From our perspective as rosemary plants, we are generally resilient and aromatic shrubs, prized for our culinary and ornamental value. However, even we can fall victim to specific ailments and invaders that compromise our health. Understanding these issues from our point of view is key to providing the correct treatment and allowing us to thrive.
Our tender new growth is particularly appealing to small, soft-bodied insects called aphids. They cluster on our stems and undersides of leaves, piercing our tissues to suck out our vital sap. This weakens us, causing leaves to curl and distort. A similar but microscopic threat is the spider mite, which thrives in hot, dry conditions. They also feed on our sap, leaving behind a tell-tale stippling of yellow dots on our leaves and fine, silken webbing. To treat these, a strong spray of water can dislodge many of the pests. For persistent problems, insecticidal soap or neem oil applications are effective and gentle on our beneficial insect pollinators.
Our most dreaded ailment is not caused by an insect, but by a fungus, often Pythium or Phytophthora. This condition, known as root rot, occurs when our roots are forced to sit in consistently waterlogged, poorly draining soil. We are Mediterranean plants, built for gritty, well-draining earth. Soggy conditions suffocate our roots, making them unable to absorb water or nutrients. The fungus then attacks the weakened roots. Above ground, our foliage turns brown, brittle, and we begin to die back. Prevention is paramount: ensure we are planted in exceptionally well-draining soil and avoid overwatering. If caught early, you may save us by removing the affected parts and replanting in fresh, dry soil.
Sometimes, a white, powdery substance appears on our leaves. This is powdery mildew, a fungal disease that thrives in conditions with poor air circulation and high humidity, especially when our roots are drier than the moist air around our foliage. While it rarely kills us outright, it is unsightly and stresses us by interfering with photosynthesis. To combat this, improve air flow around us by pruning nearby plants and ensure we are watered at the base, not on the leaves. Treatments include spraying with a homemade solution of baking soda and water or a commercial fungicide suitable for edible plants.
Scale insects are particularly sneaky. In their juvenile "crawler" stage, they move about before attaching themselves to our stems and the undersides of our leaves. They then form a hard, protective shell over themselves and remain stationary, feeding on our sap. A severe infestation can cause yellowing leaves, stunted growth, and a general decline in our vigor. They can often be physically scraped off with a fingernail or a soft brush. For larger infestations, horticultural oil sprayed onto the insects will suffocate them effectively.
