What pests or diseases affect lavender?

Lavender is generally considered a low-maintenance plant and is quite resistant to many common garden pests and diseases due to its strong aromatic oils. However, it's not entirely immune. The vast majority of problems arise from improper growing conditions, particularly related to drainage and humidity.

Here are the most common pests and diseases that can affect lavender:

I. Diseases (Mostly Fungal, Related to Moisture)

1. Root and Crown Rot (Phytophthora spp., Fusarium spp., spp., Rhizoctonia spp.):

   • Cause: This is by far the most common and fatal problem for lavender. It's caused by various soil-borne fungi (or fungal-like pathogens) that thrive in wet, waterlogged, or poorly drained soil, especially during cool, wet periods or over winter. Overwatering is the primary culprit.

   • Symptoms: Wilting, yellowing, or browning leaves (often starting from the base), stunted growth, and a general droopy appearance even when the soil is moist. The roots may appear black and mushy. The entire plant can collapse rapidly.

   • Prevention/Treatment: Prevention is key! Plant in extremely well-drained soil (amend with grit/sand), use raised beds if necessary, ensure good air circulation, and avoid overwatering. Once a plant has severe root rot, it's often difficult to save; removal and disposal of the infected plant (and potentially surrounding soil) are usually recommended to prevent spread.

2. Botrytis (Gray Mold):

   • Cause: A fungal disease that flourishes in high humidity and poor air circulation.

   • Symptoms: Gray, fuzzy mold appearing on leaves, stems, or flowers, often starting at the base of the plant. Infected parts turn brown, yellow, and eventually die.

   • Prevention/Treatment: Ensure good air circulation by proper plant spacing and pruning. Remove and dispose of any infected plant parts immediately. Avoid overhead watering, especially in humid conditions.

3. Septoria Leaf Spot (Septoria lavandulae):

   • Cause: A fungal disease that is common in high humidity and when water sits on lavender leaves.

   • Symptoms: Small, dark spots with lighter centers appear on the leaves. Severe infections can lead to leaf yellowing and drop.

   • Prevention/Treatment: Similar to other fungal issues, focus on good air circulation, avoid overhead watering, and remove affected leaves.

4. Shab (Phomopsis lavandulae):

   • Cause: A fungal infection, though less common in home gardens than root rot. It can be devastating to commercial crops.

   • Symptoms: Shoots suddenly wilt even without drought, and stems may turn brown, developing small black spots (pycnidia). The disease can take months to years to show symptoms.

   • Prevention/Treatment: There is no cure. The only way to stop the spread is to dig up and destroy (burn if possible) infected plants. Good plant hygiene and buying from reputable nurseries are important.

5. Alfalfa Mosaic Virus (AMV):

   • Cause: A virus that can affect many plants, including lavender. It's primarily spread by aphids and contaminated tools.

   • Symptoms: Stunted growth, yellow patches or rings on leaves that may become distorted or contorted.

   • Prevention/Treatment: There is no cure. Control aphids (the main vector), sanitize gardening tools, and remove and destroy (do not compost) infected plants immediately to prevent spread.

6. Xylella Fastidiosa:

   • Cause: A highly destructive bacterium spread by sap-sucking insects (like leafhoppers). More common in specific regions, particularly the Southeast and Southwest U.S.

   • Symptoms: Varies, but can include leaf scorch, wilting, and dieback, leading to plant death.

   • Prevention/Treatment: Difficult to control once infected. Managing insect vectors and removing infected plants are the main strategies.

II. Pests (Generally Minor, Lavender is Often a Deterrent)

Lavender's strong scent generally deters many common garden pests, but some may still appear:

1. Aphids:

   • Impact: Small, soft-bodied insects that suck sap from new growth, causing curling or distortion. While they may appear, they rarely cause significant damage to lavender compared to other plants.

   • Control: A strong spray of water can dislodge them. Encourage natural predators like ladybugs. Insecticidal soap can be used for severe infestations.

2. Whiteflies:

   • Impact: Tiny, white, winged insects that congregate on the undersides of leaves, sucking sap and secreting sticky honeydew.

   • Control: Similar to aphids, a strong water spray or insecticidal soap can manage them.

3. Spittlebugs (Froghoppers):

   • Impact: Nymphs create frothy "cuckoo spit" on stems, usually in late spring/early summer. They feed on plant sap but rarely cause serious damage. The foam is more of an aesthetic nuisance.

   • Control: Usually not necessary. A strong spray of water can wash them off.

4. Spider Mites:

   • Impact: Tiny arachnids that can cause fine webbing on leaves, stippling (tiny dots), and yellowing. More common in hot, dry conditions or indoors with low humidity.

   • Control: Misting plants with water can deter them as they dislike humidity. Insecticidal soap or neem oil can be effective.

5. Leafhoppers:

   • Impact: Small, wedge-shaped insects that suck sap and can transmit diseases like Alfalfa Mosaic Virus or Xylella.

   • Control: Generally difficult to control. Focus on prevention of disease spread through tool sanitation.

6. Rosemary Beetle:

   • Impact: A metallic green and purple beetle that feeds on the foliage and new shoots of lavender, rosemary, and sage. More prevalent in Europe than in the Americas.

   • Control: Hand-picking is often effective.