On NH27, the two-lane span of highway from Lucknow to Barabanki is thick with unruly trucks making their way thr­ough the smog into what is ess­entially sweet country. ­Sugar mills and fields of cane regularly dot the passing landscape. Also at hand is a less sweet cousin, filling plantations that sprawl for acre after acre in eye-popping green. Banana, that rich source of potassium and phallic wit, is one of the major crops of this area—as it is in Bihar and Maharashtra, and a lot of south India. The jovial fruit is, however, attracting more than ­humour these days.

The news, in fact, is a little grave—and is not being much talked about officially so as not to cause a scare. Along with the commercially viable banana varieties from the West, India seems to have imp­orted a debilitating fungal infection. It’s ruining not just the crop, but the soil its­elf. And the farmers don’t know that yet.

Mineet Mishra, a 28-year-old farmer, has about an acre where he predominantly grows the Grand Nain strain, source of the...