Fruit Mineral Analysis--An Update
Fruit mineral analysis is a difficult topic, because of the abundance of published papers and the often conflicting or inconclusive results. Many papers published are from outside the region, yet the answers sought for the Pacific Northwest cannot be fully addressed by research done elsewhere.
The English have extensively developed the use of fruit mineral analysis to segregate lots of fruit by storage potential. It is hoped we can apply this method here. In Washington, fruit from specific blocks can be stored for a long time, year after year, almost in spite of the harvest date. Fruit mineral analysis may provide a link in determining the reason for this, and could become a "report card" for the grower. Fruit which tested as having a good balance of nutrients could be stored, and weak fruit could be segregated and marketed early.
The information for this report stems in part from my sabbatical leave in 1986-1987 in East Malling, from the scientific literature, from researchers in the Pacific Northwest working on this subject and from industry personnel.
A number of apple and pear disorders are associated with low fruit calcium levels. Varieties of apples which are more chilling sensitive (Cox, Jonathan, Spartan, etc.) appear more susceptible to calcium related disorders. Also, increasing the amount of calcium in the fruit reduces the incidence of disorders. In most cases, researchers have been able to find reasonably good correlations between low calcium and disorders. Other aspects of fruit quality, such as sugars, acidity, texture, etc., either were not examined, or the connection was not present. Fruit firmness and disorders seem to be the most closely linked with calcium.
In Washington Drs. Frank Peryea and Joan Davenport of the Washington State University Wenatchee Tree Fruit Research and Extension Center, have begun to study the relationship of whole fruit mineral content and fruit quality in Red Delicious. After conducting an exhaustive study of Red Delicious harvested in 1986 and stored for 6 months in Controlled Atmosphere (CA), they found no correlation with fruit quality. However, they were able to conclude that Reds and Goldens sprayed with calcium had less Bitter Pit than fruit left unsprayed. They are continuing their work this year on both Reds and Goldens from the 1987 harvest.
Drs. Eugene Mielke and Tim Facteau of Oregon State University, stationed at Hood River, have been examining the relationship of calcium to Newtown apple and d'Anjou pear quality. Their results are not clear. In some years, in some orchards, there seems to be a relationship between low calcium in fruit cortex and disorders. In most years too much variation occurs from orchard to orchard, from tree to tree, and even from fruit to fruit to make predictions on how fruit will behave in storage.
Dr. Daryl Richardson (OSU) and Dr. Tom Raese (USDA), working on d'Anjou pears, have found good correlations between low fruit calcium in fruit peel and cortex and Cork Spot and Alfalfa Greening. In addition, Raese found good correlations with low fruit calcium and Bitter Pit in Reds and Goldens. He also found that in 1987, fruit firmness, color and in some cases, acidity increased in Reds and Goldens and d'Anjous following calcium sprays. These sprays significantly reduce, but do not eliminate, disorders.
Dr. Tim Righetti (OSU) has been running a commercial analysis laboratory along with his research program. He did not find a relationship between fruit disorders and the absolute level of calcium in d'Anjou pears, Newtown apples, or Reds and Goldens. He determined that it takes a minimum sample size of 40 apples from a tree or a block of uniform trees to have any confidence in the level of calcium determined by a laboratory. More variation occurs within a uniform orchard than between orchards, even if fruit of the same size are sampled. He found that when pooling fruit from two orchards and comparing the average calcium or nitrogen level it is possible to have the same average value, yet have quite a different range of calcium values. For example, he found that the first orchard might have both high and low calcium levels, while the second orchard might have a uniform set of fruit. Although having the same average calcium values, each orchard's storage response will differ.
In summary, given the varieties of apples and pears grown in the Pacific Northwest, researchers have been unable to establish consistent correlations between fruit calcium and fruit quality. Because calcium varies from fruit to fruit, researchers have been unable to determine the specific calcium threshold below which the fruit is at a high risk for disorders or poor quality out of storage.
