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Consider Apple Nutrient Applications After Recent Warm Weather

Author: Annie Klodd. While May 2019 saw cool temperatures, low growing degree day (GGD) accumulation, and excessive rainfall, the warm temperatures in the last ten days have led to rapid growth on apple tree fruit and shoots. Many apple growing regions of Minnesota and Wisconsin accumulated up to 18 GGD last week, pushing growth along quite a bit. While we are happy that fruit development is "catching up," growers also must make management considerations based on this surge in growth.

Plant nutrient deficiencies are a risk with rapid growth after petal fall, because the rate of fruit and shoot growth may outpace the ability of the plant to uptake available nutrients from the soil. In some areas, this issue may be also be compounded with "wet feet" (saturated soil) which can interfere with nutrient uptake. While in some cases the trees are likely to outgrow temporary deficiencies, growers should check for signs of nutrient deficiencies on the leaves and decide whether applying foliar nutrients is warranted.
Iron deficiency on an apple tree. Photo: Bugwood

Identifying nutrient stress on apple trees

When discoloration occurs on apple tree leaves, the grower must first determine whether the symptoms are indicative of a nutrient deficiency or another issue like vascular damage, trunk cankers, or disease. Nutrient issues can be misdiagnosed, leading to the wrong treatment or unnecessary foliar nutrient applications.

To determine whether leaf discoloration is due to a nutrient deficiency or something else, look first to the pattern and placement of the symptoms on the tree. Note whether the symptoms are in the newest leaves, older leaves, or if the effected leaves are occurring throughout the tree canopy.

Nutrient deficiencies at this point in the season typically appear most in the newest leaves or in individual shoots that are growing rapidly. However if the symptoms are spread throughout the whole canopy, this is more indicative of a vascular problem in the woody tissue, interfering with nutrient and water transport through the tree. Vascular problems can be caused by trunk disease, winter injury, or issues underground like saturated soil ("wet feet"). Drought stress is unlikely at this point in the season in Minnesota, but it can also be mistaken with nutrient deficiency (photo).
Drought stress on apple tree leaves. Photo: Bugwood.

At this point, growers also may start to see leaves in sections of the canopy wilt and desiccate. Often, this is due to winter injury that did not manifest immediately. Winter injury on fruit trees is expected in Minnesota following cold February temperatures, but it is too early to predict the extent of the damage on every variety and region.

Managing Nutrient Deficiencies After Petal Fall

If nutrient deficiencies are suspected, foliar application of micronutrients may be used at this time as a short-term correction. However, in many instances the trees will outgrow the deficiency once growth slows, which is likely to occur in the next week with lower temperatures in the forecast. Therefore, foliar application is most appropriate if the symptoms are widespread, but may not be needed in minor cases.

Before applying foliar nutrients, identify the type of deficiency the trees are experiencing using photo keys such as this one. Depending on which nutrients are deficient and the severity of the problem, individual micronutrients or micronutrient mixes may be applied. One common strategy is to apply a 20-20-20 N-P-K foliar amendment. Foliar N-P-K will not correct any nitrogen, phosphorus, or potassium issues within the plant, but many of these products also contain micronutrients like zinc, magnesium, and iron.

If nutrient deficiencies are reoccurring in the orchard, a good soil nutrient management plan should be used to correct these issues. Foliar nutrition is not a substitute for soil nutrition. Soil and foliar nutrient testing should be used annually to monitor orchard nutrient status. For more information on orchard nutrition, read this article from Michigan State University.

Should growers do a foliar nutrient test at this point in the season?

It is wise to submit a foliar sample for nutrient testing at least once per year, along with a soil test, but the results often take between 5-10 days to be returned. Furthermore, the best time to sample leaves for nutrient analysis in July to August, according to recommendations from Washington State University. Therefore it is not practical to use them for immediate recommendations at this point in the growing season.

Some growers wish to use sap nutrient analysis in place of foliar nutrient analysis. However, there are no validated standards for sap analysis in tree fruit, so it is difficult to make nutrient recommendations based on sap analysis.

Author: Annie Klodd, Extension Educator - Fruit and Vegetable Production

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