Fire blight. The bloom period is over for Iowa apples, thus ending the period in which it's advisable to use streptomycin sprays for fire blight control. Remember, it's poor practice to spray strep after bloom, even though the label allows it, because the added sprays do no good and clearly increase the risk of developing strep-resistant strains of the pathogen. There is only one exception to this general recommendation: if large hail or very high winds (above 50 mph) cause visible wounding on blight- susceptible cultivars in orchards with a history of the disease, it is prudent to apply a strep spray within 24 hours after the damaging event. One more reminder: if fire blight appeared in the orchard last year, keep up a program of protective insecticide sprays until shoot elongation stops in order to protect against the shoot blight phase of fire blight, because insects with piercing/sucking mouthparts create the wounds that allow the bacteria to enter and blight the shoots.
Apple scab. We are nearing the second cover stage in some orchards. If your control of primary scab has been effective, it's a good time to switch over to a cover-spray program for control of sooty blotch and flyspeck.
Codling moth. Many orchards in our cooperative IPM program have reached biofix (5 moths captured in pheromone traps in the orchard), and several have reached the threshold of 250 degree-days after biofix, so they've been advised to apply an insectidice to protect against codling moth. In a few central Iowa orchards, however, captures have been so few that the biofix point has not yet been reached. In some instances, moth captures at orchards just a few miles apart have shown tremendous differences. This reinforces a key point: for reliability in an IPM program, moth monitoring must be done in one's own orchard, rather than relying on trapping data from a neighbor.
STRAWBERRIES
Tarnished plant bug (TPB). In several commercial fields during the past week, ISU scouts have reported counts of TPB nymphs exceeding the action threshold of 0.5 nymphs per fruit cluster. This means that an insecticide spray is advised, because nymph numbers are close to a level that could cause economic injury to the fruit.
Gray mold (Botrytis fruit rot). Our wet weather has been EXTREMELY favorable for severe Botrytis damage during harvest. However, if you applied three fungicide sprays during the bloom period, you'll probably experience little damage. Other factors that will help are good site selection (sufficient air movement) and fertility management to control vegetative growth.
Leather rot. This disease may be devastating in 1993 in fields with the following characteristics: poor soil drainage; standing water in the field; and inadequate mulching. The symptoms may appear on green fruit first as brown clusters or individual fruit, then as a purple to brown discoloration on ripening berries. The most distinctive symptom of leather rot on ripe fruit is a an acrid taste; the word "disgusting" barely begins to describe the flavor! A few of these berries is enough to give an off-flavor to an entire batch of jam. Selecting naturally well-drained sites, tiling heavier soils to improve internal drainage, and thorough mulching to keep the berries from contact with soil particles are very important to keeping leather rot losses down, especially in monsoon years like 1993. Ridomil 2E is registered for suppression of leather rot, but can be used only once per season. Captan and thiram are other options, but are less effective than Ridomil and nearly impossible to use during harvest due to pre-harvest and reentry restrictions. Again, the key to good leather rot control is cultural (good drainage and mulching), not chemical.
This article originally appeared in the June 9, 1993 issue, pp. 1993 issue, pp. 87-88.
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