Over one million farmers in the Philippines have been impacted by Typhoon Haiyan according to the UN.
The Food and Agriculture Organisation (FAO) says that hundreds of thousands of hectares of rice have been destroyed.
Coconut plantations which are a big earner of foreign currency were said to be "completely flattened".
Fishing communities have also be severely affected with the storm destroying boats and gear.
The FAO says they have arrived at the figure of farmers affected by looking at the pathway the storm took as it passed across the country and from an analysis of data provided by the government.
"That area is the major rice producing area for the Philippines," FAO's Jeff Tschirley told BBC News.
"They've just finished their harvest and because it is not modernised production, they are still drying it but we suspect there is a lot of rice that is vulnerable to having been impacted by the storms."
The farmers are also facing into the next planting season with much of their infrastructure such as irrigation equipment affected by the typhoon.
The Philippines has been importing less rice over the past five years, but the storm will have dramatically changed that.
"It will set them back on their goal of self sufficiency but more key is that those livelihoods will be pushed way back until they get restored irrigation infrastructure. That has been severely damaged in most cases," said Mr Tschirley.
As well as rice and sugar cane, the FAO is most concerned about coconut plantations.
Coconuts account for nearly half the Philippines agricultural exports and the country is the world's biggest producer of coconut oil. A smaller typhoon last year destroyed 500,000 tonnes.
"The sugar cane fields can be recovered relatively well even if the harvest is lost," said Jeff Tschirley.
"But numerous coconut plantations have been completely flattened, and with coconuts you are looking at multiple years to recover the productivity."