdf

References

Cornell University Fact Sheet, http://nysipm.cornell.edu/factsheets/treefruit/diseases/as/as.asp

University of California Agriculture & Natural Resources: Statewide Integrated Pest Management Program, http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7413.html

University of Minnesota Extension, http://www.extension.umn.edu/garden/yard-garden/trees-shrubs/managing-apple–scab/

Plant Health Instructor, http://www.apsnet.org/edcenter/intropp/lessons/fungi/ascomycetes/Pages/AppleScab.aspx

Seasonal Checklist for Holistic Orchard, http://www.groworganicapples.com/organic-orcharding-articles/holistic-orchard-seasonal-checklist.php

American Phytopathological Society (APS) Apple Scab Factsheet,

http://www.apsnet.org/edcenter/intropp/lessons/fungi/ascomycetes/Pages/AppleScab.aspx

This edition of POP Tips compiled by POP Outreach Intern, Alyssa Schimmel, and Orchard Intern, Sophia Taylor.
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