Chlorosis in field grown maples

James Altland, Ph.D.
North Willamette Research and Extension Center (NWREC)
Oregon State University

Introduction

Top: Healthy red maple foliage; Middle: problem is developing; Bottom: severe deficiency with characteristic interveinal necrosis.
A common problem of field grown red maple (Acer rubrum) is a foliar chlorosis that develops in late summer. Maples begin growth with no sign of chlorosis through mid summer. Then, just as the Farwest Show comes around in late August, chlorosis sets in. One nursery jokingly terms this condition “Farwest Show Blight”.

The Oregon Association of Nurseries recently funded a research project to determine what causes this chlorosis, and more importantly, which soil factor or combination of soil factors cause the aforementioned chlorosis.

This article describes how the research was conducted, a brief synopsis of our results, and management guidelines for preventing maple chlorosis.

Review of manganese (Mn)

Our research verifies what many nurserymen (and women) suspected, in that foliar chlorosis is primarily a result of manganese (Mn) deficiency. What’s more important and more interesting, is what causes the deficiency and what management practices will prevent the problem. Because Mn is at the crux of the issue, let’s review some information covered in a previous Digger article (June, 2003).

Mn can occur as Mn2+ or the oxidized form of Mn3+ (at high pH, it can also be found as Mn4+). Only Mn2+ is available for plant uptake. Mn is not mobile in plants, so deficiency symptoms occur on new growth first.

Mn plays three major roles in plant growth and development. In photosynthesis, it is involved in electron transport within photosystem II. In N metabolism, it affects the reduction of nitrate to ammonium, in which case it probably is involved with the enzyme nitrate reductase. Most important to the context of this research is its role as a precursor to production of aromatic ring compounds, most importantly auxin. Mn deficiency reduces auxin levels and causes hormone imbalance. Change in the ratio between auxin and other plant hormones could lead to many of the Mn deficiency symptoms, including inhibited lateral root development and decreased root extension.

Mn has also been shown to play a vital role in carbohydrate production. Carbohydrates are molecules containing carbon, hydrogen, and oxygen that are used by plants for energy storage. Carbohydrates are especially important for storing energy over the winter when trees are dormant. Mn deficiency is most pronounced in the root system. For example, Mn deficiency reduced bean carbohydrate levels 77% in foliage, 59% in stems, and 88% in roots (Vielemeyer et al., 1969). By the time visual symptoms of Mn deficiency are obvious in foliage, roots have already been adversely affected.

Mn deficiency occurs late in the growing season and is often ignored. Current wisdom states that because deficiencies show up after most tree growth has occurred, tree quality is not affected. However, foliar symptoms of nutrient deficiency often occur long after plant growth is affected. Our research shows that this is a crucial aspect to Mn management and a point critical for remembering: foliar chlorosis is a latent symptom of Mn deficiency, and by the time it is visible in the field growth has already been adversely affected.

Each plot sampled consisted of 20 consectutive trees in a single row. 75 plots were sampled from throughout Oregon's bareroot shadetree industry.
Research method

‘Red Sunset’ maples (Acer rubrum) with one-year old tops (two-year old roots) were selected as the test subject for this study. Soil and foliar tissue samples were collected from 75 plots in 24 bareroot shadetree nurseries. Soil samples were analyzed for all the parameters listed in Tables 1 and 2. Trees were also measured for height, caliper, foliar chlorophyll content, and overall quality.

The results

Trees at every nursery appeared healthy when samples were collected in June. All plants were growing vigorously and had dark green foliar color. Tree quality was measured on a subjective scale from 1 to 10, where 1 is a tree of poor quality with severe chlorosis, and 10 is a tree of high quality with no signs of chlorosis. When samples were collected in June, virtually all trees had ratings of 9 or higher.

Many nurserymen and crop consultants have long suspected Mn deficiency as the cause for maple chlorosis. Despite excellent foliar color, trees differed greatly in levels of absorbed manganese (Mn). Mn levels in maple leaves throughout Oregon ranged from 10 to 535 ppm.

By late August, trees at some nurseries appeared healthy and vigorous, while others were chlorotic, stunted, and lacked vigor. Tissue analysis revealed a host of deficiencies in these plants. This is not surprising. Absorption and assimilation of any given plant nutrient is complex, and dependent on many other factors including soil moisture, pH, and soil concentration of other nutrients. When one nutrient is deficient, it will often cause decreased uptake or assimilation of other nutrients.

Our data overwhelmingly point to poor Mn absorption early in the year as the cause for late season maple chlorosis. Several soil nutritional factors affect Mn absorption, most importantly pH, Mn, sulfate, and ammonium to nitrate ratio. To see these data presented graphically, click here.

Management practices

Soil tests

Using historical knowledge, narrow down the possible planting sites for red maples to those areas where you think pH is lowest. Collect soil samples from these areas, and then select the area with the lowest pH. Our data clearly show that as pH increases, Mn availability in soil and absorption in plants decreases dramatically. Planting red maple in fields with sufficiently low pH (Table 2) is 95% of the battle.

