Root rot is a common, often fatal condition affecting rosemary plants, typically triggered by excessive moisture and poor soil drainage. Rosemary (Salvia rosmarinus), native to the arid Mediterranean, is highly susceptible to this soil-borne fungal disease, often caused by pathogens like Pythium or Phytophthora. The infection cuts off the plant’s ability to absorb water and nutrients, leading to a visible decline. Recognizing the signals above ground is the first step in diagnosis.
Visual Indicators of Distress (Above Ground)
The initial signs of root rot often mimic severe underwatering. A primary indicator is general wilting or drooping of the evergreen foliage that does not improve after watering. The leaves, normally firm, may soften and appear droopy. This occurs because damaged roots can no longer transport moisture, essentially causing a functional drought.
Foliage discoloration is another clear symptom, typically starting with older, lower leaves. These needles turn yellow, progressing to brown or black, often starting at the tips. In advanced stages, entire branches may suddenly turn brown and die back. Stems near the soil line might also become soft, dark, or weak as the infection spreads into the root crown. The combination of wilting and discolored foliage is a strong signal that the plant’s vascular system is compromised.
Confirming Root Health (Below Ground Appearance)
Definitive diagnosis requires carefully removing the rosemary from its container or the ground to inspect the root structure. Healthy rosemary roots should appear light-colored, ranging from white to a pale, woody tan. They should feel firm and pliable, with a robust structure. This contrasts sharply with the appearance of diseased roots.
Affected roots will be dark brown or black, appearing waterlogged and decaying. They lose structural integrity, becoming soft and mushy to the touch. A severe infection causes the root tissue to slough off the central core easily when pulled or rubbed. The soil and roots may also emit a distinct, foul, sour, or musty odor, signaling anaerobic decomposition.
Action Plan for Affected Plants
Immediate action is necessary once root rot is confirmed. Begin by gently removing the plant and washing away all soggy soil from the root ball with lukewarm water. Use sterilized scissors or pruning shears to perform ‘root surgery,’ cutting away every piece of dark, soft, or mushy tissue. Only the firm, light-colored, healthy roots should remain.
Brown or dead foliage and stems should be pruned away to reduce demand on the compromised root system. The plant must then be repotted into a new, clean container with ample drainage holes, or replanted in a location with improved drainage. Use a fresh, sterile, well-draining potting mix, ideally amended with materials like perlite or coarse sand to mimic the rosemary’s gritty habitat. Establish a reduced watering schedule, allowing the top two inches of soil to dry out completely before watering.