I believe the best approach is POE:
A) Susceptibility to certain plant diseases is genetically determined.
-> makes sense, and can be inferred to some extent from the example cited:
"In 1970, for example, southern leaf blight destroyed approximately 20 percent of the United States corn crop, leaving very few varieties of corn unaffected in the areas over which the disease had spread"
Impact of southern leaf blight on corn and its varieties (put in place to support the author's argument for the need of diversity)
(B) Eighty percent of the corn grown in the United States is resistant to southern leaf blight.
- Obviously we cant infer this, as we do not know the alternative reasons why the remaining 80% were not affected. It could be as simple as not being located in the same geographical proximity as the region where southern leaf blight was wrecking havoc

(C) The extinction of wild food plants can in almost every case be traced to destructive plant diseases.
- Cant be inferred, there is a point regarding "wild food plants" highlighting the need to conserve it. However it is not related in any way to the southern leaf blight - the only destructive plant disease mentioned in the passage

(D) Plant breeders focus on developing plants that are resistant to plant disease.
- No mention of this
(E) Corn is the only food crop threatened by southern leaf blight.
- We know corn crop was impacted by southern leaf blight - whether it is the "only" food crop - we dont have enough info about the same.