Can Nutrient Sprays Improve Apple Fruit Color?
By Tim Smith, WSU Extension
After the passing of Alar, a product that helped maintain fruit quality on the tree while you waited for red color to develop, there have been a series of products that have tried to take its place.
- The home garden “wonder-fertilizer” product that was sprayed on with warm (not cool!) water. This came & went fairly quickly, despite colorful advertisements in industry magazines. It turned out that the original work showing a color boost compared two neighboring orchards. The treated one was organic, and had low N levels, the untreated one was relatively vigorous, with higher N. The treated orchard had redder fruit, which was attributed to the use of the product. Some still cling to this stuff, hoping for effect. No harm, except with initial product cost.
- Copper sprays. Some still use these, though no researcher has been able to duplicate the results obtained in split-block orchard trials. There are some soils in NC Washington that are quite low to deficient in copper. Perhaps color would be improved if this deficiency was identified and corrected. This product group has been replaced by……..
- Potassium nutrient sprays. In the early 1990’s, research in Canada, on low-K soil & drip irrigation, showed an increase in red color when K was added to the drip irrigation water. Actually, what I believe they showed was, the K deficient trees had poor color, the addition of K to the irrigation water corrected the K deficiency, enabling the fruit to have more normal red color. Though the researchers never intended this result to be interpreted this way, this data triggered a stampede in Pacific Northwest USA toward potassium as a coloring aid.
Some view this nutrient program with alarm, as we are concerned about the Ca:N, Ca:K and Ca+K:N ratios in fruit skins in relation to bitter pit and other fruit calcium disorders. Some researchers have used K nutrient sprays on fruit as a research technique to induce or increase bitter pit. I don’t know how much K-induced bitter pit we have seen in the past few years, though some have wondered. I have seen bitter pit induced by soil applications of K, so I do not recommend K unless there is a possible deficiency. K deficiency is rare in the irrigated, higher K soils of the Pacific Northwest USA. We recommend that a low soil test be backed up with a low leaf analysis before you start a K nutrient program. When applying K to the soil, you should not try to catch up all in one year; add relatively light rates over a few years. This may not be true at all in non-irrigated areas elsewhere. Potassium is a critical macro-nutrient, but should be applied to pome fruits for good reasons, and with a good plan in mind.
- Hang the fruit on the tree longer. If you want to see really red Galas, just leave them on the tree until it is well past the picking harvest time. The fruit becomes ever-more red, but of increasingly lower quality as time passes. This is one danger of over-emphasizing red color in the market. You force some people to hang fruit past the point of optimum edibility.
- Reduce the nitrogen in the tree to below optimum recommended levels. This has been a long-term trend in most PNW orchards, sometimes due to poor economics, sometimes due to good horticulture. This has worked very well. Trees with lower N have very much more colorful fruit. Less of it, more alternate bearing, and smaller, but more colorful. The exception to this are green apples (example: Granny Smith) where N is applied as a spray during the growing season to maintain green color and fight the development of yellowing or red blush. We do this only because the USA grocery store wants a green apple, very green, no yellowish or blushed apples need apply.
So, in our experience, time of harvest and level of N makes the most difference to red and yellow color.