Water keeps stagnating in one low corner and rotting my crop, how do I drain it?

A low corner where water collects and sits will keep rotting the roots there because the soil stays starved of air, so the answer is to give that water a way out. The simplest step is to cut a drainage channel from the low spot to a field drain, channel, or pond so the water flows away instead of pooling. If the whole field is uneven, land leveling, especially laser leveling, removes the low pockets so water no longer collects in one place. Where the low patch cannot drain by gravity, some farmers raise the planting into beds or ridges so the crop roots sit above the wet zone, and grow the patch with a crop that tolerates wet feet. On heavy soils that drain slowly, adding organic matter improves structure over time. For a patch that floods every monsoon, a planned drainage outlet is the lasting fix. Your agriculture department can advise on drainage layout, and confirm whether leveling support or any subsidy is available in your area.