Russian forces have riddled Ukrainian fields with mines and destroyed equipment in areas they once occupied, in what returning farmers and the Kyiv government allege is a campaign by Moscow to hobble the country’s agricultural industry. Russia’s invasion already has decimated shipments of corn, wheat and sunflower oil from what was until recently one of the world’s biggest food exporters. Prices have soared, adding to global food-price inflation and heaping misery on developing countries that had been dependent on imports from the region. The extent of damage to some farms, together with port disruption and a shortage of fertilizer, demonstrates...