ENVIRONMENTAL PROBLEMS
Symptoms: Brown leaf tips
Possible Culprits: Excessive fertilizer, watering, high level of fluoride, boron, or copper in the water
Susceptible Herbs: All plants
Solutions: Change watering and fertilizing habits. If symptoms persist, test water quality.
Symptoms: Poor growth, yellowing leaves, new growth shrivels and dies
Possible Culprits: pH imbalance, lack of nutrients, poor drainage
Susceptible Herbs: All plants
Solutions: Check drainage; test soil. Amend with appropriate additives or repot plant to provide better drainage.
PEST PROBLEMS
Symptoms: Stunted and deformed leaves and stems; plant parts covered with sticky, dark substance
Possible Culprits: Aphids: suck sap from plants, producing a sugary “honeydew”
Susceptible Herbs: Calendula, mint, oregano, and rosemary
Solutions: Spray plants with water or insecticidal soap. Or wipe affected areas with a cotton swab soaked in rubbing alcohol.
Symptoms: Wilted leaves, possibly coated with sooty mold; plant loses vitality and eventually dies
Possible Culprits: Whiteflies: feed on undersides of leaves and can cause extensive damage
Susceptible Herbs: Calendula, lemon verbena, and rosemary
Solutions: Spray plants with water. If indoors, lower growing temperature to decrease whitefly activity. Encarsia formosa, a species of tiny predatory wasp, can be an effective control outdoors.
Symptoms: Yellowish or silvery leaves; severe yellowing and rusty spots; fine webbing on leaves and stems
Possible Culprits: Spider mites: tiny pests puncture plant leaves and stems and feed on sap
Susceptible Herbs: Mint, oregano, parsley, rosemary, sage, and thyme
Solutions: Apply horticultural oil or insecticidal soap. Wash plants with a mild solution of dish detergent and water. Prune heavily infested branches and isolate infested plants.
Symptoms: Holes at edges of leaves; plants are eventually defoliated
Possible Culprits: Japanese beetles: metallic green insect with copper-colored wings that feed on leaves
Susceptible Herbs: More than 200 plant species including basil, borage, and foxglove
Solutions: Remove from leaves by hand. Control larvae with parasitic nematodes and the bacteria milky spore.
Symptoms: Yellow foliage; plants lose leaves, weaken, and die
Possible Culprits: Scale: minute, sap- feeding insects
Susceptible Herbs: Wide range of plants, including bay and rosemary
Solutions: Remove with a soft-bristled toothbrush dipped in rubbing alcohol or insecticidal soap spray. Plant herbs to attract beneficials. Horticultural oils may help.
PEST PROBLEMS
Symptoms: Irregular holes in the middle or at the edges of leaves; plants may be defoliated; shiny slime trails on and around plants
Possible Culprits: Snails and slugs: pests eat seedlings and soft-tissued parts at night and on cloudy, damp days
Susceptible Herbs: Bee balm, calendula, sage, and sorrel
Solutions: Remove by hand daily, then weekly, when numbers drop. Edge beds with copper strips or diatomaceous earth. Trap with boards and rocks, or sink beer-filled saucers set into the ground with the rims at soil level.
DISEASE PROBLEMS
Symptoms: Yellow, drooping foliage; plants turn brown and die
Possible Culprits: Verticillium wilt: a fungal disease
Susceptible Herbs: Mint and many other plants
Solutions: Plant disease-resistant varieties.
Symptoms: Weakened stems with flowers develop a fluffy gray or white growth, which spreads to fruits
Possible Culprits: Botrytis blight: a fungal disease
Susceptible Herbs: Rosemary, scented geranium, and many other plants
Solutions: Destroy infected plant parts. Promote air circulation around plants by cutting back or removing plants that crowd each other.
Symptoms: Yellow or white spots on surface of leaves; crusty orange or yellow bumps on undersides; plants become stunted
Possible Culprits: Rust: a disease caused by 4,000 or so related fungi
Susceptible Herbs: Germander, mint, and yarrow
Solutions: Destroy infected plant parts. Promote air circulation around plants by cutting back or removing plants that crowd each other.
Symptoms: Gray or white powdery growth on leaves; new leaves are distorted in shape; poor growth and low yield
Possible Culprits: Powdery mildew: a fungal disease that thrives in hot weather
Susceptible Herbs: Bee balm, catmint, germander, and lemon
Solutions: Promote air circulation around plants by cutting back or removing plants that crowd each other. Spray affected plants with sulfur, lime-sulfur, horticultural oil, or a weak solution of baking soda and water.
Symptoms: Yellow foliage; brown coloration along leaf edges; plants wilt and become stunted; roots soft and waterlogged
Possible Culprits: Root rot: a fungal disease
Susceptible Herbs: Oregano, rosemary, sage, tarragon, and thyme
Solutions: Use sterile potting soil when propagating plants. Avoid overwatering, but do not let plants dry out between waterings. Remove and destroy affected plants.
Symptoms: Yellow stunted leaves; wilting
Possible Culprits: Nematodes: microscopic wormlike creatures feed on roots, leaves, and stems and spread readily in water and on tools
Susceptible Herbs: Calendula and parsley
Solutions: Remove and destroy infested plants. Amend soil with compost, organic fertilizers, and products containing seaweed.