Basil or sweet basil (Ocimum basilicum) is one of the most popular herbs in our summer gardens, but in recent years, gardeners worldwide have reported difficulties growing it well. The plant appears fine when first purchased, but declines soon after. What’s going on?
The sad truth is that sweet basil has been hit almost simultaneously by two new diseases, basil fusarium wilt (Fusarium oxysporum basilici) and basil downy mildew (Peronospora belbahrii)… and individually or together, they cause a lot damage.
Basil Fusarium Wilt (Fusarium oxysporum basilici)
Fusarium wilt arrived first, reaching much of North America between 2004 to 2009; a bit earlier in some other parts of the world. It is characterized by dark brown stem cankers that eventually encircle the stem, cutting off sap flow to the leaves. Then the stem withers and collapses and the foliage turns black. When it hits young seedlings, rot starts at the base of their stem, causing rapid death.
There is no cure for basil fusarium wilt once symptoms are apparent. However, since fusarium is readily spread by contaminated seeds, you can go a long way in preventing wilt by choosing seeds certified free of the disease. Also, there is an increasingly wide range of cultivars of sweet basil that are resistant to fusarium wilt, most with the original disease-resistant strain, ‘Nufar’, as one parent. Plant these and you can essentially forget about this first disease.
I’ve been growing ‘Nufar’ basil after a first incident with wilt about 10 years ago and haven’t had a shadow of a problem since… with fusarium wilt that is. Then last summer, my basil plants were hit by something even newer and quite possibly worse: basil downy mildew.
Basil Downy Mildew (Peronospora belbahrii)
Of course, I was asking for it. I knew that buying basil plants was far riskier than growing them from seed, but I had accidentally lost my basil seedlings earlier that spring (they dried out while I was away on a trip). Wouldn’t it be faster to simply buy a few basil plants? So I went to a local public market, bought three healthy plants from an apparently reputable merchant… and within a month, I knew I had made a serious mistake.
Basil downy mildew is an even more recent introduction to basil disease lineup: it first showed up in Florida in 2009 and in many areas around the world, has only appeared in the last 3 years.
The first symptom of basil downy mildew is a slight yellowing of the cotyledons (seedlings) or lower leaves (more mature plants). This yellowing is concentrated between the leaf veins. It’s visible if you know what you’re looking for, but could easily be mistaken for a slight case of sun damage. Later, a thin layer of dark gray down appears underneath the leaves which then turn even yellower. Finally dead brown patches (necrosis) appear. In wet weather, the disease rapidly travels throughout the rest of the plant, although the upper leaves still remain less affected than the lower ones. The crown and roots are not affected.
To date, no sweet basil (O. basilicum) with good basil downy mildew resistance has been found*. In tests, most are found to be “highly susceptible”. Purple basil, a deep purple form of sweet basil of which there are numerous cultivars, is a bit more resistant, rating “moderately susceptible”, as does one green leaf cultivar ‘Midinette’. ‘Red Rubin’, although a purple basil, is “highly susceptible”.
*Update: A powdery mildew resistant sweet basil was introduced in 2018-2019 under the name Amazel™.
Other basil species, such as lemon basil (O. citriodorum), are either less susceptible to this disease than sweet basil or even highly resistant (hoary basil [O. americanum] is in this group). However, they don’t taste like sweet basil and therefore don’t really make good substitutes.
What to Do?
With two diseases simultaneously killing off sweet basil plants all over the world, what the average basilphile can do? Here are a few suggestions:
1. Avoid buying basil plants. If these diseases are spreading at the speed of lightning, it’s largely because they are so prolific in a production greenhouse environment. And it is almost impossible to properly sterilize a greenhouse once it has been contaminated. It is therefore wise to assume that any basil plants, whether sold in garden centers, supermarkets or farmers markets, are already infected. Instead, grow basil yourself, from seed.
2. Buy seed from a reliable merchant, because both diseases can be transmitted via contaminated seeds. Some merchants use steam treatment to destroy disease spores. Richters and Johnny’s Select Seeds are in this group. Look for a mention saying something like “Tested free of downy mildew and fusarium.
3. Use one of the fusarium resistant strains of sweet basil such as ‘Aroma 2’, ‘Envigor’, ‘Gecofure’, ‘Gemma’, ‘Geniale’, ‘Nufar’, ‘Plenty’ et ‘Poppy Joe’s Basil’, at least until basil strains that are resistant to both fusarium wilt and downy mildew are found or developed, That way you’ll keep at least fusarium wilt at bay.
4. Plant basil in full sun and space the plants well. This well help the leaves to dry out more rapidly after rain or dew and that will reduce the risk of infestation.
5. Water the soil, not the leaves since both diseases spread mostly when the foliage is moist.
6. Destroy any infested plants you find. Healthy leaves of infested plants remain edible, though, and be harvested and used.
7. Always use a 4 or 5-year rotation in the home garden because the spores can overwinter in the soil where basil was grown the previous year and can live in the soil for a number of years.
8. If possible, grow basil in pots rather than in the ground. Decontaminating garden soil is next to impossible, but, by changing the soil every year and cleaning the pot thoroughly (a wash in soap and warm water will suffice) before using it again, you’ll go a long way in preventing the diseases from attacking your seed-grown plants.
9. There is little use in applying fungicides. Few fungicides, whether organic or synthetic, home-made or commercially produced, have been found effective against these diseases and the few that are have are not available to home gardeners. And do you really want to spray over and over again all summer to try and stop these disease? Growing basil from seeds in fairly sterile conditions will give you better results with far less effort.
Yes, growing healthy basil is more complicated than it used to be, but if you’re just a bit more careful in how you grow it, you can still have great success!