In the southeastern United States the potential for a drought during the growing season is a very real probability. The length and severity of droughts vary greatly and cannot be predicted, so planning is critical in order to minimize the effects of a drought. However, the potential for a drought is such that current recommendations for fruit orchards include irrigation as an integral part of fruit production and not as an option. With perennial tree fruit crops it is best to take a proactive position rather than waiting for a drought before taking action. Many orchards are poorly located where water is not readily available. Also, in mature orchards, where fruit trees are relatively deep rooted, installation of an irrigation system during a drought period is impractical and usually not as effective.
The effect of a drought on tree fruit production can result in reduced yields for several years, assuming that the trees survive the drought. With fruit trees, the flower buds for the following season are initiated during the previous summer, and prolonged drought can inhibit the initiation of flowers for the next season. A drought can also reduce the quantity and quality of the current season's fruit crop as well.
To better understand the effect of drought on fruit production it is necessary to understand the fruit's seasonal moisture requirements and to understand how drought affects fruit growth and development. Fruit trees have different patterns of shoot and fruit growth, so drought during different times of the season will result in varying responses. Growth of all plant organs is dependent on an adequate supply of carbohydrates. Carbohydrates, the main food source in trees, are produced in the leaves during photosynthesis. Photosynthesis requires sunlight, carbon dioxide, and water. Stomates, small openings in the underside of leaves, allow carbon dioxide to enter the leaf and water vapor to exit. Maximum photosynthetic rates and, therefore, maximum carbohydrate production occur when the stomates are open. When plants experience drought stress the stomates close to conserve water and photosynthesis and carbohydrate production is seriously reduced. Under drought conditions, afternoon apple leaf photosynthetic rates may drop to 20% of normal. Therefore, avoiding drought stress with irrigation will result in greater production of carbohydrates for increased tree and fruit growth.
Shoots on mature apple trees normally grow for about 50 days, but on young trees shoot growth may continue for nearly 100 days. Drought stress during mid-season is more likely to reduce shoot growth on young trees than on mature trees. The majority of trunk enlargement and root growth occurs during late summer and fall, so a late-season drought can reduce both tree and root growth. This is crucial for young trees when the goals are to produce the largest tree possible on which fruit production will occur and to develop a well-anchored tree with an extensive root system for moisture and nutrient uptake.
Following harvest, carbohydrates accumulate in the woody portions of the tree, primarily the larger branches and trunk, as well as in the roots. These carbohydrate reserves are used during the following spring for early-season growth of roots, shoots, leaves, flowers, and/or fruits. Late-season drought stress can reduce fruit set for the following season. Apple flower buds are primarily initiated during about the first 50 days after bloom. Early-season drought stress, especially when combined with heavy crop loads, can result in poor return bloom. Field observations indicate this may be particularly true for spur "Delicious" trees.
Apple fruit growth is fairly uniform during the entire season. Fruit enlargement for the first 50 days after bloom is largely due to cell division, thereafter fruit growth is due to cell enlargement. Prolonged drought stress any time during the season may adversely affect fruit growth, but early-season drought is most detrimental because ultimate fruit size is related to cell numbers.
Color. In addition to fruit size, fruit quality can also be related to water availability. Because sugar is the building block for the red pigment in the apple skin, anything that reduces photosynthesis may delay or reduce red color development.
Storage Potential. Starch accumulates in apple fruits from about 50 days after bloom until fruit maturity and is converted to sugar in the maturing fruit. Stored starch is the energy source used by fruit tissues in storage. Harvested fruits having little starch will not store well and will soften earlier than expected.
Nutrient Disorders. Calcium uptake by roots requires adequate soil moisture. During dry seasons, fruit calcium levels are often low, and corkspot, bitterpit, and internal breakdown in storage may be severe. There is also some evidence that fruit rots before and after harvest may be most severe in fruits with low calcium concentrations.
Preharvest Fruit Drop. Preharvest apple drop is influenced by environmental conditions, but field observations do indicate a consistent relationship between drought and fruit drop. During the 1993 drought many Virginia apple producers delayed harvest until red color developed. A high percentage of fruit dropped in some orchards before harvest. In this case, severe drop may have been related primarily to the drought-induced late harvest.
In 1989, Virginia experienced a severe drought followed by heavy rain in early September. By mid-September fruit drop was severe in many orchards. Therefore drought, followed by late-season rain and cloudy weather, also may induce fruit drop due to reduced carbohydrate from the cloudy weather and drought combined.
Due to the potential reduction in fruit yield and tree growth and survival, it is important that growers know what actions can be taken to minimize the effect of a drought on tree fruit production. However, during a serious drought nothing can take the place of a well-designed and properly installed irrigation system.
One of the best ways to help minimize drought stress in an orchard is to provide a vegetation-free strip down the tree row before growth begins in the spring. This strip is usually implemented and maintained with a herbicide program. For optimal results, the vegetation-free strip should extend from the trunk of the tree out to at least the drip line of the tree on both sides of the tree row. Frequent mowing of the vegetation in the orchard should also be avoided. When grass is mowed before seed stalks form, the grass vigorously grows back after being mowed, further reducing soil moisture in the orchard. Options to mowing the vegetative row middles include using sublethal rates of herbicide applied during the spring and early summer. This practice is commonly referred to as chemical mowing and is used to suppress vegetative growth of the orchard floor for 6 to 8 weeks.
