June Diseases for the Grower Radar

Diseases for the fruit grower radar and their management strategies are discussed as we wrap up May and begin June.
June Diseases for the Grower Radar - News


Be vigilant and scout for apple scab! Photo: K. Peter, Penn State

The 2017 season has definitely been interesting and keeping everyone on their toes. We are seeing a lot of diseases already in the field: powdery mildew, apple scab, rust, peach leaf curl, and fire blight, to name a few. The mild, wet weather is perfect for disease development and growers need to be on their toes for potential issues in the orchard.

Apple Scab - Primary infection period update

We've spotted early scab infections, which appear as light gray-brown velvety lesions on leaves. Folks are advised to scout their orchards for any signs of infection. If you need a refresher for the ins and outs for scouting for apple scab, the how-to and a scouting sheet can be found on the Penn State Extension Tree Fruit Production website under the tab for "Monitoring for Diseases". In addition, last year we developed a scouting video .

The number of primary scab spores being released from the overwintering leaves has drastically decreased over the last week. During the peak spore dispersal, our spore counts were averaging 15,000 - 20,000 spores (weeks of May 1 and 8); this week, we are averaging around 500 spores. Although we are on the downside of the number of primary spores being available, scab infection is still a concern particularly in apple blocks with a history while we experience warm, wet conditions. As a reminder: 2 inches of rain washes off fungicides. Growers will want to maintain vigilance until about mid-June: this is when we typically see the last remaining spores disperse from the overwintering leaves.

Fire Blight - Be On Alert

The weather has been pretty mild so far this season, but it's been warm enough for fire blight symptoms to start popping up in the region. Although the summer heat hasn't kicked off in full force yet, growers should not be complacent when scouting for fire blight in their orchards. If infection became established during bloom, the cool weather only slowed the bacteria from replicating to high numbers; it did not stop the disease. When warm weather is present, the bacteria will replicate in full force. Some helpful nuggets of fire blight management wisdom as we enter June:

For newly planted apples blocks

It is best to be vigilant and scout regularly, especially in newly planted blocks. Some items to remember:

  • Be sure newly planted blocks that are blooming are protected with streptomycin since our conditions are perfect for blossom blight infection.
  • Do not manually manipulate ("hairdress," pinching off blossoms, etc.) young trees during wet weather. Any kind of manipulation should be done in dry weather.
  • After any kind of manipulation or pruning in newly planted blocks, be sure to cover the trees with copper. Copper should protect any kind of wounds made and limit infection.

What to do if you see fire blight

To minimize spread in young plantings, I would recommend growers use Cueva 2 qt/A, especially if you see active infections. Last year, we observed a decrease in incidence with shoot blight when we used Cueva. Folks have asked about tank mixing Cueva and Double Nickel since it was reported a few years ago there was an added benefit of including Double Nickel. We have not been able to repeat those results; last year, we did not observe an increase in control when including Double Nickel with Cueva. Consequently, we are recommending growers just stick to Cueva. Depending on the disease pressure, growers may want to apply Cueva weekly, or every 10 days depending on conditions.

Copper sprays will only be needed while the shoots remain green and succulent (until terminal bud set). Once shoots harden off, they are no longer susceptible to fire blight. It is advised Cueva be applied as a separate spray, if possible, to avoid phytotoxicity when tank mixing with other products that may influence the pH of the tank mixture. Lower pH will make copper more phytotoxic. To date, we have observed minimal fruit russeting when using Cueva as a stand-alone spray. As far as the post-infection efficacy of other products, such as Regalia and Serenade--stay tuned: we are evaluating that question this summer.

The 4-1-1 for cutting out fire blight infections

During the 2014 fire blight epidemic, Dr. David Rosenberger at Cornell recommended a "triage" method when it comes to pruning decisions once fire blight has struck, going from highest to lowest priority:

  • Young orchards 3 - 8 years old with just a few a strikes. (highest priority)
  • Young orchards 3 - 8 years old with severe strikes.
  • Older orchards with a few strikes.
  • The "walk away" group: orchards with so many strikes that most of the tree would need to be removed; severe pruning can stimulate new growth that can become infected. (lowest priority)

Folks have been told all along to prune out fire blight during the season when they see it; however, there can be too much of a good thing: It is very important to avoid excessive cutting when pruning out fire blight. Excessive cutting will encourage more shoot growth and make your fire blight problem even worse. This is especially true for older orchards where cankers may be lurking about in the tree.

Additional items to keep in mind when pruning out fire blight infections

  • Do not cut out infections during wet weather since bacteria move via water.
  • Cut out active infections early - before necrosis develops (limits the spread of bacteria).
  • Pruning is most effective when incidence is low.
  • Focus on salvaging tree structure and young high density plantings when incidence is high.
  • Bacteria can invade healthy tissue up to ~3 feet in advance of visible symptoms, which makes tool sterilization not effective
  • Practice the ugly stub method: cut 6 -12 inches below the margin of visible infection and remove later during winter pruning.
  • Bacteria can live very well outside the plant and, to be certain you are getting rid of all sources of bacteria, it best to burn infected tissue that has been removed from the tree.

Powdery Mildew: Still be vigilant during dry periods

As folks know, 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. This is just a friendly reminder that folks still need to be actively managing the disease since trees are most susceptible from tight cluster until the shoots harden off. The powdery mildew fungus likes young tissue, which is why powdery mildew prone varieties should be protected through terminal bud set.

Bacterial Spot on Peach

Warm and humid weather are ideal disease conditions. Bactericide applications to manage bacterial spot on peach are needed on a 7-14 day interval throughout the summer. The shorter interval (weekly sprays) are needed when conditions are favorable, which is warm (75°F to 85°F) and wet/humid; 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).