Herbs Add Flavor. Mildew, Not So Much.

Downy mildew of basil (caused by the oomycete Peronospora belbahrii) was observed on greenhouse-grown herbs for sale at a large retail operation in New Jersey the last week of June. As the spores (sporangia) of P. belbahrii may spread long distances on air currents, producers, retailers, and home gardeners in the Northeast should be careful to check basil plants regularly for symptoms of this disease now and in the coming months.

Yellowing of the upper leaf surface is often the first symptom of basil downy mildew. Yellowed areas are usually bordered by leaf veins. When spores are produced, a characteristic fuzzy, dark gray to purple growth on the underside of the leaves is evident. Sporulation on the upper surfaces of leaves may be seen in severe cases. Symptoms of downy mildew on basil can easily be mistaken for a nutritional deficiency. The fuzzy growth of spores on the underside of the leaf may look as if soil had been splashed onto the leaf under-surface. Five to ten days may elapse between infection and the appearance of symptoms; the duration of the latency period depends upon temperature and light exposure.

Fortunately, Peronospora belbahrii is not known to survive the winter in the Northeast. The pathogen may overwinter in a greenhouse environment on susceptible plant material, but in the absence of live hosts it is unlikely to persist. It has a very limited host range, infecting only a handful of other Lamiaceae (mint family) plants including coleus, agastache, sage, and rosemary. Nepeta curviflora (Syrian catnip, or “za'tar chachla”) has been experimentally infected, so it is possible that other species of Nepeta may also be susceptible. There are several relatively new resistant cultivars of sweet basil available. These include ‘Prospera’ and ‘Amazel’ as well as the Rutgers DMR series (‘Devotion’, ‘Obsession’, ‘Passion’, and ‘Thunderstruck’); ‘Eleonora’ and ‘Everleaf’ have partial resistance.

One way to protect greenhouse grown herbs from the spread of mildew is delivering CO2 to the plants in aqueous form using CO2 Delivery Solutions™.

Perimeter Protection™

CO2 Delivery Solutions™ is a novel system that delivers CO2 to plants via misting an aqueous CO2 solution directly on to the plant. Providing CO2 to herbs such as basil and cilantro enables an increase in photosynthesis resulting in faster growth and more biomass yield. Faster growth means more harvest turns and more biomass yield means more product to sell.

CO2 Delivery Solutions™ also provides protection against the spread of powdery mildew. The aqueous CO2 solution is acidic, with a pH of 4.5 - 5. Therefore, when the solution is misted on the plant, the pH on the plant surface drops. Once the CO2 transfers into the leaf (within a couple of minutes), the pH bounces back towards neutral. We recommend misting for 5 to 15 seconds every 15 to 20 minutes during the light cycle for photosynthesis, and all day & night if you are concerned about mildew spread. This fluctuation in pH occurring three to four times an hour creates an unfavorable environment for epiphytic microbial pathogens such as downy and powdery mildew and E. coli to multiply and spread. We call this phenomenon Perimeter Protection™.

For more information on how Perimeter Protection™ works, read these previous blogs: Understanding Cannabis Pathogens with Dr. Matt Julius, Most of Colorado’s Failed Cannabis Tests Stem from Microbials and Greenhouse Strawberries - Deliciously Sweet or visit co2delivery.ca

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