Black layer is the stage in corn development at which kernel growth ceases and maximum kernel dry weight is achieved (also referred to as “physiological maturity”).

A killing fall frost prior to physiological maturity can cause premature leaf death or whole plant death. This occurred recently when temperatures dropped below freezing in some late planted Ohio corn that had yet “black-layered."

The impact of frost injury to immature corn was discussed in the September 23 C.O.R.N. newsletter ( A common misconception is that kernel black layer formation sometimes does not occur following a frost or other late-season severe stress. Kernel black layer always occurs (Nielsen, 2013).  Any severe stress that occurs during the grain fill period will cause premature kernel black layer formation and is related to the reduction in or termination of sucrose (photosynthate) availability to the developing kernels (Afuakwa et al., 1984).

Afuakwa, J. J., and R. K. Crookston.  1984.  Using the kernel milkline to visually monitor grain maturity in maize.  Crop Science 24: 687-691.

Nielsen, R.L. 2013.  Effects of Stress During Grain Filling in Corn.  Corny News Network, Purdue Univ. [online]  [URL accessed Oct. 2014].

Thomison, P.R. 2014.  Assessing yield losses in corn due to frost.  C.O.R.N. Newsletter 2014 - 32 September 23, 2014 [online]