Dull, olive green areas visible on the undersides of leaves are the first evidence of apple scab.
At 39°F, 28 hours of continuous wetting are required for infection, while at 61 to 75°F, only 6 hours are required. The Revised Mills Table in this article will help you calculate apple scab infection periods.
The apple scab fungus (Venturia inaequalis) overwinters in infected leaves that have fallen to the ground. Fruiting bodies are produced within the dead leaf tissue. As spring approaches these begin to mature and produce spores (ascospores) that are discharged into air currents and carried to developing apple buds. The fruiting bodies in the fallen leaves must be wet for the spores to discharge. The ascospores are not all discharged with the first spring rains, for they mature over a 4- to 6-week period. This period usually coincides with the time that elapses from ¼ inch green until 2 to 3 weeks after petal fall.
The first infections often occur on the leaves surrounding flower buds. Dull, olive green areas visible on the undersides of leaves are the first evidence of the disease. As the lesions (infected areas) become older, they assume a definite outline as olive-green or brown circular spots. Leaves are susceptible to infection for about 5 days after they unfold.
When the spores land on wet apple buds, leaves, or fruit, and if they remain wet for a few hours, they germinate and grow into the apple tissue. The time required for germination and penetration depends on temperature and the presence of a wet surface. At 39°F, 28 hours of continuous wetting are required for infection, while at 61 to 75°F, only 6 hours are required (based on the Revised Mills Table shown below. After the fungus has penetrated, it continues to grow and enlarge beneath the cuticle. After 9 to 17 days (development occurs most rapidly at higher temperatures), a visible scab lesion is produced. On its surface appear more spores (conidia), which are easily dislodged when the lesions are wet. The spores are splashed around by rain and blown by wind to new leaf and fruit surfaces within the tree. They germinate on wet surfaces, infect the tissue, and produce a new lesion. In this manner, several secondary infection cycles may occur in the course of a growing season. Infection is rare above 78°F.
Calculating apple scab infection periods using the Revised Mills Table
- Add lowest and highest temperatures during wet period and divide sum by 2 to get average.
- Calculate hours of wetting by either (1) beginning the count at the time leaves first become wet and ending the count when the relative humidity drops below 90 percent, or (2) adding consecutive wet periods (hours) if the leaves are again wetted within 8 hours from the time relative humidity dropped below 90 percent.
- Number of days required for lesions to appear after infection has been initiated. If conditions are unfavorable for lesion development (prolonged periods above 80°F or very dry weather), additional days may be required.
New Orchard Disease Blog
Dr. Dave Rosenberger, retired Cornell Plant Pathologist, has established a new orchard disease blog at the Cornell Hudson Valley Lab.