TREES
10. TREATMENTS INCREASING FRUIT CALCIUM 1. Soil management and balanced fertilization
10.3. Preharvest calcium sprays
Calcium sprays are one of the most effective treatment increasing fruit Ca. However, the efficiency of Ca sprays depends on the following factors:
10.3.1. Calcium spray time
Young fruit have usually higher ability to take up exogenous Ca per unit surface area than older ones (Michalczuk and Kubik, 1984). However, in some situations, uptake rate of exogenous Ca by mature fruit can be higher compared to young fruitlets (Glenn et al., 1985). This is due to the fact that during fruit growth, cracks and other surface irregularities may be formed which increases penetration rate of Ca into flesh tissues (Meyer, 1944; Wojcik et al., 1997). It is worth noting that Ca taken up by young fruit moves deeper into flesh tissues than Ca applied on surface of mature fruit. Thus, to reduce Ca-related physiological disorders occurring inside fruit such as internal breakdown and watercore, Ca sprays should be started just 2–3 weeks after petal fall. When fruit are affected by bitter pit or lenticel breakdown that symptoms occur on fruit skin and/or in outer cortex tissues, late season Ca sprays are more effective in reducing these disorders.
10.3.2. Rate of calcium fertilizer
Rates of application for most foliar Ca fertilizers range from 3 to 10 L/kg per ha.
High Ca fertilizer rates increase generally Ca uptake rate by fruit (Wojcik, 1998b).
However, at higher rates, injuries of leaf and fruit tissues occur. Young developing
leaves are particularly sensitive to injuries. Therefore, rates of the most Ca fertil-izers, especially Ca salts without additives (Ca chloride or Ca nitrate) should be lower in early season by 20–30% compared to those applied in the fall.
10.3.3. Spray technique
Apple trees on vigorous rootstocks at high spacing (6 × 4 m, 5 × 4 m) require 1000–1500 L of water per ha to cover sufficiently total surface of leaves and fruit.
Such high spray volumes are necessary due to the large volumes of tree canopy (Wojcik, 1998b). As planting density increases, tree size decreases which conse-quently leads to reduction of spray rates. Thus, in high density orchards, apple growers should use generally sprayers with low spray efficiencies. Required spray rate [R] in orchards can be calculated according to the formula:
R (L ha–1) = [H × Wt/Wi] × 330,
where H is tree height [m], Wt is tree canopy width [m] and Wi is interrow width [m]. Thus, higher spray rates are required when trees are high, tree canopy is wide and distance between tree rows is large. At spray rates below 300 L ha–1 it is necessary to lower the standard rate of Ca fertilizer by 10–20% since highly con-centrated Ca solutions (> 4%) can injury leaf and fruit tissues. When concon-centrated Ca material solutions (1.5–3.9%) are applied, sprays should be performed in the evening or at night since drying of solution from fruit surface is slow. In this way, surface-applied Ca have better conditions to move into fruit flesh tissues. Sprays of concentrated Ca solutions give good results in increasing fruit Ca if wind velocity is below 3 m·s–1and tree canopy is loose. When wind velocity is higher and canopy is too dense, it is difficult to obtain uniformity of distribution of Ca solution within tree canopy. Additionally, in strong winds, drying out of Ca solution from fruit surface is quick which finally reduces rate of exogenous Ca uptake.
10.3.4. Spray frequency
Generally, with increasing number of Ca sprays during the growing season, fruit Ca status increases; although this relationship is not closely proportional (Grande et al., 1998). Under Polish conditions, from 3 to 8 sprays of Ca per season is recommended. Spray number is dependent on apple variety, the growing season and period of fruit storage. Apple varieties such as ‘Jonagold’, ‘Szampion’ ‘Cortland’
and ‘Gloster’ that are sensitive to Ca-related disorders, should be more frequently sprayed with Ca materials compared to varieties such as ‘Lobo’, ‘Idared’ and ‘Elstar’
rich usually in Ca. In dry seasons, number of Ca sprays in apple orchards should be high since under water stress conditions, accumulation of Ca into fruit is limited.
Moreover, at high air temperatures, uptake rates of exogenous Ca by fruit are lower compared to those at moderate temperatures. Therefore, in hot seasons despite intensive Ca sprays it is difficult to obtain fruit rich in Ca. If it is predicted that fruit will be stored for long period of timing or will export on large distances, number
of Ca sprays in the growing season should be high. It is worth noting that too high number of Ca sprays results in decrease of fruit size and worsening their taste. Negative effect of intensive Ca spraying on apple quality is caused by reduc-tion in photosynthesis rate resulting from decreased stomatal and mesophyll conductances (Swietlik et al., 1984).
10.3.5. Calcium fertilizer quality
In the Polish market, there are numerous commercial Ca-containing formulations.
However, they usually contain less Ca compared to Ca chloride and Ca nitrate. Many of these commercial Ca materials have been studied to evaluate their efficiency in increasing fruit Ca. Generally, we have found that many of these Ca materials were less effective, and none of them were more effective than Ca chloride or Ca nitrate. Based on our experiments we can also suggest that the efficiency of Ca sprays in increasing this element in fruit depends mainly on amount of Ca applied. Thus, commercial materials rich in Ca should be generally more effective compared to those with lower Ca status.
Calcium chloride and Ca nitrate are often used foliar fertilizers to increase fruit Ca status. It is commonly believed that sprays with Ca chloride often result in leaf damage such as browning and death of the leaf margins (Raese and Drake, 1993). However, in many experiments we have observed no leaf injuries as a result of sprays with Ca chloride at rates from 3 to 8 kg·ha–1. Problem with sprays with Ca chloride is potential corrosion of equipment. Therefore, it is imperative that equip-ment be cleaned thoroughly after spray with Ca chloride. In some growing seasons, sprays with Ca nitrate may deteriorate fruit colour. This is found particularly in years with high air temperatures occurring during 3–4 weeks before harvest. Under these conditions, development of the blush on fruit skin is reduced. Therefore, sprays with Ca nitrate should not be applied before harvest when weather conditions do not favour forming the blush on fruit surface.
10.3.6. Weather conditions
Temperature, humidity and air velocity have significant effects on the efficiency of Ca sprays. As fruit surface is long wetted with spray solution, Ca uptake by fruit increases. At high temperatures and low humidity of air, drying of Ca solution from fruit surface increases which finally reduces rate of Ca absorption. Therefore, fruit from the canopy top exposed to high temperatures have generally lower ability to absorb exogenous Ca compared to those from the bottom and the inside canopy.
In strong winds during spraying, deposit of Ca solution within canopy is unevenly.
Moreover, when it is windy, spray solution from fruit surface dries quickly which finally reduces Ca uptake by fruit.
Taking into consideration profits resulting from production of fruit rich in Ca and the high efficiency of surface-applied Ca uptake, we claim that Ca sprays should be routine treatment yearly, applied particularly on apple varieties sensitive to Ca-related physiological disorders.