There are some pests that cause damage during bloom and fortunately, there are organic options that can be used against them that don’t harm bees (if used according to the label).
Apple, Pear
Fire Blight
Infections may happen on blossoms in warm, wet weather
Fire blight is a bacterial plant disease that causes infections through open flowers on apple and pear (plus other ornamentals such as crabapple, ornamental pear, quince, and hawthorn). Infections occur during warm, wet days. We use a model to predict fire blight risk.
You can watch fire blight predictions on the Utah TRAPs website by selecting a location closest to you, and then selecting Fire Blight in the ‘Pest’ drop-down.
The upcoming infection risks (only applies to open blossoms) is shown for the counties below for April 21 – 26:
- Cache, Iron, Uintah (and other cooler areas of the state): LOW Risk – even if open flowers, do nothing now
- Box Elder, Davis, Salt Lake, Utah, Weber: LOW Risk April 21-24; HIGH Risk April 25 and 26
As you can see above there is a LOW potential for infections on open blossoms for the next few days in all of northern Utah, even with the rain that is predicted for Wednesday and Thursday. Fire blight does needs moisture to cause infection, but only under the right temperatures.
But for the warmer northern Utah counties, conditions change on Friday and Saturday, when the risk of infection increases to HIGH, and rain is forecasted for Sunday. In this case, you may choose to protect trees with open blossoms that:
- Had fire blight last year
- Are young, loaded with flowers, and especially if they are planted in a high-density orchard
Treatment
For commercial producers, most areas (except the southern part of Utah County, which has resistance) can use the antibiotic, streptomycin, which is the most effective option. It provides 3 to 4 days of protection. Where streptomycin-resistance has been documented, producers should use oxytetracycline (MycoShield) or Kasumin. Experts suggest that a single application can be a mix of full-rate oxytetracycline plus streptomycin, but this should option should be saved for a time of extreme risk-closer to full or late bloom-to get the greatest effect.
For backyard growers, streptomycin is the only antibiotic option available, but most owners should not need to apply this product. If deemed necessary, most garden centers carry streptomycin. It is important not to over-use this product, or the bacteria will develop resistance. As an organic option, USU has tested the products GardenPhos and Serenade (mixed together), applied to flowers before wetting events. This combination provided 66% control versus streptomycin, which provided 87% control.
Instead of spraying, we recommend monitoring trees closely starting two weeks after full bloom (which is when infections start to become visible) and removing the fresh infections. A future post will discuss pruning out new infections, which should be done on a dry day.
Apple Powdery Mildew
Apply fungicide at petal fall
The fungus that causes apple powdery mildew (Podosphaera leucotricha) overwinters on twigs, and as a result, new infections can occur early in the season. Depending on weather conditions, it can become active before bloom.
Powdery mildew spreads with high humidity, usually in the dawn or dusk hours, and does not need free water to germinate. When terminal shoots harden off and when relative humidity decreases, new infections slow down.
If your apples had powdery mildew in the past, apply a fungicide after bloom (petal fall) if you have not already treated. A second treatment 2 weeks later may be required where powdery mildew has been severe in the past.
Commercial growers should be aware that in other parts of the country, powdery mildew fungal populations are becoming increasingly resistant to the Group 3 fungicides (Indar, Procure, Rally, Rhyme). These fungicides have historically been highly effective due to their curative effects after mildew has shown up. By using a product from a different fungicide group at petal fall, use of Group 3 products can be reduced, preventing resistance from occurring in Utah.
Some varieties are more susceptible than others (such as Braeburn, Gala, Gingergold, Jonagold, Jonathon, Rome). The Delicious varieties are the least susceptible.
Treatment
Commercial: Options with good to excellent efficacy that are not Group 3 products include Flint, Merivon, Sovran, Fontelis, and sulfur (organic). Click here for more.
Backyard: Spectracide Immunox, Fertilome F-stop, and Monterey Fungi-max all contain the same ingredient (myclobutanil) and cannot be used on pear; Hi Yield Wettable Sulfur (or other sulfur brands; organic); Monterey Complete Disease Control (organic)
Codling Moth (not a bloomtime pest)
To determine site-specific dates to start codling moth treatment, we hang pheromone traps in orchards and use predictive models. Once we catch moths or see predictions, we can run the calculations and provide dates. As of now, we are gathering information for codling moth spraying times from some northern Utah locations. We expect to have this information out in next week’s post.
Peach/Nectarine, Apricot
Two Bloomtime Pests of Peach/Nectarine – Peach Twig Borer and Thrips
For peach twig borer (PTB), spray Bt during bloom; for thrips AND PTB, spray after bloom is over (petal fall), using spinosad
In the April 8 advisory, we discussed the damage that western flower thrips can cause to nectarine fruits, and to apply spinosad at night near the end of bloom as a prevention.
Peach twig borer larvae overwinter in protected areas of peach, nectarine, and apricot trees as a tiny larva. Once leaves start to emerge, these larvae leave their winter chambers to feed on foliage and flowers and increase in size. As soon as leaves fully expand, larvae will tunnel into the shoots and be hidden.
Bloom provides this small window of time that larvae are exposed and vulnerable to an organic insecticide called Bt. Growers can significantly reduce peach twig borer populations by making a spray during bloom.
Bt (Bacillus thuringiensis) is a naturally-occuring, soil-dwelling bacterium that, when consumed by caterpillars, affects their gut and causes them to stop feeding and die. Bt is organic and has no activity on bees.
Treatment
Commercial: Javelin, Dipel
Backyard: Hi Yield Thuricide, Fertilome Dipel, Monterey Bt Liquid, Captain Jack’s Bt