My friend and former UK colleague, Dr. Greg Schwab, recently posted an interesting item on LinkedIn. He writes that corn tassel wrap, and associated poor pollination, could be due to boron (B) deficiency. I think he might be right. 

I looked at my B deficiency photo collection. Several of the usual – photos of corn ears with various degrees of poor pollination, ranging from lightly to heavily barren ‘tip back’ (Figure 1) or medium to heavy erratic patterned kernel loss (Figures 2 and 3). 

By the time you see these symptoms, it is too late to do anything. Foliar symptoms are less well known but include a crinkling/zipper deformation of leaves (Figure 4) and pale interveinal areas that can become chlorotic, pale yellow to white (Figure 5). 

I also found an older International Plant Nutrition Institute photo (Figure 6) posted by my friend and former student, Dr. Jose Espinosa, that shows the leaf crinkling/interveinal chlorosis, and also a malformation occurring in the upper portion of corn plants prior to pollination. Could this be a precursor to tassel wrap?

Ear symptoms of B deficiency can be confounded with other challenges, especially nitrogen (N), phosphorus (P), potassium (K) deficiencies and drought stress.  In most crops, B impacts sugar transport in the plant, especially important to stimulation of root and shoot development – proper growth of meristematic/younger tissues. Boron is not very mobile within the plant, so deficiency symptoms tend to be found in the youngest/newest plant tissues. With corn, proper B nutrition is associated with better water use efficiency, drought tolerance and plays a significant role in pollination. Boron deficiency impacts tassel and silk formation and function, (pollen germination, silk tube growth and seed formation), possibly tassel-sink synchrony, and particularly when heat/moisture stress are coincident with silking and pollination. Finally, B deficiency negatively impacts movement of plant sugars from leaves to ears.

The largest soil B reservoirs are organic matter and tourmaline, a primary borosilicate soil mineral of very low solubility. Older, well weathered soils, low in organic matter, are more likely to be B deficient. Plant available B occurs as soluble boric acid/borate anions (H₃BO₃/H₂BO₃^-). Soluble borates are mobile in the soil – are not retained well by the soil and tend to leach. Boron’s mobility in the soil can be linked to a greater possibility of B deficiency in 2025 Kentucky corn. Greater spring rainfall likely drove greater leaching, reduced root exploration (greater compaction is corn fields) and resulted in lower B uptake. Soil moisture, high temperatures and adequate N availability likely accelerated corn growth rates, further diluting tissue B concentrations.

Boron deficiency is known to occur in Kentucky. AGR-1 (https://publications.ca.uky.edu/agr-1) gives B recommendations for alfalfa and tobacco. Recently, language on B for wheat was added to the latest version of ID-125 (A Comprehensive Guide to Wheat Management in Kentucky, (url not yet available) as follows: “Boron (B) has recently been found to limit wheat yield in field research. Soil testing for B will help wheat producers decide when to apply B. Hot water extractable B levels lower than 0.8 lb B/acre indicate a need for B addition. Mehlich III extractable B was also evaluated and was not usable as an indicator of wheat B need. When a need for B fertilization is indicated, the recommended rate is 1 lb B/acre.” The UK soil test lab uses hot water extraction.

The B nutrition of corn and soybean in Kentucky has been the subject of a few studies, beginning in the late 70’s. Then, the incidence/severity of B stress was relatively low – not much was found until several field studies were conducted between 2008 and 2011 in Russell County (Grove and Schwab, 2010). Both corn and soybean were studied, and other nutrients (P, zinc and copper) were also evaluated. Soils were Sango and Lonewood silt loams, low in organic matter and formed in thin loess over old Cumberland Plateau parent materials. Soil test B was determined using Mehlich extraction and ranged between 0.5 and 1.0 lb B/A. The soybean and corn B response data is summarized in Table 1. Only 2009 soybean data are shown.

