Disease Update: Blossoms Still Open—Fire Blight Risk Continues

Conditions are favorable for fire blight and apple scab. Additional tools for the fire blight management toolbox are discussed.
Disease Update: Blossoms Still Open—Fire Blight Risk Continues - News


Be proactive about fire blight management: Let’s not repeat 2014. (Photo: B. Lehman)

When I wrote my update on Monday, I was hoping bloom was going to be fast; however, that doesn't seem to be the case this year. In light of bloom still hanging on and fire blight conditions still a threat (high risk for April 20 and 21), this update will provide additional tools for the fire blight management toolbox.

Although streptomycin is still your best bet for managing blossom blight, there are other products that are showing some promise that could also be used while dealing with this protracted bloom, as well as potentially providing shoot blight control if something slipped through the cracks. We have one year of data for some of these products; however, it's worth mentioning to give growers options, considering how crazy this season is shaping up to be. Case in point: WeatherUnderground is forecasting summer-like temperatures (upper 80s) for the south central part of PA next week. In an effort to avoid a repeat of 2014 and 2015 when fire blight was exceptionally challenging, it's best to prepare for the worst, but expect the best. We will know for certain where we stand for the 2017 fire blight season in a few weeks. For folks in Pennsylvania who haven't reached bloom yet, this will also be of help when you get to that point this season.

Also to note for folks who use NEWA: There have been some updates to the apple disease pages. In particular, the EIP (epiphytic infection potential) from MaryBlyt has been included.

Here is an overview of products the Penn State Tree Fruit Pathology Lab has evaluated under PA conditions to date that could prove to be useful to mitigate fire blight issues:


Out of principle, we still need to include strep in the discussion. Strep is the antibiotic that is still the best option since it kills the bacteria and has partial systemic activity. Streptomycin still works in the Mid-Atlantic. We have not detected streptomycin resistance in the Erwinia amylovora population in PA; however, still limit four applications of strep during bloom (re: the label). Note: the systemic activity does not persist like fungicides and you have about a 48 hour window. Antibiotics only work when the blooms are open. Apply antibiotics as complete sprays and add an adjuvant or surfactant. Antibiotic sprays are most effective when they are applied the day before or the day after an infection event (within 24 hrs!).


Sold as Mycoshield and FireLine, oxytetracycline is an antibiotic commonly used to manage bacterial spot on peach. Oxytetracycline is bacteriostatic and limits the growth of the bacteria in the blossom. This is in contrast to streptomycin, which is bactericidal and kills the bacteria. Although we have observed the efficacy of this product is less than streptomycin, it still keeps the disease in check during moderate disease conditions. For a long bloom time, with frequent rains and periods of warmth, it wouldn't be a bad idea to rotate your streptomycin spray with oxytetracycline. Similar to strep, oxytetracycline has a 48 hour window of activity, as well.


This is a plant-based product that says on its label it stimulates the plant's own defenses to fight disease. We evaluated this product last year for the first time in blossom blight management programs at 1 and 2 qt/A. Both rates were equal in providing approximately 20% control of the disease (compared to 70% control for strep). We are evaluating this again this year to see if our results are duplicated during the 2017 conditions. Even with one year of data, Regalia is doing something to the tree in order to fend off disease when we applied it three times during bloom. In addition, it's a serious contender for also limiting shoot blight as well. We have observed decreased shoot blight severity and incidence in both the greenhouse and field experiments when using Regalia. Regalia does cause some injury to blossoms: we observed brown spotting on the blossoms and premature petal fall at both rates; it was most severe at the highest rate. Regalia is safe on foliage and fruit. To limit shoot blight post petal fall, growers may want to consider including Regalia in their cover sprays through terminal bud set. We're evaluating Regalia again this year for both blossom blight and shoot blight management.

Serenade Opti

Serenade Opti (20 oz/A) is a bacterial based product (Bacillus subtillus). We have evaluated this product by itself and in rotation for blossom blight management. In rotation with strep, it kept the disease in check during mild to moderate conditions. When used alone as three applications during bloom (without strep), we observed 8% control under similar conditions. The best position for Serenade may be during the earliest part of bloom prior to very severe fire blight conditions occurring. We're still exploring the timing applications for blossom blight and shoot blight management, particularly as cover sprays to see how well it holds up under PA conditions. Recently, Serenade ASO (4 qt/A) has become available; however, 2017 will be our first year for evaluating this formulation.


We are still evaluating this product, but have observed good control so far in field and greenhouse experiments. If you have young, dwarf trees that are a susceptible variety (i.e., Fuji, Gala, etc.): growers may want to consider incorporating Actigard (2 oz/A) in two of their strep sprays. I would recommend Actigard in the first and last strep sprays of the season. Actigard induces the plant immune response and it is labelled to be used as a tank mix with streptomycin. In our evaluations to date, Actigard works best on small, young trees compared to larger, semi-dwarf trees. Actigard doesn't persist for a very long period of time and offers about a one week window of activated plant defenses.

