Frost Damage in Almonds

Several calls were received today in regards to the symptoms of frost damage. Temperatures in several areas of California were cold enough to cause damage. Many growers have implemented frost protection strategies, but surveying the orchard still needs to occur in order to determine the amount of damage.

Damaged blossoms/almonds will be apparent by dead or darkened tissue. Symptoms will become apparent within 72 hours of the frost event, usually appearing 24-48 hours after the event. When viewing the flowers, looked for water soaked areas that might not have yet turned brown. Symptoms will range, but any damage done to the female parts of the flower will most likely lead to flower abortion/loss. Check all parts of the tree, both at eye level and in the upper canopy in order to determine the extent of crop damage.

Since frost damage is an abiotic disorder triggered by the environment, keep in mind the following patterns to help discriminate it from other bloom time issues (i.e.Brown Rot):
1). Damage is present over a large area of the orchard,
2). Damage is present across varieties - especially if both were in the same bloom stage. This may not be true if one variety is yet to bloom and was in the pink tip stage during the event,
3). Damage is associated with known topographical changes, i.e. a low spot,
4) Tissue of the blossom is darkened, with no obvious signs of fungal growth.

The tree's sensitivity to frost changes with the various bloom stages. The tree is most tolerant to frost when it is dormant or in pink tip. As the tree begins to bloom, it becomes more sensitive to frosts, with the fertilized flowers or small nutlets being the most susceptible. With fertilized embryos, the jacket may protect the ovary/embryo from damage.

Here are some pictures of frost damage:

Figure 1: Almond blossoms killed by cold temperatures. Please note the brown tissue on the inside of the blossom. This dead tissue is the ovary, or the tissues that house the almond embryo when fertilized. These blossoms will not yield a viable nut. Photo taken by Joe Connell, UC Farm Advisor, Butte County.

Figure 2: Frost killed blossoms. Note the darkened tissues of the jacket, stamen, and pistil. The flower may dry in the sum and appear crispy to the touch. This "crispiness" may take a few days to develop. Damage to any tissues of the flower will likely lead to flower abscission - even if the ovary or ovule was not affected.

Figure 3: Remnants of blossoms after the frost. Note the darkening of the pistil (female part of the flower). Most likely, the embryo was killed when this blossom with the exposure.