"The danger of a massive world famine is aggravated by Mr. Bush's recent initiative to transform foods into fuel," Castro wrote in Cuban news media, referring to U.S. support for using corn and other food crops to produce gasoline substitutes.
The brief essay titled "Bush, Hunger and Death" also alleged that Bush "threatens humanity with World War III, this time using atomic weapons."
Bush is expected to announce new strategies toward Cuba on Wednesday. White House spokesman Tony Fratto said last week that Bush would "emphasize the importance of democracy for the Cuban people and the role the international community can play in Cuba's transition by insisting on free speech, free assembly, free and competitive elections and the release of all political prisoners."
In his essay, Castro predicted that Bush "will adopt new measures to accelerate the 'transition period' in our country, equivalent to a new conquest of Cuba by force." Cuban officials have long denounced U.S. efforts to produce a "transition" from Castro's government to a Western-style representative democracy.
Ailing and 81, Castro has not been seen in public since undergoing emergency intestinal surgery and ceding power to a provisional government headed by his younger brother Raul in July 2006.
While he has looked upbeat and lucid in official videos, he also seems too frail to resume power.
Life on the island has changed little under Raul Castro, the 76-year-old defense minister who was his elder brother's hand-picked successor for decades.
Cuba staged municipal elections on Sunday, the first step in a process that will determine if Fidel Castro is re-elected or replaced next year as Cuban leader.