The debate on the realities of global warming, climatic change, or the occurrence of warming cycles continues among scientists and politicians. These arguments are often emotionally charged and intense, but warmer temperatures would surely impact the planting of current fruit crops and the cultural methods used to produce them.
Some of the effects of warmer temperatures are speculative, but others have been experienced by growers in previous growing seasons with erratic weather patterns. For example, Missouri growers are already familiar with the effects of warm August nights. In grapes and apples, cool evening temperatures produce bright red skin color and elongated fruit. In fact, Red Delicious apples are a prime example of the effect of color night temperatures on fruit development. In contrast to Washington-grown Red Delicious, Missouri-grown Red Delicious apples have an overall reddish-brown skin color and a round fruit shape due to the warm night temperatures. On cloudless days, when sunlight is intense and temperatures are near 100 degrees, patches of sunburned tissue develop, resulting in unmarketable fruit. Shade cloth is currently being used to shield the apple and raspberry fruit from intense sunlight in the southern hemisphere. Evaporative cooling (overhead irrigation) is also a common practice used to reduce fruit temperatures in the Pacific Northwest. Also, a kaolin clay product (i.e., Surround) can be applied to crops to prevent sunburn injury on fruits, vegetables, and nuts. When applied to crops, the clay reflects the light and cools the surface of the produce.
Other fruit attributes are also adversely affect by warm temperatures. Juice extracted from grapes cultivars (e.g., Concord) ripened under warm weather conditions has high pH and low acidity which requires adjustment before consumption. For apples, warm temperatures preceding harvest results in soft-fleshed fruits (i.e. mushy texture), starchy flavors, and creates off-flavors in cider. Such was the case for the 2003 apple crop when temperatures were above 100 degrees for 6 days between August 18 to 27, with the maximum temperature of 105 degrees. Because this extremely warm period occurred a few days before harvest, most of the Missouri apples were unmarketable.
Other effects of global warming on fruit production are predictable. For example, production of some North American cool-season crops such as raspberry will shift further north where there are cooler temperatures. Also, crops that favor warm climates such as blackberry may be grown in more northern locations. Field grown crops, such as day-neutral strawberries may be planted earlier or the harvest season extended later without the use of row cover materials or high tunnels that provide low temperatures protection to plants.
The planting of different cultivars of existing crops can also be expected under prolonged periods of warm winter temperatures. Fruit and nut crops require a specific number of hours of cool temperatures (usually in the range of 32 to 54 degrees) during the winter months to produce flowers and fruit. This is called the chilling requirement. When the required number of chilling hours has been accumulated, the chilling requirement is satisfied, and flowers are produced. Some cultivars require relatively few hours (less than 500 hours) of chilling to complete rest (i.e., low-chill cultivars), while others need as many as 1500 hours of chilling (high-chill cultivars). Currently, low-chill apple cultivars are grown in the southern regions of the U.S., while states like Missouri grow high-chill cultivars. If significant global warming occurs or winter temperatures increase over prolonged periods, currently cultivated fruit cultivars may not receive enough hours of chilling, resulting in complete crop loss.
Heat tolerance during the summer months could also become a much higher priority in the selection of plant material than at present. Plant tissues are injured at high temperatures, including disfiguring or discoloring the fruit tissue. Also, critical plant process cease at very high temperatures (such as photosynthesis). Other plant processes, such as transpiration increase as temperatures rise, accelerating moisture loss and increasing irrigation requirements. Respiration also increases which hastens maturity and shortens the life of the produce.
With higher temperatures during the growing season, there may also be a shift in the types of diseases that commonly infect Missouri-grown fruit. For example, outbreaks of anthracnose (caused by Colletotrichum fragariae) occur occasionally in Missouri, but this disease is frequently observed on strawberries grown in the southeastern U.S. from Florida and North Carolina to Louisiana, Oklahoma, and Tennessee. Infection of crowns, stolons, and leaves is enhanced under humid conditions and temperatures above 75 degrees . Another particularly destructive disease commonly found in hot, humid regions of the southern U.S. is blueberry stem dieback (Botryosphaeria dothidea). Growth of this fungus is favored by temperatures in the range of 83 to 95 degrees. Thus, this disease may become more prevalent on blueberry plants if warmer temperatures occur in Missouri.
If very warm temperatures impact Missouri crop production, growers will modify their current practices. The selection of heat tolerant crop germplasm will become a greater priority. Additionally, new strategies for crop protection will evolve and new technologies will be developed to allow continued crop production.
Documented historical temperature trends for many locations throughout Missouri began in 1890 with the establishment of the National Weather Service Cooperative Observer Program. The premise of the Cooperative Observer Program was to define a climate for the United States as well as to detect climatic trends for the region. The following figures display annual, winter and summer temperature trends over the past 113 years for Missouri (Figures 1-3).
Figure 1 indicates the recent warming trend Missouri has experienced over the past 10 years (9 out the past 10 years have been above normal).
When the year is broken down seasonally, winters in Missouri (Figure 2) reveal the strongest warming signal for the Show-Me state with 14 out of the past 20 winters averaging above normal. In fact, 4 out of the 5 warmest winters on record have occurred since 1990.
Alternatively, the recent summer temperature signals have been weaker with no strong warming or cooling bias. Interestingly, Missouri has not witnessed an exceptionally hot summer in nearly 30 years. Not since 1980, have we seen more than a dozen days reach or exceed the century mark in a single summer season.
REVISED: September 30, 2015