Here are some important points to consider while managing downy mildew of onion:
Question: “I found some downy in my onions today. It has been sunny, clear, and low humidity all week long. I thought downy liked humid, wet, cloudy weather. Why am I seeing it?”
Response: Yes, you are correct. Downy mildew development is favored by rain or dew on leaves, high humidity, and cool weather (below 70 degrees F). Warm, sunny weather with low humidity drastically slows down the disease. However, keep in mind that the downy mildew infection cycle on onions can take anywhere from 10 to 18 days, and it can take that long for the symptoms to show up. This means that what you just found this week could have been infected over 2 weeks ago when the weather and conditions were favorable. Weather conditions this week likely slowed down further infections (but we are not out of the woods yet!)
2. Question: “I just sprayed my onions with Orondis last week. This week I found some downy. I thought Orondis (or other fungicide) was supposed to “kill” downy and have a “curative” effect. Why did it still show up?
Response: Orondis is a great option for downy prevention and control. Here are a few things to keep in mind: There is not any fungicide that is going to give you 100% control when downy is present and conditions are favorable for disease. In fact, under extended or prolonged favorable conditions, it can be quite hard to stop the disease from spreading. I don’t want you to think you are wasting your money on a fungicide if you are still seeing some downy. It would likely be much worse if you did nothing.
3. Can I tank mix Orondis Ultra with “__________” fungicide?
Response: Orondis Ultra is a relatively new fungicide to our onion market. Many of us are trying some new tank mixes with it for the first time on onions. It is impossible to know and predict what will happen to any spray product mixture because there are so many factors to consider (rate, pH of water, use of adjuvants, temperature, humidity, sunny, cloudy, etc…..) The only way to know for sure about a new mixture is to do a “jar” test to see if it will stay in solution. I will tell you this: at the Onion Research farm, we have sprayed Orondis Ultra with Bravo and surfactant for an application, and we have also sprayed it with Luna Tranquility plus surfactant. Dr. Dutta, our pathologist has also used it with Phosphorous Acid products (Kphite, Phostrol, etc.) We have not seen any issues with these mixtures at our farm. I called the folks at Syngenta, the makers of Orondis, to see if they could tell me any additional info. They don’t have any additional tank mix info to add other than that they believe it tank mixes well with most other fungicides, and that we need to use a surfactant with it to help it get into the plant.
4. Can I spread downy on my farm with equipment or people?
Response: Yes. The worst time to spread it is probably early in the day when dew is present. If you find some, you should not go to any other fields without changing clothes and boots and washing your hands. This is also true for equipment. If you have a known spot of downy, spray that area last, and wash off your equipment before you go into another field.
5. What fungicides are “systemic” (get into the plant) and which ones are “protectant” (don’t get into plant but provide a layer of protection)?
Response: Orondis Ultra, Zampro, Omega 500, Phosphorous Acid products, and the “Ridomil” part of Ridomil Gold Bravo are either systemic or locally systemic. Bravo and Manzate are protectants. One way to further enhance your disease protection is to use both a “systemic” and “protectant” when you spray. For example, Orondis Ultra + Bravo, or Bravo + Phosphoric Acid, or Zampro + Manzate. However, do not mix Phosphorous Acid products with Manzate. Some fungicides are combinations of both systemic and protectant, like Ridomil Gold Bravo and Orondis Opti (Orondis + Bravo)