A well-managed orchard should not look like a lush green park or golf course. When new orchards are planted, a good technique is to prepare the soil a year in advance and plant a vegetative cover. Then 3 to 4 weeks before planting, apply a contact herbicide to provide killed sod-row strips into which the new trees will be planted. By planting into a killed vegetative cover moisture infiltration is greatly improved, and moisture loss by evaporation is reduced, maximizing the use of brief rain showers that may occur during periods of drought stress.
Another practice that may help to minimize the effect of a drought is to thin the fruit crop properly early in the season. By removing the excess fruit early in the season the tree does not expend carbohydrates and moisture in fruit that are later removed. Early thinning minimizes the amount of wasted energy and promotes growth of the desired fruit. Thinning at bloom time is preferred; however, in the Southeast the potential for spring frost-freeze problems limits the use of bloom thinning in order to guarantee a crop.
Growers may help minimize the effect of a drought on fruit production by controlling insects and disease pests that defoliate the trees, which reduces photosynthesis and carbohydrate production.
As mentioned earlier, the best option is to have a properly installed irrigation system in place to avoid any drought-stress problems. There are several different types of irrigation systems that should be considered for fruit trees. There are advantages and disadvantages to each type and for each specific orchard and local circumstances, and these must be considered before installation. It is best to consult an irrigation specialist and your county Extension Service agent to aid in designing a reliable system to meet individual needs. The three basic types of systems primarily used in commercial orchards in the Southeast are: (1) low-volume drip or trickle, (2) high-volume overhead, and (3) microsprinkler.
Low-Volume System. A major advantage of the low-volume drip or trickle irrigation system is, as the name implies, the low volume of water used. Other advantages include low energy use because of low operating pressures, least expensive to install, low labor requirements, and the potential to include fertilizer in the irrigation water to "fertigate."
This system's main disadvantage is that it emits only a small quantity of water each day. If the system is not turned on until there is a soil-moisture deficit, the low water output will not be enough to correct the deficit. Another consideration is that the water source must be clean and fine-filtered to avoid clogging of the emitters.
This type of system works best on heavier soils where there is a much broader wetting pattern for moisture movement from the emitter. On light, sandy soils the major direction of moisture movement is downward, resulting in a relatively small wetting zone, which may benefit only a very small percentage of the root system.
High-Volume System. Another type of system is the high-volume overhead system. This type of system can either be permanent in the orchard or portable, such as aluminum-piped systems. Unless pipe needs to be moved for a portable system, this system requires a relatively low level of management. This type of system applies a high volume of water over the top of trees and can correct soil-moisture deficits rather rapidly.
The wetting of the tree canopy can be both a positive and negative attribute. Wetting of the tree canopy in the spring can protect the blossoms and young fruit from frost under certain conditions. However, wetting of the canopy in the summer to provide soil moisture can result in greater disease pressure within the tree canopy.
This type of system also requires a high initial investment and requires a larger energy input to operate at medium to high pressures. Due to the high volume of water, soil erosion can also be a problem on sloping sites. A large, traveling gun is another variation of this system and may also be used in orchards in emergency situations. Common problems with this type of system are tree damage as well as fruit being knocked off by force of the water.
Microsprinkler. The third type of system used commercially is the microsprinkler. Microsprinklers are small, undertree emitters that apply water in patterns of various sizes and shapes. This system uses a low to medium volume of water and operates at relatively low pressure, therefore conserving both water and energy. The cost of microsprinklers is usually intermediate between that of the drip or trickle and the high-volume overhead system. Some microsprinklers may be used to correct soil-moisture deficits if operated for long periods of time.
Recent research has also indicated that some microsprinklers may be moved into the top of the tree during the dormant season to provide frost control in the spring. The major disadvantage of this system is that the water supply must be moderately clean or filtered and requires a moderate level of management.
Determining the amount of water to apply is probably one of the most difficult questions to answer. There are so many factors that enter into this question such as the soil type, environmental conditions, specific crop, and time of the year. Most people underestimate the amount of water that fruit trees use daily. As an example, research in Texas reported that 5-year-old peach trees used 36 gallons of water per day in July, which is over 250 gallons per week.
There are many different devices available for determining the soil-moisture status in an orchard. However, one of the least expensive and easiest ways for an orchardist to accurately determine soil moisture is with the use of tensiometers. Tensiometers are ceramic-tipped, plastic tubes filled with water with a gauge on the end. The ceramic tip is inserted into the soil to the desired depth and the gauge registers the relative moisture content of the soil. Two tensiometers will usually be sufficient for determining soil moisture with one placed at a 6-inch depth and one at a depth of 12 to 18 inches.
For fruit trees, which are relatively deep rooted, the lower tensiometer is used to determine moisture content for the lower portion of the root system. However, whenever irrigation is applied to maintain tree growth during dry weather, at least one-half inch of water should be applied per application to uniformly and adequately supply moisture throughout the rooting zone.
Publication date: Dec. 31, 1994
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