New Mexico apple producers can grow excellent apples. Apple quality depends greatly on good cultural practices -- pruning, pollination, fertilization, thinning, irrigation, insect and disease control,andharvesting--as described below. For detailed information on pruning, spraying, fertilizing, and other practices, ask your county Extension agent.
Indices of Fruit Maturity
There are several indices of fruit maturity. Because fruit measurements vary among cultivars, from one tree to another in the same cultivar, among orchards, and from one crop year to the next, no single index provides a consistently dependable guide for harvest maturity year after year. Use a combination of indices to determine maturity.
Ground Color. Ground color in the skin of red apples is the fruit's underlying green or yellow color coming from the amount of chlorophyll showing. Ground color is one of the most useful indices of maturity. In general, fruit advances to maturity as its ground color changes from green to yellow--the exact shade depends on the variety. The following ratings are generally used: dark green, medium green, light green, yellowish green, greenish yellow.
Ease of Separation from the Spur. In most apple varieties, fruit is ready to be harvested when it separates from the spur easily, without breaking the stem, when lifted either with or without a slight rotating movement.
Some varieties like 'McIntosh' and 'Red Delicious' tend to separate from the spur early, thereby dropping to the ground. This fruit drop may increase as a result of early frost or excess nitrogen. Other varieties such as 'Jonathan', 'Stayman', and 'Winesap' retain their fruit until it is overmature. Plant regulators such as Ethrel (Ethephon) and NAA-related Indices of Fruit Maturity
There are several indices of fruit maturity. Because fruit measurements vary among cultivars, from one tree to another in the same cultivar, among orchards, and from one crop year to the next, no single index provides a consistently dependable guide for harvest maturity year after year. Use a combination of indices to determine maturity.
Ground Color. Ground color in the skin of red apples is the fruit's underlying green or yellow color coming from the amount of chlorophyll showing. Ground color is one of the most useful indices of maturity. In general, fruit advances to maturity as its ground color changes from green to yellow--the exact shade depends on the variety. The following ratings are generally used: dark green, medium green, light green, yellowish green, greenish yellow.
Ease of Separation from the Spur. In most apple varieties, fruit is ready to be harvested when it separates from the spur easily, without breaking the stem, when lifted either with or without a slight rotating movement.
Some varieties like 'McIntosh' and 'Red Delicious' tend to separate from the spur early, thereby dropping to the ground. This fruit drop may increase as a result of early frost or excess nitrogen. Other varieties such as 'Jonathan', 'Stayman', and 'Winesap' retain their fruit until it is overmature. Plant regulators such as Ethrel (Ethephon) and NAA-related chemicals sprayed at the end of the season can alter this maturity index, as Ethrel advances fruit ripening while stop-drop chemicals prevent fruit drop.
Firmness. Fruit flesh becomes softer as apples mature on the tree. Determine firmness by cutting a thin slice of skin and flesh from the apple and using a hand-operated pressure tester to measure the pressure necessary for a 7/16-inch plunger to penetrate the slice. Repeat this process several times with randomly selected apples.
Depending on variety, apples for commercial use are harvested at different firmnesses. If fruit will be in long-term storage, harvest when it is in the higher firmness range. If fruit is for immediate shipping or use (such as for roadside operations), harvest when it is in the lower firmness range. Harvest 'Red Delicious' apples at 15-18 lb of pressure for optimum storage; harvest 'Golden Delicious' apples at 14-17 lb. Apples with a firmness greater than 18Êlb are usually immature.
Factors other than maturity such as location of fruit on the tree can affect firmness; use at least one other index in addition to firmness to determine apple maturity.
Soluble Solids Content. As apples mature, juice sugar content increases. Apple soluble solids content, a factor highly correlated with sugar content, is measured with a hand refractometer. Optimum percentage of soluble solids for commercial harvest depends on variety. Harvest 'Red Delicious' apples at about 10%. If fruit will be used immediately, harvest when soluble solids content is around 12%. In general, harvest apples in a 10-12% range for optimum cold-storage quality.
Starch Content. Starch content gradually decreases as the apple matures. Determine starch content by dipping slices of apples in a 75-ml solution of iodine crystals (2.5Êgm) plus potassium iodide (10 gm) for one minute. Drain off excess solution and let dry for a few minutes. Starch will react with the iodine and appear as a blue-black pattern. This index is not widely used because it is not very practical and must be performed on apples that have not been stored.
Days from Full Bloom. The period from full bloom to actual harvest is fairly constant for any given variety. The average number of days from full bloom to maturity in five varieties are: 'Jonathan', 135-145; 'Red Delicious,' 145-155; 'Golden Delicious', 150-160; 'Winesap', 160-175; and 'Rome Beauty', 165-175.
Color Development. To use this index, growers must be experienced with the apple variety or strain. Some varieties may be fully colored several weeks before optimum harvest time. Some cultural practices may either hasten or delay fruit color. Excess nitrogen, as well as an iron deficiency, may cause poor fruit color. Potassium applications can improve fruit color when potassium is deficient. Additional color variations may be caused by climatic conditions and soil type, as well as irrigation and various cultural practices.
Grading for 'Red Delicious' apples in the United States is primarily based on percentage of red skin color. Extra-fancy grade apples must have 70% surface area with good color, while fancy grade must be 40%. With a little practice, ratings for individual apples can be made to the nearest 5-10%.
Charts that show coloring for stages of apple maturity are available for yellow apples such as 'Golden Delicious'. 'Golden Delicious' apples progress from dark green to yellow; five ratings are generally used--dark green, green, light green, yellowish green, and greenish yellow. Compare apples in the field against colors on the chart to determine apple maturity
Seed Color. Seeds turn brown or black as they mature inside the fruit. Make ratings for seed color as follows: seed all white, brown color beginning to show on sharp end of seed, 1/4 of seed brown, 1/2 of seed brown, 3/4 of seed brown, and all of seed brown.
