Apple IPM Update

Apple scab. Orchards in central and southwest Iowa are just past petal fall. Growers who are using the IRS reduced-spray program for scab control need to apply a tank mix of a SI fungicide (e.g., Nova or Rubigan) with a contact fungicide (e.g., mancozeb, polyram, captan) at petal fall and again at first cover (a week to 10 days after petal fall). This will bring you to the end of the high-risk period for primary infection by scab.

Fire blight. Fire blight infection risk during bloom has been moderated by cool temperatures. With petal fall comes the end of the highest-risk period for the blossom blight phase of fire blight. Two things to keep in mind:

  1. The risk of blossom blight persists until the last blossoms have disappeared from trees of blight-susceptible varieties. In other words, if you are using the MARYBLYT program, "petal fall" does not occur until ALL these flowers have fallen. Because pollinating insects spread the fire blight bacterium, and the number of flower visits they make adds up throughout the bloom period, THE RISK OF BLOSSOM BLIGHT IS GREATEST AT THE VERY END OF BLOOM. A lot of growers don't realize this, and discontinue spray and monitoring programs too early, when a few blooms still remain. Remember -- the risk of blossom blight remains until the last blossoms have fallen.
  2. If fire blight is in the orchard, it's important to maintain a preventive program of insecticide sprays until the shoots stop elongating. The reason is that the shoot blight phase of fire blight, which results in the "shepherd's crook" curling of the shoot tip, is believed to result from invasion of the bacterium through wounds caused by feeding of insects such as aphids and leafhoppers.

Cedar-apple rust. Galls on cedars are still releasing spores. The high-risk period for infections on rust-susceptible apple varieties continues until about 2nd cover (about 3 weeks after petal fall).

Codling moth. Most IPM cooperators have reached the biofix point (capture of a total of at least 5 male moths in pheromone traps in the orchard). Accumulation of the required 250 degree- days between biofix and the next scheduled insecticide spray for codling moth has been slow due to the unseasonably cold weather. Keep in mind that the IPM program for codling moth covers that insect only. Pink and petal fall insecticide sprays for cat-facing insects (such as tarnished plant bug) are advisable, and growers with a history of apple maggot problems may want to use the commercially available sticky sphere traps to monitor for this insect, beginning about mid-June.

This article originally appeared in the May 26, 1993 issue, p. 80.


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