Variability in Calcium Content
Fruit calcium content varies within a tree, among trees, between orchards and from year to year. Sampling consistently gives a variation of 50% or more from one tree to the next. Even variation within a fruit can be high. In d'Anjou pears, Raese found that the shaded portion of the pear had 360 ppm calcium, while the sunlight portion had 255 ppm calcium. This variability is most significant when dealing with calcium; the other elements vary far less. Therefore, it is hazardous to obtain only the absolute level of calcium (such as milligrams of calcium per 100 grams fresh weight), since the variability is so high. There is no way to tell whether some fruits were high and others were low. One possible way of getting around this problem is to use a laboratory that analyzes many orchards. Once the laboratory personnel have accumulated test results from approximately 100 orchards of the same variety, they can rank each orchard on the amount of calcium in the fruit. The orchard with the highest calcium level is number one, and the orchard with the lowest calcium level is number 100. When looking at the top 25 orchards, the orchardist can assume that these will be less prone to disorders. The bottom 25 orchards will be highly prone to disorders in some years. It is impossible to rank the middle 50 orchards against one another, or to determine which of the bottom 25 orchards will be more prone to disorders. There are significant limitations to this procedure. First, it is only valid if fruit of the same size and maturity are compared. Second, fruit should be grown under the same conditions.
Some companies in England and the United States use fruitlet analysis to determine if calcium sprays are needed. Fruitlet analysis is far less accurate than fruit tested at harvest in predicting storage disorders.
Sampling should be done as late in the growing season as possible, since calcium levels fluctuate widely during the growing season. English studies determined that during hot weather, 20% of fruit calcium moves out of the fruit during the last 3 weeks before harvest.
Righetti has worked out what he calls his "75% rule." When orchards are ranked according to fruitlet calcium, there is only a 75% chance that blocks whose fruitless have low calcium values will also have fruit low in calcium at harvest.
Factors Affecting Calcium Levels
In 1987 in Washington, fruit underwent stressful growing conditions due to the extremely warm weather. In many instances, when irrigation could not keep up with demand serious fruit shrinkage occurred during the day as fruit supplied water to the leaves. With this water went calcium, and low soil moisture reduced soil calcium availability.
Besides climate, irrigation, pruning, fertilization, harvest maturity, fruit size and fruit set and position on tree also influence fruit calcium levels. Even postharvest fruit temperature can affect the incidence of Bitter Pit, a calcium related disorder. Research out of South Africa has shown that rapid cooling of fruit immediately after harvest reduced Bitter Pit.
English and Washington Apple Production
Why then do English growers find fruit mineral analysis so valuable, while Pacific Northwest research indicates that it can be so undependable in predicting storage performance?
The first difference is in fruit varieties. Cox Orange Pippin and Bramley seedling are extremely different from Reds, Goldens and Grannies. Washington varieties are hardier, require different storage temperatures and can last longer in storage. In addition, Washington varieties naturally produce fruit of larger size. The English Cox Orange Pippen apple can only grow to about a maximum size of 125 in a good year.
A second difference is in climate. The English climate varies from hour to hour and from day to day. Some years have an overabundance of rainfall, while others will be droughty. This is quite different from Washington's high desert climate, which has relatively consistent weather.
A third difference is what growers expect from fruit mineral analysis. The English researchers have made excellent connections between fruit mineral content and disorders. Their varieties are susceptible to many disorders caused in the orchard but appearing after harvest. Researchers have used fruit mineral analysis to predict storage disorders, rather than fruit quality attributes. The magnitude of the problem is different. In some English lots, disorders can range from 40 to 60%! Even Washington's Bitter Pit in 1987 was less than some of England's worst lots in a good year.
An important consideration is that there is not full agreement on the scientific validity of fruitlet analysis in the United Kingdom (UK). In spite of the poor correlation between calcium levels at the fruitlet stage and at harvest even in the UK, many English growers find the test results useful. Growers use the early season analysis to determine whether to apply five calcium sprays or 10 to 14. Almost every English grower sprays calcium EVERY YEAR.
A fourth difference is that UK growers use laboratories that test a great many samples and also rank orchard blocks against each other. Their minimum levels of calcium have been determined after 30 years of research. This research began at East Malling in an effort to find out how to reduce fruit rotting in storage from the Gleosporium fungus. It was found that higher calcium fruit has less rot, but it was nearly 20 years before sampling, laboratory procedures and interpretation were worked out.
Methods to Reduce Variability
The problem of variability in orchard block sampling is serious. Sampling fruit in the orchard should be done carefully and uniformly. Sending 10 cull fruits to a laboratory for analysis will not give reliable results. Forty fruits of uniform size should be considered a minimum sample size. The laboratory should pool all 40 fruits and not select fewer.