Prohexadione calcium (ProCa; Apogee or Kudos)

If you have older, susceptible semi-dwarf trees, ProCa will be your best bet to limit the shoot blight phase of fire blight post bloom. Since ProCa takes 10-14 days to kick in, applying this product with one of your streptomycin sprays is recommended. ProCa will harden off the shoots, which will make the shoots not susceptible to shoot blight. As far as rates, folks are using lower rates (4 oz/A) and several applications; however, if you have large older trees with a known history of fire blight, it's best to stick to the label and use a higher rate.

Cueva and Double Nickel

We evaluated Cueva (copper octanoate) during the 2016 season and it provided 17% control of blossom blight, as well as significantly limited shoot blight thereafter. Last year, we also observed a significant reduction in shoot blight incidence in trees that were treated with cover sprays from petal fall until early July with Cueva or Cueva + Double Nickel (Bacilllus amyloliquifaciens). Consequently, Cueva (1 qt/A) alone or tank mixed with Double Nickel (2 qt/A) is another management tool during bloom and post petal fall, especially if conditions post bloom continue to be favorable for spread of the disease. As with any copper, it can cause fruit russeting. Be mindful of conditions and varieties where post bloom copper sprays are used, especially when conditions are slow drying. To limit shoot blight, growers would want to start at petal fall and apply every 10-14 days until terminal bud set (when the shoots are no longer susceptible to infection). When using copper in your cover sprays, please be mindful what else is in your tank. Copper will become more phytotoxic when anything is added that will make the solution more acidic (adjuvants, foliar fertilizers, etc.). We have observed favorable results on dwarf and semi-dwarf trees. We have the most experience with Cueva for fire blight control; however, we're evaluating additional copper products this season for blossom and shoot blight control.

Blossom Protect

This is a yeast-based product (Aureobasidium pullulans) and works by colonizing the flower, blocking the opening of the nectaries thereby preventing Erwinia amylovora from entering the plant. We have seen control (30-40%) using Blossom Protect the last couple of seasons. Last year, we did observe significant fruit russeting on Gala due to a lengthy wetting period following bloom last year. For growers concerned about fruit finish, caution is needed when using this product, especially since rain is in the forecast for our foreseeable future.

Finally, as a friendly reminder: DO NOT spray antibiotics post petal fall. A trauma event (hail, high winds) is the exception.

We have a jam packed field season evaluating many products for both blossom blight and shoot blight management, so hopefully we'll have more tools for growers to choose from next year.

Apple scab

Reminder to use FRAC Group 7 fungicides late pink through petal fall

I am a broken record and this might look familiar…but we're in the middle of another scab infection event through the weekend. Rain continues to be in the forecast through next week. The warmer the temperatures are, the shorter the leaf wetness time needed for scab infection. As I had recommended during the winter meetings and in the April 17th disease update: from late pink until petal fall is your time to use the FRAC Group 7 fungicides (SDHI class). During this critical time, growers should consider applying a total of 2 complete sprays (4 half sprays) of products containing FRAC Group 7:

  • Aprovia (FRAC Group 7)
  • Fontelis (FRAC Group 7)
  • Luna Tranquility (FRAC Groups 7 and 9)
  • Luna Sensation (FRAC Groups 7 and 11)
  • Merivon (FRAC Groups 7 and 11)
  • Sercadis (FRAC Group 7)

Important considerations to keep in mind:

  • Please shorten your intervals when using alternate row middle sprays during this critical disease period. This is where folks get caught with apple scab: stretching their intervals too long during optimal disease conditions. My wish is for everyone to use complete sprays during this time of year; however, I know that is not feasible for a lot of people. Let's meet half way: The time it takes to complete the first half spray, the interval will be met for the second half spray…so once half spray #1 is finished, just turn around and begin half spray #2.
  • Please practice fungicide resistance management: The label for products containing FRAC Group 7 states a maximum of 4 (complete) applications per season, and no more than 2 applications in a row. Regardless if the FRAC Group 7 product is a standalone (Aprovia, Fontelis, Sercadis) or in a premix (Lunas, Merivon), ALL count toward your allotment of FRAC Group 7 sprays for the season.
  • I strongly encourage growers to save 2 of those FRAC Group 7 sprays for the end of the season in order to apply Merivon and/or Luna Sensation before harvest. These products will help with rots showing up in storage. Rots showing up months after harvest in storage are becoming a real headache for a lot of folks.
  • Always tank mix with a broad spectrum (EBDC, Captan) fungicide and rotate with another FRAC group. Other fungicides to consider rotating with a FRAC Group 7 could be Indar (FRAC Group 3), Inspire Super (FRAC Groups 3 and 9), Vangard (FRAC Group 9), or Scala (FRAC Group 9).
  • Using a strobilurin class (FRAC Group 11) standalone product (Flint, Sovran) is cautioned during this peak time of disease pressure considering there is evidence of fungicide resistance to this class of chemistry in Pennsylvania. The premix FRAC Group 11 products are recommended, such as Merivon or Luna Sensation.
  • If you are concerned about powdery mildew during this time period, the FRAC group 7 products will provide control.