Horticultural News
       
        M. Elena Garcia,  Horticulturist

Another growing season is about to begin.  As of Monday, April 19, here at the UVM Horticulture Research Center, McIntosh was at green tip stage.  Full bloom should be in about three weeks.  Other than a deficit in the amount of precipitation we have received in Burlington, we have not had any winter weather conditions that might threaten the '99 crop potential.  However, there are more 60 days until the end of June over which the potential orchard returns are initiated, followed by 90 days or more before the crop assets can be collected.  Therefore, 150 unpredictable days lay ahead. There are many horticultural practices you can employ to increase the likelihood of having a successful year.  In the last issue of this newsletter, we were discussing the role nutrients on fruit quality.  I will continue to discuss the role of other elements in fruit quality because you will be applying these nutrients within the next couple of months. 

Nutrition and fruit quality

Magnesium (Mg)

Magnesium (Mg) is another element very important to fruit quality.  When the tree is deficient in Mg, the fruit ripens prematurely and preharvest fruit drop is accentuated; and since preharvest fruit drop is a major problem associated with McIntosh, careful monitoring of Mg is highly recommended. In addition, Mg is essential for chlorophyll synthesis (Mg is at the center of the chlorophyll molecule), protein synthesis,  and nitrogen metabolism. For the overall well being of the tree, proper Mg levels are important. 
Deficiency symptoms are seen as interveinal chlorosis, starting at the terminals of older leaves, progressing to the base and midrib of the leaf and giving the typical herringbone appearance.  In extreme cases, the chlorosis becomes necrotic followed by leaf abscission.  In the most severe stages, the symptoms may resemble the marginal scotching associated with potassium deficiency. Under deficiency conditions, vigor of extension shoot growth and spur may be seriously reduced. The older and midshoot leaves are affected first because Mg is very mobile within the plant .  In deficiency situations, Mg is remobilized from older to younger tissues, consequently the old leaves are the first to be affected. The degree to which symptoms develop can vary significantly.  Only a few branches may be affected or the whole tree may look scorched and defoliated. Although not readily visible, one of the first consequences of Mg deficiency is the death of older feeder root tissues as the Mg is moved from these tissues to newly developing root tips.

Other facts about Mg:

• Leaf sample content of 0.30 to 0.50 % is considered necessary for tree fruit crops.
  
• There is a relationship between leaf potassium and leaf Mg.  The ratio between these two elements should not exceed 4 (K) : 1 (Mg).  Ratios higher than 4:1(K:Mg) are considered indicative of Mg shortage regardless of the actual Mg levels.  Heavy applications of K can induce Mg deficiencies.
  
• Besides potassium,  the presence of other competing cations such as ammonium , calcium, and manganese can decrease the rate of Mg uptake from the soil.
  
• Mg is relatively immobile in the soil.
  
• Moisture stress in the soil limits availability and uptake of Mg.
  
• Ratios of Ca to Mg are important.  Excess Mg can hinder Ca uptake.  High Mg can accentuate bitter pit.
  
• Deficiency symptoms are more pronounced in young or trees with heavy crop loads.
  
• Fruit Mg requirements are relatively high, and Mg is remobilized from spurs to the fruit and seeds as the fruit nears maturity.
  

• Soil tests should be used in conjunction with foliar analysis to determine whether low Mg leaf contents are being caused by an inadequate soil supply or  a shortage of Mg in relation to soil K or other cations.
  
• Increasing the pH increases the availability of Mg, thus maintaining the proper pH is very important in the availability of Mg.
  
• Use dolomitic limestone before planting, and on a yearly basis based on soil analysis.  However, the movement of  Mg in the soil is rather slow, so do not depend solely on ground application to supply the tree with Mg.
  
• For foliar application, Epson salt sprays at a rate of 15 lbs. in 100 gal. per acre at  petal fall, first and second cover sprays ( avoid high temperatures during drying).
  
• Crop requirements (the actual amount of Mg taken up by a bearing tree) are approximately 20 to 24 pounds/acre/year, of which half is permanently removed in the fruit.