The section of the fruit which the laboratory samples is not as important as the fruit which is sent for analysis. However, it is not possible to compare results from two laboratories if they sample different sections of each fruit. For example, fruit sent to a laboratory which tests opposite quarters of the fruit will report back a higher calcium level than one which just samples the peel or the flesh. This is because the core of the fruit has higher calcium levels than the flesh or skin. The calcium in the core migrates outward during storage, which is one reason there is less Bitter Pit in stored fruit than early marketed fruit.
Research is continuing to explore the relationship of fruit mineral content and fruit quality in the Pacific Northwest. Dr. Joe Poovaiah is continuing basic scientific research on calcium and fruit quality at Washington State University, Pullman, and applied work is continuing at each Washington State University experiment station. Research results show that calcium and other mineral nutrients are extremely important to fruit quality. Low calcium fruit in low calcium years is at risk of disorders.
There are three general directions of research underway in the Pacific Northwest. The first is to improve understanding of the relationship between nutrients and fruit quality. Low calcium fruit and Bitter Pit, Cork Spot and other disorders are related. This has been demonstrated by researchers around the world, as well as by those in the Pacific Northwest. However, it is more difficult to prove that the positive aspects of quality, storability and nutrients are directly related.
The second research direction is orchard sampling. Sampling an orchard is difficult, since fruit calcium level varies so widely. It might be useful to know if a year is going to be a low calcium year, in which case orchardists may wish to apply more calcium or modify horticultural practices.
The third research direction is to test the concept that fruit calcium can be increased through the use of soil, foliar or trunk injections in the orchard, or the application of calcium after harvest in the warehouses. Raese studied soil fertilizers and their effect on fruit calcium. His work (in progress) has been finding the lowest rates of Bitter Pit on blocks fertilized with calcium nitrate or ammonium nitrate, and the highest rates of Bitter Pit in blocks fertilized with monammonium phosphate or Triple 16. The amount of fertilizer also plays a role; higher rates produce more Bitter Pit.
Canadian scientists, Drs. Gerry Neilson and Mike Meheriuk, found that although trunk injections resulted in more calcium in fruit, there was significant phytotoxicity and uneven distribution within the tree. Agreeing with Raese and the Oregon scientists, they recommend repeated foliar sprays to increase fruit calcium.
Most scientists agree that foliar application of calcium mid to late in the season is preferable to earlier applications, although last year, Raese's experimental plots responded to earlier sprays more positively than to the later sprays. It is essential that the spray material come into direct contact with the fruit, since calcium does not move from fruit to fruit or from the leaves to the fruit.
The application of three to five calcium sprays, two in June and one in July and two in August, seems to be a fairly effective way of increasing calcium in fruit. Except for calcium sulphate, which is not very effective, most other calcium spray materials appear as effective as calcium chloride. Some other compounds may be safer or more convenient to use than calcium chloride. Apply 8 to 12 pounds per application per acre (as 2 to 3 pounds per 100 gallons) for apples. For pears apply 1 to 4 pounds per application per acre (0.5 to 1.5 pounds per 100 gallons). The number of times that the fruit is in contact with calcium appears to be more important than the concentration, so do not raise the rate or reduce the number of applications and expect to see results. High rates of calcium may also burn the fruit or leaves. Consult EB0419, 1988 Spray Guide for Tree Fruits in Eastern Washington, for more information.
Raese believes that calcium sprays may eventually assist in reducing calcium-related disorders by 75% or more in most orchards. He cautions that the orchardist must continue to improve spraying to get better coverage of the fruit. The side of the fruit facing the inside of the tree must be covered.
One final point. Calcium will never be the cure-all for poor orchard management. Dr. Ron Tukey used to say that Bitter Pit is a sign that something is wrong in the orchard. It might be pruning, irrigation, crop load, climate, etc. There is evidence that calcium sprays can help reduce some of the calcium-related disorders, while the rest will have to be taken care of by improved horticulture.
Dr. Eugene Kupferman, Postharvest Specialist
Taken from an address to the 4th Annual Warehouse Conference, Yakima, WA
Post Harvest Pomology Newsletter, 6(2): 3-7