Soil fertilizers and amendments

Do not lime the field unless pH is well below 5.0. Lime does two things to soil, it adds calcium and raises pH (dolomitic lime also adds magnesium). Calcium (Ca) and magnesium (Mg) levels should be within ranges listed in Table 2. If not, add gypsum (CaSO4) for supplying Ca and/or Epsom salt (MgSO4) for Mg. These two products are readily available, inexpensive, and have no effect on soil pH.

Sulfate influences Mn absorption and plant quality. Apply elemental sulfur if sulfate levels are deficient (Table 2). Many nutrient cations are available in fertilizers as sulfate salts. For example, zinc (Zn) can be applied as zinc sulfate (ZnSO4). If soil tests indicate other deficient nutrients in addition to sulfate, apply the sulfate salt of that nutrient.

Mn sprays

Trees above were among the nicest observed in this experiment, and they were NOT treated with foliar Mn sprays. They were planted in a soil with appropriate pH and suffiicient nutrient levels (Table 2).
Foliar Mn sprays are not effective remedies for Mn deficiency. Mn is not mobile in plants, so applying Mn to foliage will only temporarily affect those parts of the plants contacted by the spray. And yes, you can make repeated sprays (at no small cost) to continuously supply Mn to new growth. But what is often forgotten is Mn level in plant roots. Recall that Mn is critical for auxin production and carbohydrate storage in plant roots. If you are only supplying Mn to the foliage, and Mn is not translocated from that foliage (and it won’t be), plant roots will be Mn deficient despite sprays that ‘green up’ foliage.

In a recent conversation with my counterpart in Tennessee, she explained that the major concern with plants coming from Oregon is the relatively small root system on large trees. While the overall perception of Oregon nursery crops is high, most establishment problems on the east coast are attributed to insufficient root size. Production, digging, and shipping methods may dictate the size of root system delivered, but needless to say, whatever roots are shipped should have maximum stored energy (carbohydrates) for regenerating small feeder roots when lined out in customer fields. Mn deficiency limits root vigor by reducing carbohydrate storage, and foliar Mn sprays will not remedy this critical problem.

Mn soil supplements

Mn absorption increases with increasing soil Mn levels. However, appropriately adjusted pH will likely do more for Mn availability than adding additional Mn. There is a new product being trialed by several nurseries in Oregon. It is a sulfur coated Mn fertilizer prill. The concept behind this product is that sulfur will lower pH immediately around the Mn core, thus making Mn soluble and available for plant uptake.

Research has shown that monocalcium phosphate (MCP) used in a similar manner provides the same result. When applied with monocalcium phosphate, Mn is solubilized and spread throughout the soil (not sure how far) in a highly acidic solution containing dissolved Mn. MCP is formed by reacting a calcium source with highly purified phosphoric acid, and so the reaction in soil is likely chemical (not biological) and occurs rapidly regardless of environmental conditions. In contrast, soil acidification by sulfur is a biological reaction that requires warm, moist soils and time for the reaction to occur.

Plants cannot translocated Mn from a well-supplied part of the root system to a deficient part. If Mn coated fertilizers are topdressed after planting (which is how they are currently being evaluated), only those roots near the soil surface or zone of incorporation will have sufficient Mn, while the remainder of the root system will be deficient and of poor quality. Even if this section of the root system can absorb sufficient Mn for shoot growth, the part of the root system with no access to Mn will be poorly developed. Therefore, it seems unlikely that topdressing coated Mn fertilizers will be sufficient to ‘cure’ the entire tree if planted in a high pH soil. However, incorporation of the fertilizer prior to planting might be beneficial.

Key points to prevent Mn deficiency

  1. Conduct soil tests to determine which field has the lowest soil pH, and use it.
  2. Do not lime the soil! If soil test reveals Ca and or Mg deficiencies, use gypsum (Ca sulfate) and Epsom salt (Mg sulfate), respectively.
  3. Using soil tests, be sure Mn levels are sufficient for plant growth. Beyond the ranges listed in Table 2, adding additional Mn will not likely provide a benefit. Mn sprays and Mn soil supplements are not necessary when soil pH is sufficiently low.
  4. Apply elemental sulfur to adjust sulfate levels to those listed in Table 2. When other nutrient cations are deficient, supply the sulfate salt of that nutrient.
  5. Acid injection and fertigation through drip tape may offer a solution to maples planted in high pH soils. This might be an area for future research.

Conclusion

It all goes back to the simplest of nutrition practices: collect soil samples. Blindly planting maples in a field without knowing soil pH is like playing Russian roulette. Using Mn sprays and/or soil supplements is a poor remedy for planting maples in high pH soils. By planting maples in soil with low pH, you avoid the hopeless venture of trying to get Mn into a plant that is incapable of absorbing it.


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