Apple scab is treated with a fungicide labeled for scab, applied starting at the first sign of infection and repeated every 7 to 14 days, combined with raking up and destroying fallen leaves each autumn to remove where the fungus spends the winter. The same fungus infects both apple and crabapple trees, so the identification and treatment described here apply to either one.

What causes apple scab, and why does it keep coming back?

Apple scab is caused by the fungus Venturia inaequalis, which thrives when spring weather is mild, around 60 to 70 degrees Fahrenheit, and wet. The fungus overwinters in leaves that fell the previous autumn, and wind carries its spores back up into the canopy the following spring to start a new infection. A tree that keeps getting scab year after year is usually still hosting the fungus in leaf litter left on the ground rather than facing a fresh source of infection each time.

What does apple scab look like on leaves and fruit?

Small brown spots appear on the undersides of leaves first, then spread to the top surface as the infection develops. On fruit, the spots are light brown, roughly ¼ to ½ inch across, and feel rough to the touch; in severe cases the fruit becomes cracked and malformed and can drop before it ripens.

Does apple scab affect crabapple trees the same way as apple trees?

Yes. The same fungus, Venturia inaequalis, is described as a common disease of both apple and crabapple trees, so a crabapple showing the same brown leaf and fruit spotting is dealing with the identical infection an apple tree would, whether or not the crabapple is a fruiting variety.

What is the best fungicide treatment for an active apple scab infection?

A fungicide labeled specifically for scab, such as Liquid Copper Fungicide Spray, is the standard treatment, applied at the first sign of disease and repeated every 7 to 14 days, with shorter intervals during frequent rain since wet weather is what drives new infections. One pint of concentrate treats 5 to 6 large trees, mixed at roughly one gallon of spray solution per 200 square feet or up to two gallons for a single large tree.

Is there an alternative treatment method for scab beyond a standard foliar spray?

Garden Phos with Pentra Bark is labeled for apple black spot and scab as a foliar spray, and also offers a trunk-spray option applied to the lower 8 feet of the trunk for cases where reaching the full canopy with a foliar spray is impractical. Either method works best when started within a few months of the first symptoms rather than after the infection has spread through the whole canopy.

How does removing fallen leaves help control apple scab?

Raking up and destroying leaves each fall removes the fungus's main overwintering site, since the spores that start a new infection in spring come from leaf litter left on the ground rather than from the tree itself. Skipping this step in a year when scab was a problem makes it more likely the same infection returns the following spring regardless of how well the spring spray schedule is followed.

Frequently Asked Questions

Should the fungicide spray schedule change during an especially rainy spring?

Yes. A rainy spring calls for shortening the 7 to 14 day interval between sprays, since wet conditions are what allow the fungus to infect new growth, and a longer gap between applications gives it more opportunity to take hold.

Can apple scab kill an apple or crabapple tree?

Rarely on its own. The bigger cost is reduced fruit quality and repeated defoliation over several seasons, which weakens the tree gradually rather than killing it in a single year.

Is fruit with apple scab spots safe to eat?

Generally, yes. The rough brown spots are cosmetic rather than toxic, though fruit with severe cracking is more prone to secondary rot and is worth checking before eating.

Are ornamental, non-fruiting crabapple varieties just as susceptible as fruiting apple trees?

Susceptibility comes down to the variety rather than whether it produces edible fruit, since some cultivars are bred for scab resistance and others are not, regardless of whether they are grown for fruit or for spring flowers.

Should scab-infected leaves be composted, or destroyed a different way?

Destroyed rather than added to a home compost pile. Most backyard compost does not reach the sustained heat needed to kill fungal spores, so infected leaves tossed into a compost bin can end up spreading the fungus right back into the garden.