Guess The Pest! Week 5 Answer: Radiation Frost

David Owens, Extension Entomologist,

Mr. John Hockmuth logged a good guess for last week’s condition as anthracnose. There is an anthracnose that affects sycamore trees, especially in cool, wet years. Anthracnose can keep sycamores from leafing out and giving the tree a gnarly appearance. Upper canopy twigs seem to be less affected. You can read more about this disease here: However, the other condition that is a more immediate concern to the foreground blueberry bushes is a late freeze.

This from Dr. Emmalea Ernest:
It might be a stretch to call freezing weather a pest, but it certainly can wreak havoc when fruit crops are flowering in the spring. The site pictured experienced a low of 25°F in the early morning hours of April 23, 2021. The freeze damage to the bottom, but not the top of the London planetree (a species with frost sensitive leaves) shows the effect of a radiation frost event; these tend to happen on clear, calm, low humidity nights. Radiation frosts create a temperature inversion with colder air temperatures near ground level and warmer temperatures in a layer 40-70 ft off the ground. Based on the height of the freeze damage to the London planetree it looks like this inversion layer was about 40 ft from ground level.

When radiation frosts are predicted, tree fruit growers can use helicopters or wind machines to mix warmer air from the inversion layer with the cold air near the ground to keep the air around the trees above critical temperatures for damage. Smaller plants, like strawberries, can be protected from radiation frosts with row covers.

Guess the pest image

The other type of frost/freeze event, an advective frost, occurs when a mass of cold air moves into a region accompanied by wind. It is much more difficult to protect tree fruit from an advective frost. Low growing crops can sometimes be protected from advective frosts with row covers. Frost protection techniques and critical temperatures for various crops have been covered in several past WCU articles linked below, but fingers crossed for no more freezes this spring!

Freezes, Frost and Frost/Freeze Protection
Cold Damage in Fruits
False Spring Concerns – Again
Fruits and Freeze Damage During Flowering

As a side note, the April 23 freeze did not cause discernable damage to the blueberries but did significant damage to nearby peaches and Asian pears.