Don’t get caught with your plants down: watch out for fire blight conditions during bloom. Photo: K. Peter, Penn State
The 2018 season has been slow to start compared to 2016 and 2017, but the fruit trees are wide awake and diseases are ready to go. The disease I’m most concerned about is fire blight. During 2014 and 2015, our apple trees bloomed during the first week in May, which coincided with warm, wet weather that stuck around. Both of those years were also bad fire blight years. There is a chance we may encounter similar conditions during 2018. I recently saw info from the University of Tennessee about widespread fire blight incidence showing up in ornamental pears in Middle Tennessee. This isn’t a good sign; however, commercial growers have a lot of tools to use to stay ahead of a potential fire blight fiasco.
Preventing a fire blight epidemic
Monitor weather conditions closely during bloom: average temperatures >60°F and wetting events (rain, heavy dew). Commercial fruit growers are encouraged to remain on guard not only during bloom but during petal fall. In years when the warm conditions persisted post bloom for a significant period (2014 and 2015), fire blight exploded and caught everyone off guard. Based on the forecast next week, summer will arrive, and bloom might occur in a hurry.
Here is a plan for growers during bloom:
- If bloom begins slowly, consider applying a biological (Serenade Opti, Serenade ASO, Double Nickel) during 10-20% bloom. Approximately 48 hrs later, apply a streptomycin spray since more blossoms will have opened, and trees may be near full bloom.
- Streptomycin still is effective. Use it during bloom: it works. Four sprays are allowed during a season; however, save one spray if a trauma event (hail, high wind) occurs during the season post bloom.
- Incorporate Actigard (2 oz/A) in your last streptomycin bloom spray. Actigard is a plant immune system inducer and will offer about seven days protection post application. This is an expensive product; focus on the most susceptible varieties. This will be important for that tricky period post full bloom.
- Consider low rates of Apogee/Kudos (2 oz/A) beginning during late bloom/petal fall on susceptible dwarf trees that have filled their space. Our research has shown any rate (as low as 2 oz/A) of ProCa will suppress growth. However, if it is a bad fire blight year, a grower will need to determine their priority: keeping the tree alive vs. getting optimal growth. ProCa will significantly decrease fire blight severity by hardening off the shoots 10-14 days post application. Multiple applications may be needed.
- Apogee/Kudos (full rate) is a must for susceptible semi-dwarf trees.
- If Apogee/Kudos is not preferred for dwarf trees, regular sprays of Cueva (2 qt/A) beginning at petal fall is recommended. We have observed reduced shoot blight using frequent Cueva applications post bloom. Applications are only necessary until mid-late June or when the trees reach terminal bud set.
- In cover sprays post bloom, another consideration is incorporating Vacciplant or Regalia for young, dwarf trees. We are still researching the utility of these products to mitigate shoot blight, but the early results are promising.
Just a word to the wise: If this year ends up being an exceptionally challenging fire blight year, typically “resistant” varieties will get fire blight if the disease pressure is high enough.
For more information and a review for susceptible hosts (which could be lurking near your orchard): Apple Disease - Fire Blight
Managing powdery mildew and apple scab
It’s late April, and many areas are in the tight cluster stage of bud development on apples. Now is the time to begin powdery mildew control. Powdery mildew is considered a “dry weather” disease. The fungus does not like prolonged leaf wetness (i.e., apple scab conditions); high humidity and temperatures ranging from 55 - 70°F are enough for the spores to germinate. Be mindful of the dry weather days occurring from tight cluster until the shoots harden off (approximately second cover spray), which is the most susceptible time for infection since the powdery mildew fungus likes young tissue. As tissue begins to grow, the fungus will colonize young, green tissue as it emerges. Symptoms of primary infection include “flag shoots,” which may have stunted growth or die back. On very susceptible varieties, severe blossom infections can occur and fail to produce fruit.
There is the challenge of controlling for powdery mildew and scab combined with practicing fungicide resistance management during these mixed days of dry and wet conditions. During the current period of green tip through tight cluster bud development, when available scab spores are low, consider using fungicides from FRAC Groups 3 or 9, such as Rhyme, Rally, Indar, Inspire Super, Procure/Trionic, Scala, or Vangard. Syllit, which is FRAC Group M7, can also be included; however, include sulfur for powdery mildew control since Syllit is ineffective for controlling powdery mildew. Alternative management options include sulfur or lime sulfur only (both will manage scab and powdery mildew). Be sure to rotate FRAC Groups and tank mix with a broad spectrum fungicide (mancozeb, ferbam, metiram, ziram) for fungicide resistance management.
There are many products available with the FRAC Group 7 mode action. Several of these products are excellent for both scab and powdery mildew control. Growers are highly encouraged to wait to use fungicides containing the FRAC Group 7 mode of action during pink through petal fall; these fungicides are best saved for peak apple scab pressure, which is from pink through petal fall. Even if dry conditions persist during late pink through petal fall, the FRAC Group 7 products will control for powdery mildew control during this period. These products include Aprovia, Fontelis, Sercadis, Luna Tranquility, Luna Sensation, Pristine, and Merivon.
If disease conditions persist and additional fungicide applications are necessary, rotate the FRAC Group 7 fungicides with FRAC Groups 3 and 9. Four complete sprays (eight half sprays) of fungicides containing the FRAC Group 7 mode of action are permitted per season. This includes both premix FRAC Group 7 fungicides (Luna Tranquility, Luna Sensation, Merivon, Pristine) and the stand-alone FRAC Group 7 fungicides (Aprovia, Fontelis, Sercadis). Some of these products are excellent in rot control later in the season and during postharvest. I typically recommend growers use two complete sprays (4 half sprays) of the FRAC Group 7 fungicides during the early part of the season (pink through petal fall) and use two complete sprays of FRAC Group 7 fungicides (Luna Sensation, Merivon, Pristine) near harvest.
Preventing brown rot infections during peach and nectarine bloom
Blossom infections from the brown rot fungus can occur whenever pistils are exposed, and a favorable climate exists. Infections can occur during any wetting period when temperatures are between 41 and 86°F. However, optimum conditions for infection occur with wetting and temperatures in the mid-70s. During long wetting periods (several days or more) blossoms can be infected regardless of temperature. Infections that occur when conditions are sub-optimal are less severe. Blossoms and fruitlets will remain susceptible until the pistil desiccates (sometime between petal fall and shuck split). Keep blossoms protected with fungicides for blossom blight. Be sure to tank mix fungicides with a broad spectrum protectant for fungicide resistance management.