Again this year, I see some of my sword ferns (Polystichum munitum) are becoming brownish. It doesn't look fungal to me nor are there any signs of spider mites so I'm wondering not only what the cause may be but also whether it might spread to the unaffected sword ferns nearby. All the better if it is preventable or curable. The ferns get regular watering through the dry months and have excellent drainage. They are growing in bright semi-shade. Except for being unattractive, the plant seems perfectly healthy.