There has been a significant occurrence of oak blight increase caused

This problem a classic example of two principles: (1) how the location of modifiers can change the meaning of answer choices, and (2) how not every difference in the answer choices makes a difference. (I call these "false alarms.")

Let's start with a "false alarm." I have seen several suggestions in this forum that argue that "resistance against herbicides" is idiomatically incorrect, but that "resistance to herbicides" is preferred. This is totally a false alarm. Academic papers use these terms interchangeably. There is no idiomatic problem here. (In fact, the GMAT often uses obscure, less-used idioms in correct answers, baiting people into picking other answer choices that have the "expected" idiom, but that contain other, more grievous errors. This is like a corrupt car mechanic who "fixes" something that isn't necessarily broken but might sound a little funny to our untrained ears, and then "breaks" something else in the car so we have to take the car in later!)

There are very few issues in this problem that conflict with concrete grammatical rules. Instead, most of the changes create changes in meaning. Modifiers in the wrong spot can often cause illogical scenarios that can easily be eliminated. The trick is to leverage the answer choices against each other, looking for “Decision Points” that create differences in meaning.

Answer choice A and E can be eliminated because the phrase “a significant occurrence of oak blight increase” implies that there was one event (or “occurrence”) of oak blight increase, as if it spiked for a single moment. This makes no sense, since an increase takes place over time. An "occurrence of an increase" is a single, isolated event, whereas an "increase in the occurrence" of something implies a change in the rate of something occurring over time. Answer choice C has a very similar issue, since it implies that there has been a single “increasing oak blight occurrence”, once again implying a single event or occurrence.

Answer choice B is wrong because the modifier "caused by growing fungus resistance against herbicides" modifies the phrase “oak blight” -- but oak blight is not caused by fungus resistance. (Think about it: if oak blight were caused by fungus resistance to herbicides, then we could just stop using herbicides and there would be nothing for the fungus to resist, thereby eliminating oak blight!) The location of this modifier causes problems with the meaning. Instead, occurrences of oak blight are happening more often because of the growing resistance of fungi to herbicides. We want to find the answer that shows how the rate of fungus resistance is related to the rate of oak blight outbreaks.

Answer choice D is the only remaining answer. The phrase “because of” begins an adverbial phrase, describing how or why something occurs. (This is contrast to the modifier “caused by” which is an adjectival phrase modifying a noun.) Since “because of” begins an adverbial phrase, it cannot modify the noun “oak blight”, but instead modifies the clause beginning with, “There has been a…” This makes perfect sense. It describes how the increase in oak blight was caused by the growing resistance of fungi to herbicides.

The answer is (D).