Apple scab is a fungal disease which affects most commercial apple varieties, as well as ornamental crabapple trees. Caused by the fungus Venturia inaequalis, apple scab inflicts damage on both leaves and fruit. While the disease rarely fatal, the scabby lesions resulting from the disease often make fruit unsellable. The disease can also create issues in supply chains, as sometimes lesions only appear after fruit has been placed into storage.
Apple scab is easily identified by the telltale spots on leaves and fruits, though sometimes the worst effects of the disease aren’t obvious until after harvest.
Infection by V. inaequalis typically occurs in the early spring. The first signs of apple scab appear a few days later in the form of small light-green spots on the undersides of young leaves. These spots grow in size as the fungus reproduces, becoming olive in color and gaining a velvety texture. Spots are larger on young leaves, while older leaves are more resistant and have smaller spots.
As the spots grow to as much as 2/5” in diameter, they turn brown and harden. The most seriously infected leaves turn yellow, and are often dwarfed and twisted in appearance. These leaves often drop off early.
When the disease is transmitted from leaves to nearby blossoms, the blossoms may immediately abort and drop off. If the blossoms are retained, the resulting scabby lesions are difficult to miss. They may be yellow, olive, or brown, and vary in size, presenting as small blisters, round scabby spots, or discolored patches large enough to wrap around much of the fruit. The lesions are sunken and have a distinct margin.
As infection progresses, the lesions will often crack open, oftentimes resulting in secondary infection by bacteria, fungi, or insects. Severely infected fruit often drop from the tree prematurely. Fruit that is older and more developed when infection occurs will likely develop to full maturity, but may still appear unsightly enough to be unmarketable.
When fruit is infected shortly after harvest, spots may not appear until after the fruit is placed into storage. When spots appear after harvest, they are often small and dark, and are sometimes referred to as “pinpoint scab.”
Apple scab is caused by the fungus Venturia inaequalis, which recurs year after year by overwintering in infected plant debris.
The life cycle of V. inaequalis begins and ends with dropped leaves and fruit from infected trees. The fungus overwinters in debris, until warming weather in the springtime prompts the fungus to begin spore production. Fungus spores are typically first released around the time that new leaves begin to develop, with the spores being transmitted by wind and rain. The fungus favors cool weather, and the mode of transmission means that apple scab is most commonly seen after cool, moist springs.
The time from the first infection of the spring to germination of new spores is only 10 to 20 days. This allows the disease to quickly move throughout tree canopies, infecting blossoms and the resulting fruit as they emerge. The process of fungal reproduction and spreading of the disease continues until summertime conditions set in, as hot and dry weather is inhospitable to the fungus.
Apple scab is very rarely a fatal disease, but seriously undermines the profitability of orchards, as it can cause yield losses of 70% or more. It is currently considered to be the most economically costly disease affecting apples anywhere in the world.
Because apple scab infection requires the presence of infected plant material and specific weather conditions, conscientious growers can break the cycle of infection and transmission.
Latent fungal populations overwintering in dropped plant debris are triggered by moist, warming weather because these are the exact conditions which spores require to successfully infect would-be host trees.
V. inaequalis spores requires temperatures of 42 to 78 degrees to germinate, simultaneous from a period of consistent wetting ranging from 9 to 30 hours. Infection occurs quickest between temperatures of 57 to 76 degrees Fahrenheit. Cooler (or slightly warmer) conditions necessitate longer periods of moist conditions.
Because of the somewhat fussy nature of V. inaequalis, it can be readily controlled for the most part through good sanitation practices:
- During the winter, clear out all plant debris on the ground, or till the soil and ensure that plant matter fully decomposes. Stimulating microbiological activity in the soil can accelerate the decomposition process.
- Only irrigate orchards in the morning or evening to allow ample time for leaves to dry out during the day, and avoid the use of overhead irrigation.
- Prune tree canopies as necessary to ensure ample air circulation.
Fungal infections can be treated when they appear with any of a variety of common fungicides, including Daconil, ferbam, maneb, mancozeb, triflumizole, ziram, and others. Organic growers can effectively treat the disease with liquid copper or sulfur-based treatments.