Common Basil Pests & Diseases
Basil is a valuable crop grown around the world as a kitchen herb and for essential oil. It is a soft, cold-sensitive plant and is vulnerable to many pests and diseases. This sheet summarizes the most common problems and the main ways to prevent and treat them, but it is not exhaustive. If you see trouble in your field or greenhouse, contact your local Cooperative Extension office or ag agency promptly and have samples tested; for many problems, a lab test is the only way to be sure of the cause and the right treatment.
Cultural Practices for Promoting Plant Health
Most basil problems are best managed by keeping plants and soil healthy, with good sanitation and crop rotation. That includes:
- Start with high-quality seed that has been tested for Fusarium wilt fungus.
- Grow resistant varieties when available.
- Keep greenhouse spaces clean and sanitized.
- Don't plant basil in the same soil year after year.
- Scout often, and remove and dispose of sick or infested plants right away.
- Water adequately and make sure soil drains well. Drip lines and mulch save water and limit the spread of water- and soil-borne diseases.
- Avoid sprinklers, which help spread disease.
- Keep air moving with enough space between plants, fans, or both.
- Supply enough nitrogen for this leafy crop.
- Keep soil organic matter high to support plant health.
Diseases of Basil
Basil Downy Mildew (Peronospora belbahrii)
A fairly recent arrival in North America, this disease moves fast and can cause heavy losses. It shows as faint yellowing on the upper leaf surface, often in bands, rather like a nutrient deficiency, while purplish-gray spores form a dark fuzz underneath.
Prevention and control:
- U.S. growers can check where it has been reported at https://basil.agpestmonitor.org. If it has turned up near you, start with clean seed of less susceptible varieties. Newer resistant varieties hold up much better than the first resistant releases, and growing them during the period of greatest infection risk is the smartest use. There appear to be regional strains (races) of the pathogen, and race-specific data are limited, so test varieties locally to find the best fit.
- Reduce humidity and leaf wetness, the conditions the disease needs, through good airflow, drip or soaker irrigation, well-drained soil, full sun or good lighting, and sufficient spacing.
- Preventive sprays of OMRI-listed fungicides such as MilStop® SP, OxiDate® and Actinovate® have given limited control indoors and out.
For more on this disease, see our sheet Basil Downy Mildew Awareness & Prevention
Fusarium Wilt and Crown Rot
A vascular wilt, Fusarium wilt and crown rot of basil is caused by the fungus Fusarium oxysporum f. sp. basilici. Leaves curl, yellow and wilt from the crown upward; stems show vertical brown streaks; plants are stunted and may collapse suddenly. Fusarium spreads through infested soil, tools and seed, and by airborne spores.
Prevention and control:
- Have your soil tested. If Fusarium turns up, grow Fusarium-resistant basils; many are available and they work well.
- Seed lots can be lab-tested for the pathogenic Fusarium wilt fungus; ask your seed supplier about testing.
- Light infestations of Fusarium may be held back with OMRI-listed products like Actinovate®, MilStop® SP, OxiDate® and RootShield® Plus.
Gray Mold (Botrytis cinerea)
Gray mold is very destructive. Its brown-gray fuzzy growth shows up on cut stem ends after harvest or on plant debris near the crop. Crowded plants, high humidity and cool-to-mild temperatures give it the best conditions, and spores move easily on air currents, rain and splashing water.
Prevention and control:
- To prevent Botrytis gray mold, keep the area clean, use drip instead of overhead watering, and provide good air movement, heat and ventilation.
- Actinovate®, an OMRI-listed biofungicide, can give some control as a foliar spray.
Basal Root Rot, Damping Off (Rhizoctonia solani; Pythium spp.)
These diseases can strike at any stage but are most severe on seedlings and young plants, which suddenly wilt and collapse. High humidity and stagnant air make them worse. Pythium damping-off is favored by cool temperatures.
Prevention and control:
- In the greenhouse, sterile mix and clean containers lower the risk of root rot and damping-off.
