Crop Comments: Comparing Modern Drought Vulnerability to Southern Corn Leaf Blight

Four basic inputs are required for successful crop production: solar radiation, moisture, warmth (soil and air) and soil nutrients. All equally important, if any one factor is seriously limiting, crop production is greatly undermined.

The input category threatened most by wildfire smoke is solar radiation, the most common unit of measure of which is the Langley (Ly). Ly is a “unit of heat transmission, especially used to express the rate of solar radiation.” Wildfires have been in the media spotlight in the last few years. But there was another miscarriage of nature that many folks don’t recall, since it occurred over a half century ago. Called southern corn leaf blight (SCLB), its impact wasn’t just confined to the South.

By June 15, 1970, this disease had infested corn fields as far north as southern parts of Minnesota and Wisconsin. Almost as if to justify the name of this plant disease, the state of Mississippi, in 1970, lost between 30% and 40% of its corn crop to SCLB.

Although I’d studied corn production as a Cornell undergraduate, my first real contact with a major corn disease was when I was assigned to an Air Force base in South Texas in 1970. Even though agriculture wasn’t in my job description then, that’s where I first learned about SCLB. The SCLB epidemic of 1970-71 was one of the costliest disease outbreaks affecting American agriculture, destroying 15% of the crop at a cost of $1 billion ($8 – $10 billion, when corrected for 55 years of inflation).

In the vocabulary of plant geneticists, SCLB resulted from an over-reliance on cytoplasmic Texas male sterile (cms-T) lines in hybrid seed production. Combine this poor judgment call (involving cms-T) with a perfectly natural mutation of a race of Bipolar maydis (a fungus), then a perfect storm plant disease results.

This mutation, discovered in the Philippines in 1961, first appeared in the Corn Belt in 1969, damaging not only leaves but stalks, ears and developing kernels of hybrids containing cms-T genetics. A favorable environment combined with at least 85% of U.S. hybrids grown being of cms-T genetics set the stage for an epidemic. The Cms-T strain was discontinued in 1971, and hybrid seed production returned to using detasseling for the female parent.

That 1970 crop disaster served as warning to the seed production business to never purify genetics to the extent inflicted on America’s 1970 corn crop. Abandoning such “purity” and, in so doing, preserving genetic diversity ultimately reduced the likelihood of SCLB (or something similar) from ever recurring.

Although modern corn varieties are unlikely to be decimated by a plague like SCLB, they’re not coping well with threats associated with climate change – wildfire smoke (too little solar radiation), drought, prolonged hot spells and occasionally too much precipitation. The additional warmth associated with temperatures exceeding 85º F doesn’t really contribute to functional growing degree days. In addition to temperatures over 85º not benefiting corn, excess heat (above 95º) sabotages corn pollination.

Roger Elmore (University of Nebraska Cooperative Extension) explained why: “Excess heat (above 95º) impairs corn pollination by reducing pollen production and viability, desiccating silks and disrupting the critical timing between pollen shed and silk emergence. Especially when combined with drought, this leads to poor kernel set and lower yields. High temperatures also shorten pollen’s life to hours, dry out silks and create a ‘missed nick,’ where pollen isn’t available when silks emerge, resulting in blank or poorly filled ears.”

One of the biggest concerns observed on corn ears in recent years is “tip die-back.” According to Dan Quinn, Purdue University Extension agronomist, tip die-back occurs when ears do not fill kernels from the base of the ear all the way to the ear’s tip. Thus, ear tips may exhibit missing and/or incomplete kernels.

There are two main reasons for tip die-back: poor pollination, causing the absence of kernel formation and kernel abortion. The silks that emerge last from an ear during pollination, and the kernels that fill last during grain fill, are located on the tip of the ear. Any significant stresses exhibited shortly before, during and shortly after pollination can negatively impact these “youngest” kernels.

Quoting Quinn: “If many of your corn ears exhibit tip die-back, examine the tips of the ears closely to understand the cause of the problem. Poor pollination can occur when stressful conditions occur a few weeks prior to and during pollination (e.g., silk and tassel emergence). For example, drought conditions can delay silk emergence and cause poor synchrony between pollen drop and silk emergence. In addition, since the last emerging silks are located at the tip of the ear, pollen drop (which occurs only for seven to 10 days) can be completed prior to silk emergence, thus causing the ovules (potential kernels) at the tip of the ear to never be pollinated.”

Another climate hiccup threatening corn is lost solar radiation caused by pollution, an issue plaguing Chinese growers longer than wildfire smoke has stymied their American counterparts. Thus, Chinese plant scientists determined to build a less shade-sensitive corn. They addressed this matter in a paper titled “Quantitative Relationship Between Solar Radiation & Grain-Filling Parameters of Maize.” Written by Yushan Yang, Ph.D., et al., the paper’s said workers stressed the need to understand factors driving changes in corn grain filling as being essential for effective prioritization of increasing maize yield.

Quoting the paper: “Grain filling is a significant stage in maize yield formation. Solar radiation is the energy source for grain filling, which is the ultimate driving factor for final grain weight and grain-filling capacity determining maize yield. Here we confirmed the quantitative relationships between grain filling parameters and photosynthetically active radiation (PAR) by conducting experiments using different shading and plant density conditions and cultivars in 2019 and 2020 in Xinjiang, China.”

The researchers wrote that relative changes in PAR were significantly and positively correlated with relative changes in yield and grain filling rate. I’ll jump to their conclusion: “We believe that with increased likelihood of factors continuing to decrease PAR, plant geneticists should place more emphasis on developing corn strains that are less susceptible to erosion of effective solar radiation.”

by Paris Reidhead