Apple scab of apple and crabapple: what the extension services record
What The Morton Arboretum, UC IPM and the University of Connecticut say about apple scab: the Venturia fungus, the rose-family hosts, the leaf and fruit spots, the overwintering on fallen leaves and the wet-weather conditions the sources give for infection.
Checked against the sources at the bottom of this page on October 9, 2026. Rules, fees and pay change: the source has the last word.
This page describes what the named sources say. It is not a diagnosis of any tree and does not say what should be done with one. None of the three pages read lists apple scab as regulated or notifiable. Each section names its source, all read on October 9, 2026. The full list of diseases is on the tree diseases page. Another rose-family disease with its own page is fire blight.
Cause and hosts
All three sources name the fungus as Venturia inaequalis. The Morton Arboretum calls apple scab one of the more serious diseases of apples and ornamental crabapples and says it mainly affects members of the rose family: apples and crabapples (Malus), common pear (Pyrus), cotoneaster, firethorn (Pyracantha), hawthorn (Crataegus) and mountain-ash (Sorbus). The University of Connecticut calls it the most serious disease of apples in that state and says most of the commonly grown apples and crabapples are susceptible. UC IPM, the University of California's pest management program, says apple scab is a serious disease of apples in California and also a problem on ornamental crabapple. It separates it from pear scab, caused by V. pirina, and says neither fungus affects the other's host, so both have quite limited host ranges.
What the sources describe on the plant
| Plant part | What is recorded | Source |
|---|---|---|
| Leaves | Small velvety brown to olive-green spots that enlarge, darken and become more or less circular with feathery, indistinct margins; young leaves curl and distort when infections are numerous. Connecticut describes olive-green spots with fuzzy margins that become distinct and greenish black. | The Morton Arboretum; University of Connecticut |
| Leaf onset | UC IPM says scab first appears as yellow spots, then dark olive spots, sometimes velvety underneath, and leaves may twist or pucker and turn yellow and drop when severe. | UC IPM |
| Flowers and young fruit | Infection of flower stems can cause flowers to drop, and Connecticut says early infections can drop blossoms and young fruit. | UC IPM; University of Connecticut |
| Fruit | Lesions become corky, cracked and rough (Arboretum). Connecticut says fruit is susceptible at any time during development and late-season infections show as small black specks called pin-point scab. UC IPM says severely infected fruit becomes distorted and usually drops, and cracks allow entry of secondary organisms. | All three |
Disease cycle and weather
All three say the fungus overwinters in fallen infected leaves. The Arboretum says the first spores mature at about the time of budbreak, are ejected into the air after rain, and are produced over much of the growing season, with the most produced near the end of flowering on apples and crabapples. Connecticut says the spores are forcibly discharged during rainy periods just as buds begin to open and air currents carry them to the emerging leaves. UC IPM says rainfall or sprinkler irrigation is necessary to release the spores. The Arboretum describes secondary spores that re-infect new leaves through the summer, carried by splashing rain, irrigation and wind, and says several secondary cycles may occur if wet weather continues. UC IPM gives 8 to 17 days after the first infection for the secondary spores to form.
The wetness figures differ by source. UC IPM says infection is most rapid between 55 and 75 degrees Fahrenheit and the leaves or fruit must stay wet for a minimum of 9 hours at those temperatures; if spring is dry from green tip through fruit set, scab usually is not a problem. Connecticut says the cooler the temperature the longer the leaves must stay wet, about 17 hours at an average of 45 degrees and about six hours at 65 degrees. The Arboretum says the disease is most severe in spring and early summer when humidity is high and temperature is moderate, and that fewer hours of wet leaf are needed at high temperatures than at low.
Harm to the tree
The Arboretum says apple scab can lead to defoliation and that years of repeated defoliation can lead to decline of the host tree. Connecticut says the disease causes loss by reducing fruit amount and quality and tree vigour, which can increase winter injury and susceptibility to secondary diseases and insects. UC IPM says scab can destroy an apple or pear crop and that defoliation follows severe early leaf infection.
What the sources say about resistance and control
The Arboretum calls planting resistant apples and crabapples the best way to prevent apple scab and says many species and cultivars resist the fungus. UC IPM says breeding for scab resistance is ongoing in New York and says scab-resistant crabapples are available. This page gives no treatment steps, product names, rates or timings, and does not repeat the cultivar lists. None of the pages names a quarantine or a report line. For who may apply pesticides see pesticide licences.