Memorial Day Apple Disease Outlook

Despite the cold bloom at petal fall, NY will be having a pre-Memorial Day heatwave. The last few apple seasons have been characterized by a cool bloom and hot post-bloom / thinning period. We’ve just come down from a heatwave and temperatures will be cool with highs in low 70s soon during the week.

Fire Blight: I’m already seeing the first blighted clusters and ooze droplets on untreated trees in Geneva. Trees managed for fire blight are not showing symptoms. Flower/fruitlet populations on untreated trees were low all bloom but have been quickly rising over the weekend. If you still have any bloom remaining, protect your crop early this week and consider making application of Prohexadione CA to slow any shoot blight that might be a result of early-season systemic infections. Finish strong this season and hopefully you’ll not get fire blight this year with these Prohexadione CA use guidelines:

• In the two weeks following bloom, scout for, and prune out, fire blight strikes promptly. Destroy pruned strikes by burning or leaving them out to dry. It is best to prune well back into healthy wood, at least 12 inches behind the water-soaked margin. Take care as summer pruning may stimulate active shoot growth leading to new susceptible tissues that could later become infected. If fire blight reaches the central leader, the tree should be removed. An effective removal practice is quickly cut the tree off below the graft union hang on the wire and dry out until you have time to remove it. Dead trees will not continue to ooze, but if trees are oozing, they should be promptly removed to avoid inoculum spread. Fortunately, the location may be safely replanted in later years.
• Post-bloom applications of prohexadione-calcium (Apogee or Kudos) for shoot blight management should be considered, especially on vigorous blocks of highly-susceptible apple varieties during shoot elongation which begins in late bloom.
  • For maximum effectiveness, prohexadione-calcium (Kudos, Apogee, etc.) should be applied at 6-12 oz/100 gal (3-6 oz/100 gal for tree <5 years) when trees have 1-2” of shoot growth. A second application should be made 14-21 days later.
• Preventative applications of copper can be used post-bloom and during the summer to protect against the spread of shoot blight infections. Copper must be applied before infection occurs as it will only reduce bacteria on the surface of tissues. It will have no effect on existing shoot blight infections and may cause fruit russet in young developing fruit. Apply with adequate drying time and use hydrated lime to reduce the potential of phytotoxicity from copper. Remember terminal shoots can outgrow protective residues of copper. A low-rate fixed copper program consists of applications on a 7–10-day schedule during high-risk weather until terminal bud set.
• It may be possible to save plantings on resistant rootstocks that have a moderate amount of shoot blight. Apply prohexadione-calcium (Kudos, Apogee, etc.) at the highest rate for the planting (6-12 oz/100 gal) and allow 5 days for the product to take effect. Afterwards, prune out existing and newly developing shoot blight every two weeks for the rest of the season, but remove any trees where fire blight has reached the central leader. If pruning stimulates additional shoot growth, a second application of prohexadione-calcium could be warranted.
• If you need to interplant apple trees in existing orchards where trees were killed by fire blight and removed, replant these missing trees ‘skips’ in late fall to better synchronize next season’s bloom with established trees.

Apple Scab: The heatwave is good for keeping apple scab down. The pathogen’s ability to grow and infect apples is favored by cool wet weather like we had earlier in the season. More than five days of temperatures greater than 85F can kill of sporulating lesions resulting from ascospore infections. The hot weather from the end of the week should give you management edge and allow you to leave captan out of applications during the thinning period and use other single-site fungicides for disease management.

Powdery Mildew: Like fire blight, powdery mildew populations can surge in this warm dry weather. In unmanaged ‘Idared’ orchards in Geneva, powdery mildew seems to be so heavy that it is blighting shoots and clusters. The early season rains likely provided the moisture for the populations that are now exploding in this warm dry weather. Captan and Mancozeb have little effect on Powdery mildew and can be left out of applications made during the thinning period. The best fungicides chemistries for managing powdery mildew include the DMIs and the QoIs, which are safe for use during the thinning period.

Prolonged Wet Weather and Captan.

For most of the spring, we’ve had prolonged wet weather, and while we are finally getting a break from the continual rain; it’s fairly cool this week with thunderstorms and rain and trees have young developing fruitlets. For another week or two, let’s take caution with captan. Tank-mixing captan with foliar nutrients or spray adjuvants that enhance penetration (such as Regulaid) may severely damage fruit finish and cause spotting and necrosis of the foliage. The potential for fruit finish and foliar phytotoxicity increases when applications are made under slow drying conditions (e.g. high relative humidity, light rain, etc.). Given the potential for fruit finish and foliar damage, the prevalence of adjuvants in tank mixes, and the susceptibility of young developing tissues, captan should not be applied from petal fall to 2nd cover. If you need to apply a protectant during this type of weather for resistance management, use Mancozeb if you are still in the PHI window for your variety. Suflur is another option for a protectant and will be helpful for apple powdery mildew. It’s not likely to be a problem in cooler weather but could still be problematic in complex tank mixes as it can cause injury. Will you create a resistance problem if you make 1-2 applications of a single fungicide alone? It’s highly unlikely, and it would be better than making a potentially damaging tank mix application during thinning and early fruit development. Make sure to not exceed allowed single site product applications in one season.