Think of it - a forest debarked, clean bare, blanched white with disease. He is describing the spiritual condition of the Laodicean church of the last days. An army of cankerworms has been eating away at the lives of multitudes of backslidden Christians. Outwardly they appear as trees planted by living water - but inwardly they are corrupted, weak, diseased. They are drying up spiritually.
Divorce is a cankerworm.
So is adultery. So are drinking and drugs. So is worldlimindedness. So are dirty television and filthy movies. So is prayerlessness.