Frost damage evaluation on apple buds

Last week, in the early mornings of May 8th to 10th, there were frost incidences in many areas of Ontario's fruit growing areas. Most of these areas did stay above the critical temperature of -2.2 but the coldest areas had temperatures drop down to -2 to -3 which could cause some damage (Table 1). At this time before thinning applications begin, it would be beneficial to know what percentage of fruit buds are alive. For a full crop of apples, only 10% of buds that are distributed evenly in the canopy are needed for a full crop of apples.

Table 1: Critical Spring Temperatures in °C for Apples
  Silver Tip Green Tip ½ inch Green Tight Cluster First Pink Full Pink First Bloom Full Bloom Post Bloom
10% kill
-9.4
-7.8
-5.0
-2.8
-2.2
-2.2
-2.2
-2.2
-2.2
90% kill
-16.7
-12.2
-9.4
-6.1
-4.4
-3.9
-3.9
-3.9
-3.9

To achieve a good estimation of bud survival, collect enough branches to bring in 100 buds. Collect these buds from five representative trees from different areas of the orchard and from different heights in the canopy. Label the branches with tags or flagging tape by variety and location of where you collected the branches. Taking the branches inside and place them in a bucket of water (Figure 1).

Figure 1. Branches collected form an orchard and placed in a bucket of water inside a heated building to force buds to bloom.

Figure 1. Branches collected form an orchard and placed in a bucket of water inside a heated building to force buds to bloom.

Cut open the buds with a sharp blade by making sequential horizontal cuts, starting from the tip of the bud progressing down to the base of the bud (Figure 2). After each cut examine the pistols for brown and black tissue which is an indication of a dead bud. After being cut for ten to 15 minutes, bud tissue will start to brown from being exposed to the air. If buds are farther along in development than tight cluster, it may be easier to examine the pistol by making a longitudinal cut (Figure 3). Figure 4 shows a bud at early pink cut longitudinally with a dead pistol.

Record if damaged buds were king buds or side buds. If a high percentage of king buds are damaged this could make thinning more difficult as the size difference in king fruitlets and side fruitlets helps to kill the side fruitlets but let the king fruitlet remain.

Figure 2. Apple buds cut open horizontally, revealing healthy buds.

Figure 2. Apple buds cut open horizontally, revealing healthy buds.

Figure 3. A bud cut open longitudinally revealing a healthy pistol.

Figure 3. A bud cut open longitudinally revealing a healthy pistol.

Figure 4. A bud cut open longitudinally revealing a dead pistol.

Figure 4. A bud cut open longitudinally revealing a dead pistol.

Sources:


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