Apr 19, 2019
Apple scab infection forecasted for Pennsylvania growers

According to Penn State University, conditions are favorable for apple scab, rust and brown rot infections on April 19-20, in Pennsylvania.

Fungicide applications are encouraged to keep green tissue and stone fruit blossoms protected. Begin to think about fire blight management.

If the forecast proves accurate, we are in for another major scab infection event April 19. Our apple scab spore counts from the overwintering leaves continue to increase and will most likely be peaking soon. Many locations, including Adams County, are already observing pink bud on many varieties. With temperatures predicted to be in the 60s and 70s for the next 10 days, bloom is right around the corner for many growers in Pennsylvania. At this time, vigilance is needed for preventing apple scab and other fungal diseases (rust, brown rot) becoming established and prepare for the fire blight prevention. The following are nuggets of wisdom to keep in mind.

Apples: Late pink through petal fall

Use FRAC Group 7 fungicides

  • As we near bloom, we are also nearing the period where the greatest number of mature overwintering apple scab spores will be available for dispersal. Typically, the maximum number of spores available will peak for approximately two weeks (late pink through petal fall). Disease pressure will be at its greatest if it is warm and wet during this period.
  • Growers are encouraged to use fungicides containing the FRAC Group 7 mode of action from late 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 time 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.
  • For fungicide resistance management purposes, four complete sprays (eight half sprays) of fungicides containing the FRAC Group 7 mode of action are recommended 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 2 complete sprays (4 half sprays) of the FRAC Group 7 fungicides during the early part of the season (pink through petal fall) and use 2 complete sprays of FRAC Group 7 fungicides (Luna Sensation, Merivon, Pristine) near harvest.
  • For fungicide resistance management, tank mix with a rainfast mancozeb. These include Manzate Pro-Stick, Roper Rainshield, Dithane Rainshield. If you are using a non-rainfast mancozeb, such as Koverall or Penncozeb, add a spread-sticker adjuvant to your spray tank. Mancozeb will also help with rust protection. The rainfast aspect of mancozeb will allow some buffer for protection during persistently rainy periods.
  • Please remember: if disease conditions for apple scab persist from late pink through petal fall, growers must shorten their interval when using alternate row middle sprays. What this means: If you spray complete sprays every seven days; half sprays need to be applied every 3.5 days.

For the northern parts of Pennsylvania

Where trees may be at tight cluster: You can be conservative:

  • Mancozeb 3 lb/A plus FRAC group 3 (Rally, Indar, Procure/Trionic, Rhyme, Inspire Super)
  • Mancozeb 3 lb/A plus sulfur (refer to the product label)
  • Alternative options: Potassium biocarbonate 3 lb/A; sulfur

Fire blight considerations from pink through bloom

The question I get asked the most these days: what kind of fire blight season are we going to have? Since we are at pink in Adams County, I think I can start to predict the season. Based on how the day time temperatures have been warm (but not too warm) and the night time temperatures have been cool (but not too cool), I’m guessing this will be a protracted bloom season. The temperatures for the extended forecast appear to be steady and moderate; temperatures Erwinia amylovora prefer. This does not bode well for those who worry about fire blight. Remember the 2014 season? Some of the worst fire blight the state has experienced? That year was a protracted bloom. I’m concerned history will repeat itself. However, there is no need to panic. Knowing this information, we have the knowledge and tools to prepare for the worst and hopefully, expect the best. I honestly hope I’m wrong.

Options for fire blight management at pink

There is a 2ee label for Apogee to be used at pink for fire blight management. This 2ee applies to CT, ME, MD, MA, NH, NJ, PA, RI, VT. Research out of Cornell has shown that applying 3 – 6 oz/A of Apogee at pink helps with blossom blight management, as well as subsequent shoot blight management.

Options for fire blight management at early bloom

During early bloom, when the king blossoms begin to open, applying a biological, such as Serenade Opti (14 oz/A), Serenade ASO (4 qt/A), or Double Nickel (2 qt/A), will keep these first blossoms protected. With the fewest numbers of blooms open, these products will do the job. During this time, Actigard could be included (2 oz/A) with your blossom protection spray. For the youngest, most fire blight susceptible trees, Actigard is recommended. We have observed during our greenhouse studies the defense signal Actigard activates can persist for a long time (up to 2 weeks). Also, we do see the benefits of an additive effect: incorporating Actigard into more than one blossom spray.

Options for fire blight management during full bloom

  • Once the laterals begin to open, and you observe more than 50% open blossoms on your trees, streptomycin will be necessary.
  • If bloom drags on, Actigard will be an excellent investment for keeping your trees protected. I recommend incorporating Actigard with one of your early streptomycin sprays.
  • If 5-7 days lapse between streptomycin applications and you still are observing significant bloom on your trees, consider another streptomycin application.
  • If bloom persists, consider applying Apogee or Kudos, either at a low rate (2 to 4 oz/A) or a higher rate for older trees (6 – 12 oz/A). This will help with subsequent shoot blight.

Non-streptomycin options for fire blight management

  • For those who chose to use OMRI labeled options for fire blight management, your best bet is Blossom Protect (Westbridge) during full bloom. Of the alternatives we have evaluated for blossom blight prevention, Blossom Protect has shown the most promise. Multiple applications will be necessary since Blossom Protect only works on open blossoms. Under wet bloom conditions, the product will russet fruit. If fruit finish is a priority, this is a consideration to keep in mind.
  • If you choose to use an OMRI labeled copper, choose a copper that has a high rate of metallic copper per unit. Copper at bloom will cause fruit finish issues; however, if fruit finish is not critical, copper is an excellent alternative for blossom blight management. We are still conducting research to determine the ideal metallic copper rate; however, our early studies indicate multiple applications of 8 – 10 oz/A of metallic copper gives adequate control.

Don’t forget:

Open stone fruit blossoms still need protection from brown rot

Stone fruit blossoms are still a-plenty in the orchards, and we are experiencing ideal conditions for brown rot. 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. Generally, 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 using products with the active ingredient iprodione (FRAC group 2; Rovral, Meteor) mixed with captan. This will give you adequate protection. Save the other FRAC groups (1, 3, 7, 9, 11) for your preharvest sprays later in the season. Alternative management options to manage blossom blight include sulfur.

For commercial fruit growers, please note: When controlling for disease, weather and tree growth conditions need to be monitored at a local level within one’s own orchard. Before chemical products are applied, be sure to comply by obtaining the current usage regulations and examining the product label. Product information can be easily obtained from CDMS.

Kari Peter, Penn State University

Available scab spores are increasing: keep trees protected. Photo: Kari A. Peter/Penn State University

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