Mar 21, 2012

Apple disease situation in this year of early onset of bloom

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° F and the threat of early season infections remains high.

The bud stage of early cultivars such as Gala is ½-inch green and of Red Delicious is ¼-inch green whereas the bud stage for Rome Beauty, Golden Delicious, Cortland and Stayman ranges from silver tip to ¼-inch green. With temperatures at 60° to 70° 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 Alan Biggs and to 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
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
From Mills (1944). Jones (1980), and MacHardy & Gadoury (1989), as amended by Stensvand et al. (1997).

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 43F, 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.

By Noemi Halbrendt, Penn State Fruit Research and Extension Center

This article is available here.