Apple Disease Situation in this Year of an Early Onset of Bloom

Posted: March 20, 2012

Primary infection periods for apple scab started two weeks early this year. Beginning with this report, the status of disease infections will be updated weekly at this Fruit Times website. To receive these reports as soon as they are posted, please click on " Subscribe to E-Mail Updates."
Scab Infection Periods

Scab Infection Periods

Releases of mature apple scab ascospores were detected from March 16 to 18 at the Penn State Fruit Research and Extension Center in Biglerville, PA (Figure 1). Dormant copper sprays are recommended before green tip and hopefully growers were able to get orchards covered. This week, temperatures are predicted to approach 70 degrees F and the threat of early season infections remains high. The bud stage of early cultivars such as Gala is ½” green and of Red Delicious is ¼” green whereas the bud stage for Rome Beauty, Golden Delicious, Cortland and Stayman ranges from silver tip to ¼” green. With temperatures at 60 to 70 degrees F, spore germination will progress faster and fewer hours are required to cause infection (Table 1). With the recent rains, ample hours of wetness are predicted to allow for infection throughout the week. Refer to the winter meeting presentation by Dr. Alan Biggs and to Dr. Alan Biggs’ previous Fruit Times article on “Disease Management Strategies Following the Challenging 2011 Season" for further information on the apple scab life cycle and apple scab management.

Table 1. The original 1944 Mills Table for predicting apple scab infection, and its 1980 revision by Jones, have been used for many years by researchers and growers. The latest revisions by Machardy and Gadoury (1989) works for both primary and secondary infections. The light, moderate, and high infection designations of previous tables are no longer used because severity of infection is also influenced by factors other than wetting time, such as inoculum level, amount of susceptible tissue, and cultivar.
Modified Mills Table. Minimum Hours of Leaf Wetness Required for Infection by Scab
Temperature (F) Hours
From Mills (1944). Jones (1980), and MacHardy & Gadoury, as amended by Stensvand et al.
34 41
36 35
37 30
39 28
41 21
43 18
45 15
56 13
57 12
50 11
52 9
54 8
55 8
57 7
59 7
61-75 6
77 8
79 11

MacHardy and Gadoury found that scab spores are scarcely discharged after dark, which changes the way growers should count wetting hours. As long as scab was controlled the previous season, and inoculum is low (likely to be the case in a commercial orchard), you can start counting hours of wetness at sunrise, if the rain begins after dark. If the rain begins during daylight hours, start counting hours when the rain starts.

MacHardy and Gadoury also found that the fungus can cause infections more quickly at low temperatures than the older Mills Tables predicted (at 43°F, 18 hours vs. 25 hours).

When there are chilly spring rains, growers should probably be particularly concerned about low-temperature infections. When calculating temperatures, it's best to take hourly readings and average them.

Prepared by Dr. Noemi O. Halbrendt, Penn State Fruit Research and Extension Center, Plant Pathology Research Associate