When the weather is cooler and the humidity is high, brownish spots may appear on some bulb foliage. This is often botrytis, a fungal disease. Bulbs that prefer some shade are particularly susceptible to this problem. The spots will turn gray-brown, then black as the problem becomes worse. Leaf spot, another fungal pest, could be the problem.
Root rot can be a pest of the calla, too.
It is often caused by a soil that is too alkaline. To avoid this, increase soil acidity and drainage. If you lift the rhizomes, dust with sulfur. Either way, improve air circulation and try to keep the water off the foliage. I'd try to avoid using a fungicide as long as possible, but if the problem worsens, take affected foliage to a nursery and ask for help in identifying the problem.