   Winter Freeze Damage - August 22, 2007 Jeff Schalau, Associate Agent, Agriculture & Natural Resources, Arizona Cooperative Extension, Yavapai County While many of us have max/min thermometers and read daily high and low temperatures, we seldom record them. In Arizona, we are much more likely to record precipitation data than daily temperature data. Daily precipitation data is relatively easy to access for many areas via the Rainlog.org cooperative precipitation monitoring program. Daily temperature records are not as easy to access, especially when trying to reconstruct plant-damaging conditions several months after the fact. You may ask yourself, who cares? Well, in late spring and early summer, the Extension offices began receiving calls describing plant damage symptoms that could have been caused by infectious disease. However, many of these symptoms were on plants that are usually disease-free. There was tip damage leaf burn on an array of hardy evergreens and lots of leaf damage to plants like oleander, wax-leaf privet, non-native cactus and succulents. If it were an infectious disease, brought on by the warmer growing season temperatures, the symptoms would have progressed and some plants probably would have died. At the time, we were not able to explain the cause of the symptoms and many damaged plants subsequently recovered. After pondering these disease-like symptoms and discussing them with plant scientists and Master Gardeners, we decided it must have been cold injury. During December 2006 and January 2007, we experienced average winter max/min temperatures for our area. Then, between January 14 and 17, temperatures dropped by 15 to 20 degrees for a three to five day period. The temperature records were not easy to find, but I was able access daily temperature data for Prescott and Flagstaff through the Arizona Meteorological Network (AZMET). This sudden dip in temperature was the likely culprit for many of the plant disease symptoms that appear during spring 2007. The ability of plants to withstand freezing temperatures is affected by diurnal temperature fluctuations and day lengths prior to a freeze. A gradual decrease in temperature over a period of time increases the ability of plants or plant parts to withstand cold temperatures. A sudden decrease in temperature usually results in more damage than if the same low temperature was to slowly develop over a period of weeks. Furthermore, short durations of warmer temperatures in midwinter can deacclimate some plants that are more prone to freeze injury. Freeze damage symptoms may not be evident until several months later when temperatures warm and growth initiates. Here are some examples of symptoms observed in late spring of 2007. Deodar cedar trees had dead needles on the outer edges of new buds but also had live green needles deeper in the center of the buds. Leyland cypresses had yellowing foliage in patches on the outer branches of the tree. Non-native prickly pear cactus and yuccas also had dead or damaged patches on leaves. To complicate matters, oleanders are frost sensitive and had damage symptoms, but we also heard about the introduction of oleander leaf scorch (a bacterial disease) in the media this year and people were concerned. It can be difficult to diagnose these symptoms, especially when they are observed months after the damage may have occurred. If symptoms progress, it is likely a disease organism causing the damage. When symptoms appear and remain static, then a non-living or abiotic cause is likely. Once insects, herbicides, other chemicals (deicers, fertilizers, etc.), and mechanical damage are ruled out, then you are left with weather. Extreme heat or cold as well as wind can damage plants and the symptoms can appear similar. Wind damage also causes tattered and holey leaves. To make matters even worse, most of the time, a given set of symptoms can have two or more causes making diagnosis even more difficult. Quite often, the diagnosis that I can provide for a client is tentative with multiple possibilities. Subsequent observations often narrow the field, but diagnosis of plant disease and environmental injury is seldom cut and dried. The University of Arizona Cooperative Extension has publications and information on gardening and pest control. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 ext. 14 or E-mail us at cottonwoodmg@yahoo.com and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site: http://cals.arizona.edu/yavapai/anr/hort/byg/. |