Noninfectious Disorders
FIELD GUIDE TO INSECTS AND DISEASES OF ARIZONA AND NEW MEXICO FORESTS
Frost
Figure 284. Figure 284. Aspen leaves succumb to June frost.

Frost damage of many tree species including aspen, locust, spruce, and oak has been observed throughout the Southwest. The tolerance of foliage and small branches to cold temperature depends on their “hardiness,” which is closely correlated with the time of year and species. Temperatures just above freezing and short photoperiods induce frost hardiness. The sudden onset of below-freezing temperatures (frosts) in spring or early autumn can severely damage unhardened tissues, killing young shoots and leaves or needles.

Figure 285. Figure 285. Newest and most sensitive growth of fir killed by frost.

As temperatures rise in late winter and early spring, plants deacclimate until, by the time growth begins, they can no longer tolerate more than a few degrees of frost. Leaves and stems killed by spring frost are usually small and succulent at the time of injury. More mature leaves and stems can tolerate lower temperatures. After thawing, damaged tissues at first appear water soaked and soon become shriveled and reddish brown, dark brown, or black, depending on the species. Dead leaves and shoots break off or abscise during the next several weeks. New shoots and leaves begin to grow almost immediately and soon mask the early season damage.

References:  83, 93