The most common and most likely answer to this crossword clue is the 5 letter word ERGOT
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More clues leading to the result ERGOT
Buddy explains!

The straight part
We know that "plant disease" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case plant disease = ergot.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does ERGOT mean?
ERGOT noun- Any fungus in the genus Claviceps which are parasitic on grasses.
- The sclerotium (wintering stage) of certain fungi in the genus Claviceps, appearing as a deformed grain in certain cereals and grasses infected by the fungi.