A cob of sweetcorn with whole rows of kernels missing, or kernels that trail off into bare cob near the tip, is almost never a disease or a feeding problem. It's pollination, and specifically, it's the shape of the row it was grown in. Sweetcorn is wind-pollinated rather than insect-pollinated, which puts it in a different category from the feijoas and passionfruit most home gardeners have already learned to worry about, and it means the layout of the planting itself decides how well the crop fills out.
Each sweetcorn plant carries its male flowers in the tassel at the top and its female flowers lower down, wrapped inside the husk of the developing cob. Every strand of silk poking out of that husk is attached to a single potential kernel, and that kernel only fills if a grain of pollen lands on its silk and travels down to fertilise it. A cob with a full, even set of kernels means nearly every silk on it caught pollen; a gappy, patchy cob means most of them didn't.
A cob with a full, even set of kernels means nearly every silk on it caught pollen; a gappy, patchy cob means most of them didn't.
The problem with a single long row is that it asks the wind to do something it's bad at: carry pollen sideways, along the row, from tassel to silk, over and over. Most of a tassel's pollen falls within a metre or two of where it was shed, and wind direction on any given day is only ever roughly consistent, so a lot of it lands on bare soil or leaves instead of on a neighbouring cob. A block spreads the same number of plants across width as well as length, so pollen shed from tassels in the middle of the block has other plants' silks close by in every direction, not just fore and aft. That's why a block of at least four plants by four plants is the usual minimum recommended for sweetcorn, and why it out-pollinates a row of sixteen plants stretched out in a single line even though both use the same number of seedlings.
Spacing inside the block matters too. This site's plant database lists sweetcorn's spacing at 30cm, and that figure holds regardless of whether the block is square or long and thin, since it's set by how much root room and light each stalk needs rather than by pollination. The pollination benefit comes purely from the block shape, not from planting any closer together than 30cm would already have you.
Sweetcorn's planting window in New Zealand runs from around September in the warmest northern gardens through to November further south, with most of the country sowing in October or November for a harvest 70 to 90 days later, which lines up well with a Christmas or New Year picking if it goes in this spring. If space is genuinely too tight for a full block, hand pollination is a reasonable fallback: cut a ripe, pollen-shedding tassel off one plant and shake or dab it directly over the silks of nearby cobs, repeating over the few days each plant tassels. It's fiddly on a garden scale bigger than a handful of plants, which is exactly why block planting is worth building the layout around from the start rather than treating as optional. It's also worth knowing that different sweetcorn varieties cross-pollinate each other, so growing two varieties side by side, or sweetcorn near a decorative or field corn planting, can affect kernel quality even inside an otherwise well-pollinated block.