Generally, R1 leaf tissue B concentrations responded positively to B addition. The R1 leaf tissue B sufficiency concentration range, from AGR-92 (https://publications.ca.uky.edu/agr-92), is 20 to 60 ppm B for soybean and 5 to 25 ppm B for corn. So, without added B these crops evidenced either insufficient (corn) or borderline insufficient (soybean) B nutrition. Yields were generally good, but yield responses were mixed, despite the generally low level of tissue B, especially for corn. Taken together, the corn yield response to B addition was very significant on these soils. We observed a good deal of poor pollination in association with the large yield responses. We did not observe tassel wrapping, but we weren’t looking for it. The 2010 corn 2 yield response was complicated by a B by zinc interaction, where zinc addition raised yield potential and caused a much stronger yield response to B that was not evident in the absence of zinc. Soybean did not give a yield response to added B in 2009, nor in 2010 (data not shown). That said, soybean tissue analysis might serve as a ‘canary in the coal mine’, allowing earlier detection of B insufficient fields intended for corn in the next season.

Now – Consider evaluating this year’s tassel-wrapped corn for B nutrition stress by taking leaf tissue samples. But those leaves need to be green. Take them as close to the node with the uppermost ear as you can but move upward if you must in order to get green leaf tissue. Given the immobility of B in the corn plant, if tissue B was low before, it will still be low in the upper green leaves. If all leaves are brown/dead, wait until your next soil test results are available.

Prevention – Start with your soil testing program. Soil testing isn’t perfect, but it is a good place to start. The critical corn soil test B (STB) value (the value triggering a B application) isn’t well understood for Kentucky soils. Our previous experience with alfalfa, wheat and tobacco suggests the likely critical STB level is around 0.5 to 1.0 lb/A (0.25 to 0.50 ppm B) and this also depends on the extraction procedure (there are several). If your soil test B value is low, then apply 1 to 2 lb actual B/A. There are several good dry B products (boric acid, Granubor, etc.) available to use as part of a dry fertilizer blend. Needed B may also be sold co-granulated with potash (usually KCl, potassium muriate) or phosphate (usually MAP, monoammonium phosphate). These can work if you need the potash or phosphate, respectively. Spring, rather than fall, application is favored because of B’s tendency to leach with excessive rainfall.

Soluble B sources (borax, Solubor, etc.) are available. These can be applied at any time, pre-plant, and both pre- and post-emergence, but probably tend to be used later in the season. Soluble B that falls on the soil is less likely to be “fixed”, chemically, than some other spray-applied nutrients. Soluble B products are often compatible with pesticides (herbicides, fungicides, insecticides) that are sprayed over corn at different times, but careful jar testing is strongly recommended. One application of 0.5 to 1 lb actual B/A should be enough for the season.

Detection – Use plant tissue analysis to monitor/diagnose B nutrition status. At present, AGR-92 (https://publications.ca.uky.edu/agr-92) gives a B sufficiency range of 5 to 25 ppm B regardless of the corn stage of growth. Yield response-wise, even less is known about using plant tissue analysis and then a later timing of foliar B. Though it is likely that earlier sampling is better, it is also probable that earlier sampling is less likely to detect deficiency of a soil mobile/plant-immobile nutrient like B (i.e. tissue B levels are fine until the soil B reservoirs run dry).

In summary, I’m not entirely sure that B deficiency is the cause of corn tassel wrapping, but there is some coincidence between symptoms and poor pollination outcomes and what is known about corn B deficiency. I don’t believe in coincidences. As our future corn production seasons might more often be a combination of wetter springs and warmer summers, I do think that paying more attention to corn B nutrition is warranted. Please share your own observations. I’d be pleased to learn more.

References

Grove, J.H., and G.J. Schwab. 2010. Micronutrient nutrition for corn and soybean: Emerging issues in Kentucky. Proc. 40th North Central Extension-Industry Soil Fertility Conference. 17-18 Nov. Des Moines, IA. 7 pp.