The what and why of controlling Downy Mildew
Peronospora is one family among five families of pathogens that cause downy mildews. Peronospora's specialty is basil downy mildew, a destructive pathogen which, if uncontrolled, may cause complete crop loss.
Basil is the golden crop for Upstart Farmers, to whom it has endeared itself as an easy-to-grow, high-demand and high-volume crop.
Haydn Christensen of Bayberry Fresh in Fort Collins, Colorado, has had experience growing basil and controlling basil downy mildew.
He kindly donated some of his time to answer my questions.
Are there certain conditions or times of year that basil downy mildew is especially bad?
"Basil downy mildew likes hot and humid conditions so any time of year that its hot and muggy, rainy, wet, etc. It needs a 12 hour leaf wetness period to germinate so even if you have heavy dew overnight it can flourish."
What are the symptoms/what problems does it cause?
"Early on it can be hard to identify since it causes the leaves to yellow similar to a deficiency or any other ailment. However a very easy to distinguish characteristic is the black/brown sporulation on the underside on the leaf. At this stage it is easy to identify and action must be taken immediately. If the disease progress leaves will become necrotic and the whole plant will die."
How do you recommend controlling it?
"The only chemical controls proven to work controlling early outbreaks are systemic fungicides such as Revus (translaminar). I don't think any organic controls have any effect on the disease once spores have germinated. My recommendation is to monitor aggressively for it and not bring any plant material into the greenhouse. If yellowing leaves are found and basil downy mildew is suspected in the greenhouse apply a chemical or organic protectant to prevent it from spreading and quarantine or destroy infected plants.
"When removing infected plants it is very important to bag them carefully or remove them in such a way to avoid spreading the spores around. For instance, when I removed my infected plants, I made a hole in the wall downwind from the uninfected plants in order to remove the disease from the greenhouse without having to pass the uninfected plants. You can also run exhaust fans at night to help reduce leaf wetness from dew, but if its raining/foggy outside there is little you can do.
"I think as far as control goes it is extremely important to restrict plant material and have a good seed source since this can effectively keep the disease from ever entering your growing operation. I no longer allow plants of any kind in because even plants other than basil can be carrying spores if they came from a nursery where they grew by infected plants."
Note: Haydn grows hydroponically at Bayberry Fresh. If your farm is an aquaponic system, you need to research any fungicides that you use to make sure they're safe for fish.
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