Horticultural News

Fruit Quality

Improving fruit quality was one of the major areas identified by Vermont apple growers during the December 10 Apple Industry Summit into which more resources need to be allocated. Good fruit quality results from combining information on tree growth and development into tree management decisions.  A fundamental concept of tree growth which must be transformed to orchard cultural practices to affect fruit quality is the concept of tree nutrition. Throughout the season, I will provide information on horticultural practices that improve fruit quality. I will begin by focusing on calcium because calcium is perhaps the most important mineral element in determining fruit quality, especially fruits that are stored for long periods of time such as apples.

Calcium (Ca)

• Fruit low in Ca tend to ripen earlier than those with an adequate supply of this element.  Therefore, Ca deficient fruit will not store well and will be of inferior quality when packed and marketed.
  
• Uptake of Ca from soil is restricted to the area just behind the root tip.
  
• Soil absorption of Ca is limited, thus the need for foliar sprays.
  
• Ca is one of the most immobile mineral elements.
  
• As Ca moves from the roots up to the growing tissues, Ca becomes bound to cell membranes and walls of the first tissues where it is needed, and there it stays permanently.  Young tissues located at the periphery of the tree such a fruits, do not receive much of the ground applied Ca.
  
• Most physiological disorders are triggered by weather conditions that cause moisture or temperature stress.
  
• Ca plays an important role in reducing stress by maintaining cell integrity.
  
• Sprays of calcium chloride have been successful in reducing or controlling some fruit disorders
  

Ca-related fruit disorders
Remember that the lack of Ca does not cause these disorders, but it may only exacerbate the problem

How can you tell if your fruit is Ca deficient?

• Obtain a fruit analysis during the growing season.  The problem with fruit Ca analysis is that the Ca status changes quickly in reponse to a treatment application and to environmental stresses such as water stress.
  
• Observe your fruit and pay attention to the history of the block, especially if you have had a history of bitter pit or cork spot.
  

General principles that promote high Ca fruit:

1. Although Ca is usually one of the most abundant elements in the soil, the apple roots are very poor at absorbing it.  Competition from other cations such as K, Mg, and NH₄ reduce Ca root uptake.  So, do not over-fertilize with these elements and compounds.
   
2. Quick responsesto Ca treatments are only obtained when Ca is put directly on the fruit because Ca moves very slowly in the plant. 
   
3. Use practices that  discourage too much vegetative growth.  When there is competition  for Ca between vegetative growth and fruit, the vegetative growth is a much stronger competitor. 
   
4. Avoid excessive dormant pruning because this encourages vegetative growth.  However, summer pruning may prove advantageous because of the reduction of vegetative growth. 
   
5. Encourage good pollination.  Seed number affects fruit Ca.  It appears that seeds help direct Ca to the fruit.
   
6. Do not grow excessively large fruit.  Fruit Ca decreases with fruit size for any given cultivar.
   
7. Avoid water stress.  For roots to absorb Ca, water must be present, and under severe water stress, the leaves can withdraw water and Ca from the fruit.
   
8. Maintain the pH at 6.2 to 6.5.  Availability of elements in the soil depends on the pH (See chart on how soil pH affects availability of plant nutrients in the '98 March issue of this letter http://orchard.uvm.edu/uvmapple/newsletter/1998/vtapple030198.html)