Another Close Call

Posted: April 21, 2014

I received a phone call from Dr. Greg Peck at Virginia Tech’s lab in Winchester, VA following last week’s freezing temperatures. Greg said that he was seeing a surprising amount of freeze injury to apple flower pistils. I mentioned that our minimum temperatures hadn’t dropped below the critical values for apple buds between half inch green and tight cluster, (23° F and 27°F, respectively).
Critical temperatures for various stages of bloom.

Critical temperatures for various stages of bloom.

Greg suggested that Mid-Atlantic apple buds may not adhere to the critical temperatures established in 1964 to 1970 in Washington State. We inspected some flower clusters at the Penn State Fruit Research and Extension Center (FREC), and after finding some damage, we decided to assess the situation more thoroughly. 

Jim Schupp, Tom Kon and Melanie Schupp, Penn State Fruit Research and Extension Center

We collected entire branches of several varieties, measured them with an Equillifruit disk to determine the number of fruit needed to set a full crop, dissected the blossoms, and counted the number of king and side bloom with dead pistils. Table 1 shows that there was more flower mortality than expected, but there are still many times more viable blossoms than needed to set an optimal crop. 

There were a couple of surprises in the data. Flower mortality was higher in Buckeye Gala than in Crimson Gala (35% and 7%, respectively). Yorks were at half inch green, considerably behind other varieties in bud development, and therefore considered more tolerant to freezing when compared to varieties with more advanced flower bud development. York flower mortality was similar to Golden Delicious and Fuji, and far worse than Crimson Gala, which was the variety with the most advanced bud stage. Flower buds of Honeycrisp were among the least damaged.

Our initial inspection of flower bud mortality was done in 4 rows of Golden Delicious with low blossom density (Table 2). Blossom mortality was very high in these trees, and had us very nervous at first about the prospects for a crop. The trees in these rows had been used for chemical thinning studies in 2013, and because of the experiments, many of these trees were over-cropped last fall. Flower bud mortality was much higher here than in adjacent rows of Golden Delicious with a moderate 2013 crop. We surmise that apple blossom mortality may be greater in trees that went into winter under stress from being over-cropped in 2013. The trees with extremely poor return bloom (1-3 blossom clusters per tree) had 87% flower mortality, while the trees with a more representative low return bloom had 58% flower mortality. Compare these values with 30% -- that of moderately-cropped trees of Golden Delicious from the same block, as shown in Table 1.
Several factors may determine the severity of blossom mortality observed, including: site, bud development stage at time of freeze event, lowest temperature observed, the duration of low temperature, cultivar, cropping history, and tree health. It may be worthwhile to assess blossom mortality in your own apple orchards.

Peach and sweet cherry flowers were also evaluated. Peach flowers seem to have come through with minimal losses. Cherry flower mortality is severe and variety dependent. We hope to have a more thorough report on cherry flower mortality later this week.

Table 1. Assessment of blossom mortality and cropping potential of six apple cultivars at PSU's Fruit Research and Extension Center in Biglerville, PA on 4.19.2014.

Cultivar/Strain Dead king blossoms
Dead side blossoms
Total dead
No. of viable blossoms/
No. fruit for full crop /limb Percent of full crop
Buckeye Gala 42% 35% 36% 281 27 1041%
Crimson Gala 7% 7% 7% 271 28 968%
Honeycrisp 5% 12% 10% 423 35 1209%
Golden Delicious 24% 32% 30% 247 29 852%
Brak Fuji 20% 22% 22% 280 47 596%
York 19% 30% 27% 314 33 952%

Table 2. Assessment of blossom mortality of 'Golden Delicious' with poor return bloom at PSU's Fruit Research and Extension Center in Biglerville, PA. 4.19.2014.

of trees
Dead king blossoms
Dead side blossoms
Total percent dead
No. of viable blossoms
Low 59% 57% 58% 78
Extremely Low 70% 87% 84% 16

Contact Information

James Schupp
  • Professor of Pomology
Phone: 717-677-6116