Thai, Lemon, Holy and Purple Basil: What Actually Changes Between Varieties
Each basil's flavour comes down largely to one or two aroma compounds, and against downy mildew, breeding counts for more than the name on the label.
Put a leaf of Genovese basil next to a leaf of Thai basil and the difference is obvious before you taste either. Most of it comes down to aroma chemistry. In a 1996 field evaluation of 42 basil cultivars at Purdue University, the aroma oil of ‘Genovese’ was 77% linalool, the main compound in sweet basil oil, while one Thai entry was 90% methylchavicol, which smells of anise. Lemon basils get their scent from citral.
So choosing a basil is mostly choosing an aroma. The second choice, if you garden where basil downy mildew has arrived, is how well the plant resists it, and there the species and the breeding count for far more than the colour or the name.
Basil is several species sold under one name
Most basils in seed catalogues are one species, Ocimum basilicum. NC State Extension notes that “Thai and Genovese basil are cultivars of this plant”, and most purple basils belong here too. Holy basil, or tulsi, is a separate species, Ocimum tenuiflorum, previously known as O. sanctum. Lemon basils are classed as O. × citriodorum, a hybrid. Names can mislead. In the Purdue trial a cultivar called ‘Holy Sacred Red’ is listed as ordinary O. basilicum, with a sweet basil aroma.
What each basil brings, compared across two studies
The table lines up Purdue’s aroma data against a peer-reviewed 2010 HortScience field trial in New Jersey, which scored downy mildew from 0 (no symptoms) to 3 (heavy, dense sporulation) on 21 September 2009. At least six cultivar names appear in both, though seed sources may differ.
| Cultivar | Species | Main aroma compounds (Purdue, 1996) | Downy mildew, 0 to 3 (New Jersey, 21 Sept 2009) |
|---|---|---|---|
| Genovese | O. basilicum | linalool 77% | 3.00 |
| Italian Large Leaf | O. basilicum | linalool 65%, methylchavicol 18% | 3.00 |
| Cinnamon | O. basilicum | linalool 47%, methylcinnamate 30% | 2.50 |
| Red Rubin (purple) | O. basilicum | linalool 70%, methylchavicol 10% | 1.00 (3.00 at a second site) |
| Lemon Mrs. Burns | O. × citriodorum | linalool 61%, citral 16% | 1.00 |
| Spice | O. americanum | β-bisabolene 33%, 1,8-cineole 32% | 0.00 |
In Purdue’s table, linalool is the larger share even in the lemon basil, and the cultivars named ‘Anise’ and ‘Licorice’ were also linalool-led, with methylchavicol at 12% and 13%.
Even “Thai” isn’t a single chemistry. Purdue grew four Thai-type sweet basils (O. basilicum), and their methylchavicol share ranged from 13% to 90%. Holy basil varies just as much. Purdue’s holy basils were led by β-caryophyllene, while a 2014 paper on an Alabama field trial found one accession rich in eugenol and another rich in β-caryophyllene, with the balance shifting by harvest date. The authors describe the chemotypes as “morphologically indistinguishable”, though the oil typically carries “an appreciable note of cloves and sometimes licorice”. If the anise or clove note matters to you, smell a plant before you buy a tray of them.
Which basil for Thai cooking?
A Northern Illinois University guide to Thai garden plants says Thais stir-fry holy basil “with red hot chili and any kind of meat, except fish”. Its companion page puts the basil it calls sweet basil into coconut-milk curries such as red curry, or eats it raw with Thai salads. So holy basil suits the stir-fry, and a sweet or Thai-type O. basilicum the curry.
Purple basils are mostly about colour. The purple comes from anthocyanins, and Purdue notes the pigments are “genetically unstable”, which is why some plants develop green patches as the season goes on.
Which basils hold up against downy mildew?
Sweet basil is the most vulnerable. Basil downy mildew, a fungus-like pathogen spread on the wind and in seed, was first confirmed in the United States in 2007. It was identified in Australia in 2017, in Queensland, and a 2018 Queensland government report recorded spread to every state other than Tasmania and Western Australia.
It starts as yellowing on the upper leaf that stays boxed in by the veins, and UConn notes it can look like a nutrient deficiency. In humid weather, purplish brown to greyish fuzz appears on the undersides. NC State suggests removing infected plants, and UConn advises against saving seed from them.
In the New Jersey trial, “all sweet basils evaluated in this study were susceptible”. Purple basils were no exception. ‘Red Rubin’ rated 1.00 at the main site but 3.00 at a second site in Pittstown, as did ‘Rubin’ and ‘Opal Purple Variegated’. Lemon and lime basils showed far less disease, and three spice basils showed no symptoms at all. Spice basil is edible, but the authors note these resistant types aren’t acceptable as sweet basil because of their distinct look and aroma, so treat it as its own herb.
An NC State factsheet lists the Thai basil ‘Queenette’ among varieties with less severe symptoms “in a previous study”, likely this one. The 2010 paper’s ‘Queentette’, likely the same cultivar, scored 3.00, the top of the scale, at both sites. The trial data don’t support counting on it in a mildew-prone garden.
Do the new resistant sweet basils work?
Mostly, yes. Rutgers University bred four downy mildew resistant sweet basils, ‘Rutgers Devotion DMR’, ‘Obsession DMR’, ‘Passion DMR’ and ‘Thunderstruck DMR’, released to commercial growers in 2018 with home garden seed from 2019. In a 2019 Cornell field trial with no fungicides, the Rutgers lines gave 82% to 94% control compared with a susceptible variety, and ‘Prospera’ gave 100%.
Resistant plants can still show some disease. The trial notes that symptoms “eventually” appeared on all resistant cultivars, though the share of affected leaves stayed low. On flavour, the evidence is mixed: an informal Cornell tasting preferred the susceptible variety, while NC State’s volunteer trial rated Rutgers Obsession best among resistant types.
Good habits help any variety. UConn advises generous spacing, full sun, watering early or under the leaves, and clean seed.
Growing basics that apply to all of them
For sweet basil, NC State advises full sun, moist but well-drained soil, and planting out after your last frost. Pinch the tips and any flower buds to keep plants bushy; Rutgers breeder James Simon notes that flowering “can make the leaves taste bitter”. Holy basil also wants sun but does poorly in waterlogged soil, and NC State warns it “can be considered an aggressive weed” outside its native range, so a container is the tidy option. For basil on a windowsill, see our guide to indoor herbs.
What this means for you
- Choose by aroma. Linalool-rich types for classic sweet basil, methylchavicol for anise, citral for lemon.
- For Thai cooking, grow holy basil for stir-fries and a sweet or Thai basil for curries.
- Where downy mildew is about, pick a bred-resistant sweet basil or a lemon basil, and don’t rely on purple basils or Queenette.
- Give every basil sun, space and dry leaves, and pinch out flowers as they form.
Which basil would you grow if you could only have one, and has downy mildew reached your garden yet? Hit reply and tell us.
How we made this: AI tools helped us research and draft this post. We checked every figure, quote and link against its original source before publishing. Figures are as of 8 November 2026.
Grown with intention by Dynamic Nexus Garden.