When we make a diagnosis of freeze injury in July, it causes more than a few heads to turn. The fact of the matter is that in July we are still seeing the aftermath of freeze injury that occurred in March and April or possibly earlier. Admittedly, this past spring was an odd one in the Midwest. Temperatures were summerlike in March, long enough for plants to start their spring growth. Then right about the first week of April, there was a sudden drop to near-freezing temperatures. Cold injury resulted.
Much of this injury was immediately visible (acute injury) as dead stem tips, brown buds, loss of fruit crop, and water soaked foliage. As growth resumed, many plants began to develop new foliage, filling in the dead area. In some cases, however, sections of plants did not recover. If buds survived the freeze, they opened based on stored foods; but something kept water from reaching those leaves. The leaves dried up after they emerged. Cambium and sapwood injury prevented water flow. So what has caused this injury?
Plants deacclimate from winter hardiness slowly as spring temperatures rise. When a late frost occurs, as it did in early April this year, plant tissues are injured because they are no longer winter hardy. This freeze injury is not a hardiness issue. Affected plants are certainly winter hardy. It may be that affected varieties start to deacclimate sooner and are thus more susceptible when low temperatures follow extended warm spells. According to Sinclair and Lyon, in Diseases of Trees and Shrubs, “sapwood, cambium, and phloem tissues deacclimate to different degrees.” The sapwood is least tolerant of cold, so it freezes first.
The most dramatic cases this year seem to be with evergreens. On one Plant Clinic arborvitae sample, the splits in the wood were severe, blowing open the stems to white wood, showing like exposed bone. An identical case was sent to our lab via e-mail from Iowa asking whether we had seen such injury. Obviously, the foliage above these areas can no longer receive water from the roots, resulting in large areas of brown foliage. In another case on juniper, foliage was brown and Phomopsis tip blight detected. A closer look revealed small fissures in the wood. When the bark was removed, large splits and cankers were revealed, probably the result of earlier frost or freeze injury. In both cases, the client was concerned about plant disease. It is true that canker fungi invade injured tissue. However, in both these cases, there was no fungal pathogen involved. Given time, fungi invade these wounds, but the primary problem is abiotic (noninfectious).