astrophytum myriostigma

Cause

Astrophytum myriostigma, like other cacti, is prone to rot that appears at the base of the plant as a black, discolored patch with soft, pulpy plant tissue behind it, and to a root rot in which stems and roots turn brown, soften and spread; this rot is invited by the moist, poorly drained soil that the sun-loving, dry-habitat Bishop's Cap cannot tolerate, especially when overwintered in cold wet compost.

Solutions

• Remove the discolored black patches and soft pulpy tissue at the base of the plant, allow the cut surfaces to dry and callus, then spray with a copper-, sulfur- or neem oil-based fungicide.
• Repot the trimmed plant into a fresh, sharply drained cactus mix of 1 part commercial potting soil to 1 part sharp sand and do not water for at least a week after repotting.
• Cut watering back to just enough to prevent shriveling in winter and let the soil become almost totally dry between drinks in spring and fall, so the base of the plant never sits in cold, wet compost.

Cause

Astrophytum myriostigma plants under three years of age or 2 inches in diameter (under 5 cm wide) are especially susceptible to injury from overwatering because their small, underdeveloped root systems cannot tolerate waterlogged soil; in spring and fall the soil should become almost totally dry before watering, in summer it should dry only moderately, and in winter the plant should be watered only enough to keep it from shriveling.

Solutions

• Allow the soil to become almost totally dry between waterings in spring and fall, and only moderately dry in hot summer weather, before giving a thorough soaking.
• Cut winter watering back to just enough to prevent shriveling, with cooler temperatures requiring still less water, and never let the pot stand in a saucer of drained water.
• Repot into a sharply drained cactus mix of 1 part commercial potting soil to 1 part sharp sand (plus bone meal and ground limestone per the source), and withhold fertilizer from newly potted plants for the first year.