- Root-protecting products such as Actinovate®, OxiDate® or RootShield® Plus add another layer of control.
Leaf Spot (Pseudomonas spp.; Colletotrichum spp.)
Dark spots or lesions on leaves and stems point to leaf spot (sometimes called black spot), but a lab test of a sample is the best way to confirm which organism is involved. High humidity, crowded plants and overhead watering all help these diseases spread.
Prevention and control:
- Bacterial leaf spot can't be cured once it takes hold, so prevent soil from splashing onto leaves. Drip watering and good airflow are the main defenses.
- Mulch helps too.
Pests of Basil
Aphids
Aphids are a common warm-season pest: tiny, soft, usually wingless insects with long antennae. They feed on tender new growth, causing distorted, puckered leaves, spread viruses, and open the door to secondary problems. You can see them without magnification, and they leave a sticky, shiny residue called honeydew.
Prevention and control:
- Insectary plantings of daisy-family and carrot-family flowers nearby attract and support aphid predators such as parasitic wasps, lady beetles, lacewings, mantids and hoverflies.
- When numbers are low, a hard spray of water knocks many aphids off.
- For larger outbreaks, several sprays are available; OMRI-listed options include AzaGuard® and PyGanic®
Japanese Beetles
A serious problem for field growers in the East, these metallic green beetles about ½" long can do a lot of damage fast. Once limited to states east of the Mississippi, they are moving steadily west. They chew large, ragged holes in leaves. Most of their life, roughly 10 months a year, is spent as grubs in the soil before adults emerge in summer to feed and mate.
Prevention and control:
- Reduce exposure by planting in succession, keeping each planting under row cover, and harvesting whole plants.
- Traps can help, but only if used carefully: set them out for a day or two every couple of weeks, upwind and well away from the crop.
- Treating the soil with both beneficial nematodes and milky spore can knock back the grubs and reduce future outbreaks.
- OMRI-listed options include row cover, AzaGuard® and PyGanic®.
Leafhoppers
Leafhopper feeding damage on basil.
Andreas Vietmeier, Landwirtschaftskammer Nordrhein-WestfalenFachbereich 62 – PflanzenschutzdienstSachbereich Gemüsebau, CC BY-SA 4.0, via Wikimedia Commons
Leafhoppers, of which there are many species, trouble both field and greenhouse crops. They are small (⅛–¼"), pale and wedge-shaped. Look for fine stippling or speckling on the upper leaf surface. Besides the cosmetic damage, they transmit viruses.
Prevention and control:
- Pyrethrum, neem and horticultural oil sprays work against leafhoppers. OMRI-listed options include row cover, AzaGuard® and PyGanic®.
Root-knot Nematodes
Nematodes are microscopic roundworms; most are harmless or helpful, but some injure roots and reduce the plant's ability to take up water and nutrients. Affected plants wilt, yield poorly, discolor, and show other signs of nutrient shortage.
Prevention and control:
- Building soil organic matter favors competing organisms, including beneficial nematodes.
- Soil solarization also helps control them.
Slugs
Slugs feed at night and are most active in wet weather. They leave holes and scrape marks on leaves, plus a slimy trail.
Prevention and control:
- Cutting back on overhead watering reduces their numbers.
- A clean, debris-free greenhouse leaves them fewer places to hide during the day.
- Sluggo® is an OMRI-listed bait for controlling slugs.
Whiteflies
Whiteflies, in many species, are among the most common greenhouse pests. They are tiny (about 1/16"), with white bodies and wings, and gather on leaf undersides to feed and lay eggs, damaging leaves and spreading disease.
Prevention and control:
- Yellow sticky cards trap whiteflies and show how bad an infestation is.
- The parasitic wasp Encarsia formosa can be released to control greenhouse whiteflies effectively.
- Pyrethrum, insecticidal soap and horticultural oil sprays can also work. OMRI-listed controls include AzaGuard® and PyGanic®. Always follow the label when using any pesticide or fungicide.
