Jamieson, Fausset and Brown on Zechariah 14:18

18. if ... Egypt go not up--specified as Israel's ancient foe. If Egypt go not up, and so there be no rain on them (a judgment which Egypt would condemn, as depending on the Nile's overflow, not on rain), there shall be the plague ... . Because the guilty are not affected by one judgment, let them not think to escape, for God has other judgments which shall plague them. Maurer translates, "If Egypt go not up, upon them also there shall be none" (no rain). Psa 105:32 mentions "rain" in Egypt. But it is not their main source of fertility.