Yes, your basil can look this good again thanks to improved disease-resistance in recent hybrids. This is Prospera® DMR. Photo: veseys.com
Did your sweet basil (Ocimum basilicum) turn black and die last summer? Millions of plants did around the world, with the continued spread of two serious and usually fatal (to the plant!) basil diseases: basil fusarium wilt (Fusariumoxysporum basilici) and basil downy mildew (Peronospora belbahrii). Individually or together, they cause a lot damage.
Both diseases seem to have come out of nowhere: basil fusarium wilt (Fusarium oxysporum basilici) or FW between 2004 and 2009 and basil downy mildew (Peronospora belbahrii) or DM by the mid-2010s. They’re now well established on every continent except Antarctica. Undoubtedly they transferred from a wild species and found sweet basil an appropriate host.
Note that most other species of basil—lemon basil (O. × citriodorum), American basil (O. americanum), etc.—appear to be moderately to highly resistant to both diseases. In general, the less a basil looks, smells and tastes like sweet basil, the less susceptible it will be to both diseases.
FW causes dark brown stem cankers that cut off sap flow to the leaves. Then the stem withers and collapses and the foliage turns black. It can attack both seedlings and more mature plants. It’s a soil- and seed-borne fungus. It seems to be the lesser of the two diseases these days, as serious seed companies are now supplying seed tested to be free of spores. So, unless your soil is already contaminated (the spores can survive freezing temperatures), you might be able to keep it at bay by buying seeds from serious suppliers.
DM is a funguslike organism not so conveniently controlled. Carried by the wind for very long distances and also by water splashed on basil leaves during rainfall, spraying or watering, its spores can really get around! Judging from the state of the basil plants I’ve been seeing in local community gardens, it’s rampant.
The first symptom of DM is a slight yellowing of lower leaves, concentrated between the leaf veins. Later, a thin layer of dark gray down appears underneath the leaves which then turn even yellower, then necrosis (dead brown patches) appears. 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.
Developing Resistant Cultivars
Researchers have spent a lot of effort in developing disease-resistant cultivars. Some varieties of sweet basil (O. basilicum) have a natural resistance that has been enhanced by careful breeding: crossing resistant plants together, then selecting those with the best resistance for further crosses. No, the improved basils are not GMOs (genetically modified organisms): this is just the same old crossing and selection process that humans have been using for thousands of years to domesticate and improve crops more appropriate to human use. (Every farmer knows that if you want a strong work horse, you breed two strong work horses together.)
Rutgers University of New Brunswick, New Jersey (the Rutgers series of hybrids) and Cornell University of Ithaca, New York (the Prospera@ series) have been at the forefront of these new developments in the United States. Both series are identified with the abbreviation DMR (downy mildew resistant). In Eurasia, resistant cultivars have mostly been coming from Israel.
Basil Disease Comparison Chart
Here are some cultivars with known resistance to the two diseases:
|Downy Mildew||Fusarium Wilt|
|‘Everleaf Emerald Tower’||IR||IR|
|‘Poppy Joe’s Basil’||S||IR|
|Prospera@ Compact DMR||HR||IR|
|Prospera@ Italian DMR||HR||IR|
|‘Rutgers Devotion DMR’||HR||HR|
|‘Rutgers Obsession DMR’||HR||HR|
|‘Rutgers Passion DMR’||HR||HR|
|‘Rutgers Thunderstruck DMR’||HR||HR|
|‘Sweet Aroma 2’ (‘Aroma II’)||S||IR|
|‘Sweet Aroma 4’ (‘Aroma IV’)||S||IR|
High resistance (HR): usually no symptoms or only minor damage.
Intermediate resistance (IR): shows less severe symptoms or damage than susceptible plant varieties.
Susceptible (S): shows no serious resistance to the disease.
What to Do?
With two diseases simultaneously killing off sweet basil plants all over the world, what is the average basilophile to do? Here are a few suggestions:
- Use one of the disease-resistant strains of sweet basil listed above.
2. 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’s very difficult 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.
3. 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 Herbs and Johnny’s Select Seeds are in this group. Look for a mention saying something like “Tested free of downy mildew and fusarium”.
4. Plant basil in full sun and space the plants well. This will 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 most readily when the foliage is moist.
6. Destroy any infested plants you find. Healthy leaves of infected plants remain edible, though, and be harvested and used.
7. Always use a 4- or 5-year rotation in the home garden to control fusarium wilt, because the spores can overwinter in the soil where basil was grown the previous year and can live there for a number of years. Basil downy mildew does not overwinter outdoors in temperate climates, but rather in greenhouses.
8. If possible, grow basil in pots rather than in the ground, specifically for controlling fusarium wilt. Decontaminating garden soil infected with fusarium spores is next to impossible, but, by changing the soil every year (you could use it for other plants) and cleaning the pot thoroughly (a wash in soap and warm water will suffice) before reusing it, you’ll go a long way towards preventing it 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 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 diseases?
Yes, growing healthy basil is more complicated than it used to be, but if you give disease-resistant varieties a try, you can still have great success!