Fire Blight, Apple Scab, Rust, Plus Others: Be On Alert!

Conditions are optimal for fire blight, apple scab, and rust infections. Growers are encouraged to keep trees protected. Other diseases for the fruit grower radar are also discussed.
Fire Blight, Apple Scab, Rust, Plus Others: Be On Alert! - News


Protect any trees still in bloom: Fire blight conditions ideal April 29 through May 2. (Photo: K. Peter)

There was no easing into the 2017 season. Conditions have been favorable for many diseases, which will probably make this year a very busy year for me. As a pathologist, this should make me happy; however, there can be too much of a good thing. Here are some good nuggets to help you navigate this crazy time as we finish up April and kick-off May:

Fire blight: Conditions excellent April 28 - May 2

There are still very many apple trees blooming right now. The forecasted rain and the warm temperatures make conditions perfect for fire blight: April 28 - May 2 will be ideal for infection events. The most challenging fire blight days will probably be April 29 and May 1. For a friendly review of the minimum requirements for blossom infection and the order in which they must occur:

  1. Flowers must be open with petals intact (flowers in petal fall are resistant)
  2. An accumulation of at least 198 degree hours above 65°F
  3. A wetting even as dew or rain
  4. An average daily temperature of 60°F

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 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. 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.

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!). DO NOT spray antibiotics post petal fall: A trauma event (hail, high winds) is the exception.


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 plants 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, copper can cause fruit russetting. Be mindful of conditions and variety 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.

Apple scab: Infection periods April 27 - 29

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 and the warmer temperatures allow for a very short time of leaf wetness hours (6 hours) needed for scab infection. The disease pressure is at its greatest now: we are at peak dispersal of mature scab spores from the overwintering leaves. We monitor weekly for spore release from the overwintering leaves we collected from last season that are kept in one of our research blocks. This week our counts range from 4,000 to 40,000 spores being released from leaves (roughly estimated). We typically stay at this peak for one to two weeks, and I anticipate we will still have high numbers next week. What does this all mean? The overwintering spores have reached maturity and moisture will allow a lot of spores from overwintering leaves to be released to wreak havoc in the orchard. After this peak release, the numbers will drastically drop off with the last few mature spores probably being released until early June.

Growers are encouraged to apply fungicides prior to rain to protect trees from infection. As I had recommended during the winter meetings and previous disease updates: 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 and they end up with "alternate row middle scab" on their trees. 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) 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.

Rust: The galls are active

This is just a friendly reminder the rust galls are active. The products applied now to combat apple scab will also manage rust, especially if an EBDC is included in the tank mix. At this time, a comparable alternative to managing rust is still elusive.

Rust Review

  • Each rust spot on your apple leaf is caused by one rust spore coming from the galls on the cedar trees.
  • Once infection is established on the leaves/fruitlets on the apple tree, the spores produced from those infections will not continue to infect the plant tissue on the apple tree. The spores developed later in the season on that infected plant tissue will fly out and land on the cedar tree and infect it, thereby establishing the cycle all over again.
  • Severe rust infections on the leaves can defoliate the tree, as well as infect and deform fruit.
  • Rust management typically occurs from when the galls appear until when a dry period persists after petal fall. The moral of the story: once you see rust spots on your apple tree, it's too late for protection. Hence the need to get protection on early to prevent the disease.

Powdery Mildew: Beware of the dry periods

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 dry days and protect your trees. Trees are most susceptible from tight cluster until the shoots harden off (approximately second cover spray). The powdery mildew fungus likes young tissue, which is why powdery mildew prone varieties should be protected through terminal bud set. Even if dry conditions persist during late pink through petal fall, the FRAC Group 7 products (such as Luna Sensation, Luna Tranquility, Merivon, Sercadis) will be effective for powdery mildew control during this time period.

Cherry leaf spot: Conditions favorable

Cherry leaf spot is similar to apple scab when it comes to infection conditions: warm and wet. I often refer to cherry leaf spot as the "apple scab" of cherries. The cherry leaf spot fungus prefers moderately wet conditions (hours of leaf wetness), with temperatures above 60°F. Optimal temperature range for the spread of this fungus is between 60 to 68°F. Serious infection of a tree occurs in years with many rainy periods and cooler summers. Like scab, we've had excellent conditions for cherry leaf spot infections. Fungicide options post petal fall for tart cherries include: captan, ferbam, ziram, copper, sulfur, Fontelis, Gem, Indar, Luna Sensation, Merivon, Syllit, Topsin M. Rotate products by FRAC group to practice fungicide resistance management.

Bacterial spot on peach: Shuck split onward, sprays needed

It didn't take long to experience great bacterial spot weather. Bactericide applications to manage bacterial spot typically begin early shuck split and continue on a 7-14 day interval throughout the summer. When conditions are favorable, which is warm (75°F to 85°F) and wet, shorter intervals are needed (i.e. weekly sprays); a longer 14-day interval is acceptable during extended periods of dry weather. Primary management tools include oxytetracycline (FireLine, Mycoshield) and copper. A few years ago, I included information about the ideal rates of coppers for bacterial spot from Rutgers in a Fruit Times article posted on April 24, 2015. As a reminder: the window of activity for oxytetracycline is 48 hours. Other products we have evaluated to date that show promise and can be used in rotation include Serenade Opti (14 oz/A) and Regalia (1 qt/A).

A good reference from Cornell's Scaffolds Fruit Journal

The March 27, 2017 Disease Update in Cornell's Scaffolds Fruit Journal has a great update about fungicides for scab. It is well worth the read.

A note of encouragement

This might be a tough season since disease conditions have been a doozy so far - and it's not even May. There is a long way to go until harvest…In the immortal words of Star Wars Jedi Master, Obe-Wan Kenobi: May the Force be with you.