Cold temperatures, foliar nutrients, and scab

The good thing is that insect and fire blight activity remain pretty low. On the other hand, we have potential for an extended scab event starting today and lasting through the weekend. This is a tough one to plan around. Ascospore maturity is creeping up, and significant wetting, especially today, can discharge a lot of spores. Hours required for infection to occur are likely pretty high, given the cold weather, but it does look like an infection will happen. Orchards should be covered with a fungicide, and a material with kick-back activity applied when the winds die down (Monday?) if there is any question of coverage.

While spraying, any orchards between tight cluster and pink should receive the prebloom nutrient cocktail that will help buds to recover from cold and enhance fertilization during bloom. Zinc and boron are important minerals which will help trees recover from cold damage, and nitrogen is needed at this time of year when rapid growth taxes tree reserves. Refer to Mary Concklin’s quick summary here for more information:

Speaking of cold, many of us are concerned about the cold that’s coming this weekend. Remember that fruit buds, even at bloom, are hardy to about 28°F, and more closed buds are even hardier. This is expected to be an extended cold period, though, with high wind that could dessicate buds (although a coating of snow ought to keep some moisture in). I think we’ll be on the safer side of the tightrope, given the expected temperatures I’m seeing (30F in Champlain valley and Southern Vermont, 27 inland where buds are more closed, give or take). I’ll talk next week about how to check buds for damage after the fact.

Plan on draining water lines and sprayers today. Better safe than sorry.

Finally, I’ll share some advice I received from Jim Wargo @ Valent USA regarding plant growth regulator use to increase fruit set on apples that have suffered from cold events:

PROMALIN Plant Growth Regulator is well-known as a tool to improve fruit set after a frost. The active ingredients in PROMALIN replaces the hormonal signal that is lost when developing seeds are killed or damaged by low temperatures during bloom, allowing the fruit to grow with reduced or no seeds, and often to normal size. Promalin does not repair or heal damaged flower parts. It simply replaces the hormonal signal that normally comes from the developing ovules (seeds) when pollination and ovule fertilization has been accomplished. But what about cold injury that occurs prior to bloom – at tight cluster to early pink? Anecdotal observations suggest applying Promalin within 24 hours of frost events during these earlier growth stages is less effective or not effective at all. My guess is there are two possible reasons wh. 1) Applying Promalin (a hormonal signal) at a time which is out of step for the normal signaling process that begins after pollination (bloom – petal fall) is not effective at “tricking” the tree. And 2) Promalin applied prior to bloom gets metabolized (broken down) by the plant cells and is no longer present when needed at the post pollination/fertilization time. For perspective, the use pattern of another PGR, ProVide, comes to mind. ProVide (GA4+7) is basically Promalin without the 6BA and is used to reduce skin russeting and scarf skin disorders on apples. Weekly ProVide applications starting at petal fall are needed to sustain sufficient levels of GA4+7 in the plant over the time period when fruitlets are most susceptible to russet. The applied GA’s get metabolized over time which is why multiple applications of low rates work better than a single application of a high rate e.g. 12 oz/A at petal fall. One application of GA4+7 just doesn’t stick around long enough to have the same effect as multiple lower rate applications do.

One possible work around for flowers that are damaged prior to bloom is to delay the application of Promalin until full bloom. In other words, do not apply it within 24 hours of the frost event that occurs at tight cluster – early pink. Wait until the trees come in to full bloom, apply a pint of Promalin at that time and then another pint at petal fall. The two applications at bloom and petal fall are within label guidelines. This approach is theoretical, but if we are trying to trick the trees, we need to have the hormones available and in sufficient quantity at the time the tree is deciding what to do with that fruitlet – hang it on or let it go. The tree is not making that decision at pink or prior to that – it’s too early. My recommendation for a 2nd pint at petal fall is to prolong the level of hormones in the plant to further enhance fruit set. As a disclaimer, if flower clusters are damaged beyond repair, no PGR treatment will work. In order to have any effect, injury must be restricted to the reproductive parts of the flower e.g. stigma, style, anther, ovary. If receptacle tissue is damaged, then all bets are off.

ReTain Plant Growth Regulator – How ReTain works to enhance fruit set is completely different than Promalin, and the two should not be used interchangeably. ReTain inhibits ethylene and in turn extends the effective pollination period – defined as the number of days during which pollination is effective in producing a fruit. It is determined by the longevity of the ovules minus the time lag between pollination and fertilization. ReTain will do nothing for flowers that have been damaged by cold injury. In addition to benefitting varieties that are inherently predisposed to poor set e.g. SweeTango, Evercrisp, ReTain may have benefits when pollination conditions are less than ideal – in particular, challenging weather conditions (cool temperatures, strong winds and rain during the bloom period). Honeybee activity is limited during rains and heavy winds and when temperatures are below 65F. This is where ReTain can help, by extending ovule longevity, it broadens the window of opportunity for pollination/fertilization of the ovules by 3 – 5 days. The idea is to get past the challenging weather conditions and still have flowers that are capable of setting fruit.

Recommendations for using ReTain for enhancing fruit set:

  • For varieties that naturally have a long bloom period or in years with poor pollination weather during bloom apply ½ pouch of ReTain when 25 – 30% of king flowers are open and ½ pouch at 75% king bloom. Do not use a surfactant
  • For varieties that have relatively shorter bloom period or in years with more ideal pollination weather apply 1 full pouch of ReTain when 20% of the king bloom is open. Do not use a surfactant

Here is a real-life example of how I decided to make a decision regarding PGR use in my own orchard. I experienced bud damage on my earliest varieties (Empire, McIntosh) from frost events at tight cluster. I was preparing to spray Promalin at bloom, but when the trees started entering king bloom on 5/5, I noticed the damage was not nearly as severe as I had thought. However, the forecast for the next ten days here in CT does not bode well (high/low temps) 60/42, 54/34/, 44/35, 51/42, 50/38, 51/36, 54/41, 56/48, 63/55. The only day that looks to have any decent potential for bee activity is Friday May 15th! I’ve decided to apply ReTain instead of Promalin during bloom to extend flower viability in hopes it warms up to >65F eventually. Mother nature has the final say, all we can do is wait it out. ReTain gives us the ability to broaden the pollination window and play the waiting game…

Along with the PGR applications, I would strongly suggest growers utilize the Warren Stiles foliar nutrition program for enhancing fruit set – urea (3 lb./100 gallons dilute) and boron (0.1 -0.3 lb. B/A) at tight cluster to pink. Followed up by another shot of urea (5 lb/100 gal) and Boron (0.1-0.3 lb B/100 gallons) at petal fall. The Boron will help with pollen germination and pollen tube growth. The urea provides N which is a precursor to polyamines. Polyamines have been shown to extend the effective pollination period by increasing ovule longevity. Zinc chelates may also be applied prebloom – up through tight cluster stage.



James Wargo

Territory Manager, Northeast

Valent USA



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