GMAT Sentence Correction (SC) | Now the fourth most important crop worldwide

Hi Experts /chetan2u ,

If we just focus on the following part:
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Now the fourthmost importantcrop worldwide after wheat, rice and corn,potatoes are expected
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On the basis of bolded words can we say the the noun should be singular..?
Additionally, in the non-underlined portion we have singular nouns ( wheat,rice and corns).
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