By Kathryn Carter, Tender Fruit and Grape Specialist

Peaches, plums, pears and cherries are bloom in many areas in the province.  Overnight temperatures for the past two days ranged from 0° to -4.4° C in Niagara, Norfolk and Harrow, and frost injury is starting to show up in some orchards.   

Susceptibility to Frost

The critical temperature (or killing temperature) is defined as the temperature that buds can withstand for a half-hour before permanent damage occurs. Generally, there is a range of temperatures over which damage occurs with increasing number of buds and flowers damaged at lower temperatures until all the fruit buds are killed. As a general rule, temperatures of -2.22° C at the pre-bloom to bloom stage, will result in 10% bud loss to fruit trees. While temperatures of -4.44 °C will result in 90% bud loss. Cold events are cumulative, so multiple freeze events that cause low mortality to buds, can add up over time.  

Other factors affecting frost injury

There are many different factors that also impact the severity of frost injury. The level of frost injury is directly related to the cold intensity (colder temperatures=more damage up to the critical temperature that kills all buds) and the duration of the event (cold events that last hours=greater injury). Stage of development, also impacts the susceptibility to frost injury, and variation in fruit bud maturity on the tree can result in some fruit buds getting killed, while buds that are less developed remain undamaged. During radiation frosts temperatures near the soil surface will be colder then temperatures high up in the canopies, resulting in more frost injury at the bottom of the tree then the top.  Within a flower, the pistil (female) is the most sensitive to cold temperatures, followed by stamens (male), then petals.

How much fruit set do you need?

It isn’t necessary to have all the fruit buds to set fruit in order to have a full crop, as a result, orchards that have a small amount of cold injury can still have enough fruit buds for a full crop. The number of fruit buds needed for a crop varies depending on the commodity: a full crop of cherries requires over 50% bud survival, while pears, apricots, plums and peaches only need 10% bud survival in order to have a full crop.

In sites with significant cold injury, lack of crop may result in the need to amend production practices (thinning, fertilizer etc.).  It can be easy to over-estimate the impact of frost injury and under-estimate the amount of crop. Make sure to evaluate the extent of the damage in the orchards prior to making decisions about production practices.

Assessing frost injury

It takes several hours for symptoms of frost damage to develop. Discolouration of frost damaged tissues intensifies with time, so it can take several hours for the symptoms of damage to be apparent.

It is best to evaluate damage by collecting shoots with flower buds and bring them indoors to warm up. Looking at buds in the field can give you a “feel” for the damage, but it is much more objective and accurate to determine exact levels of damage inside, with a good light and magnification if necessary.

Collecting 100 buds for evaluation will provide a good assessment. Collect 10 shoots (each with approximately 10 buds) from differing heights in the tree (high and low) and different locations within the orchard or orchard block. Separate cultivars (and locations/height within the tree) by bundling with flagging tape and a label.

Bring the samples back from the orchard and place the base of the shoots into a bucket (or a can) with water. Allow them to warm up to 21° C for a minimum of four hours, which will allow the damaged tissue to develop the characteristic brown-dark or brown-black color due to oxidation of phenolic compounds released by the injury.

Then cut the buds or blossoms on a vertically, using a very sharp blade.  Stone fruit flowers (peaches, nectarines, cherries and plums) have a single pistil (the female part of the flower). If the pistil is brown or black (Figure 1) after the freeze the fruit will not develop. A green pistil indicates it is healthy and the fruit is alive. (Figure 2).  Buds killed within the last day or two will have pistils that are the same size or only slightly smaller than live pistils, over time the pistil shrinks in size and darker in colour. Peach and nectarine flowers have fuzz on the pistil, which can make it more challenging to see the injury. When frost injury occurs post bloom (shuck stage) and the fruit is small, there is slightly more protection from the cold provided the fruit does not touch the sides of the floral cup. When the fruit fills the shuck, the shuck stops providing protection.

Figure 1 a) Sweet cherry flower killed by frost at bloom (note black pistil). B) Healthy sweet cherry flower (green pistil)

Pears

Darkening of the pistil is also an indication of frost injury in pears. The pistils of pears are buried inside the base of the flower, meaning it is necessary to tear the flower apart (cross section vertically) to see if the centre of the flower is brown or black. Blossoms within a pear flower cluster often vary in their stage of development.  When checking pears for frost injury, check individual blooms separately, as some flowers may have damage, while others remain undamaged.

If evidence of frost injury is present, then contact crop insurance immediately.