It was a bumper harvest this year and barns in southern Ukraine are filled with great golden mounds of grain. Yet farmers in Ukraine’s prime agricultural belt say they are facing ruin, and experts warn the world may be staring at another global food shock as Russia and Ukraine, both major exporters of grain, intensify attacks on ports and shipping in the Black Sea. We don’t know how we will survive, said Viktor Zvirishyn, 57, a farmer who lives in a village in the southern region of Mykolaiv.

There’s almost nowhere to store it, he added. Because the grain cannot get out through Ukrainian ports and Black Sea shipping routes have become increasingly precarious, no one wants to buy the grain, he said, and prices have plummeted. The invasion displaced thousands of farmers and disrupted grain exports via the Black Sea, causing a global food shock in 2022.

Ukraine managed to recover partially from that, regaining some lost territory and negotiating a Black Sea grain deal with Russia, via mediation by Turkey and the United Nations, that allowed exports to resume and farmers to start over. But this year, Russia began attacks on Ukraine’s last functioning ports and shipping in the Black Sea, again closing down the main export route for Ukrainian grain. Moscow has also extended drone attacks on crops, equipment and farm buildings in frontline areas.

Ukraine extended its own attacks to include port facilities in southern Russia, and Kyiv has hit at least five Russian tankers carrying grain in August, according to Lloyd’s List, a London-based maritime journal. The result is that grain exports from both Russia and Ukraine have been drastically reduced. Ukraine exported only half a million metric tons of grain from Aug. 1 to Aug. 18.

That is just 20 percent of normal levels when the ports are fully functional, Taras Vysotskyi, Ukraine’s minister of agriculture, said in a recent briefing. Russia’s grain exports were at 1.4 million metric tons from Aug. 1 to Aug. 20, less than half what they were for the same period a year earlier and the lowest in 10 years, according to a bulletin from the Russian Grain Union, a trade group. Both countries are seeking other export routes.

Ukraine is using road and rail, and shipping terminals on the Danube River, but Russia has also started attacking parts of those networks. The Danube already has limited capacity this year because of drought and can handle only smaller volumes. The effect of the blockage is hurting Ukraine’s farmers most of all, said Vicki Aken, country director in Ukraine for the American nonprofit Mercy Corps, which works to help farmers like Mr. Zvirishyn.

If farmers cannot export their grain, they will probably lose the harvest for lack of storage, Ms. Aken said. In a statement last week, she issued an urgent call for a new grain deal, though there is no sign of any talks. Ukraine’s farmers have risked their lives for this harvest, Ms. Aken said.

They should not now have their grain stranded — or their livelihoods collapse — because the world has failed to keep a safe route to market open. Mr. Zvirishyn has rebuilt one room of his house to live in, using bricks scarred by shrapnel from the ruins. He has had to store his grain in a local silo, and the rest is piled in the open under tarpaulins. He had paid off his debts, he said, but had no money for fuel for his tractor to prepare the land for the new sowing season.

I am almost out of money now, he said. I don’t know how I will continue. Nearby in the same village, Roman Pradun, 35, limped on a crutch through his barn and bent to lift a handful of grain from a mound of harvested wheat, displaying its quality.

He said he had bills to pay but would be selling at a loss at current prices. He was lucky to have storage for his g...