Infections with downy mildew can be delayed by starting with "clean" planting material. In the case of onion sets, a warm water treatment before planting is one way eliminate any contamination of the planting material. However, this method demands precision, otherwise the treatment is not effective.
With the warm water treatment, it is important that the temperature remains above 40°. After a minimum of one hour at 40°C, the onion sets have been decontaminated. A temperature of 43°C during the warm water treatment has been shown to have no negative effect on germination. However, higher temperatures quickly reduce germination. Treating onion sets with this method requires thorough preparation; besides heating the water, it is important to dry the onion sets afterwards. Tests have shown that planting the onion sets immediately without drying reduces the germination by up to 80%. After drying, a bioassay indicated a germination rate of approximately 97%
Systemic infection and infection sources
The first infections with downy mildew are usually found on waste heaps. The spores spread from this location. Prompt covering of waste heaps prevents contamination of the surroundings. With the onion sets, the first-year onions are contaminated and can cause systemic infections during the second year. Downy mildew then grows with the new shoots and creates spores that spread through the air. The control of downy mildew focuses on interrupting the infection route. This can be done with a warm water treatment before planting and possibly with a heat treatment, such as burning, during cultivation. A study conducted in 2009 showed the following:
- Burning onion foliage destroys downy mildew spores. The greatest control effect is achieved at the top of the canopy, where spores are produced that are usually responsible for spreading into the surroundings;
- After burning, the formation of new blemishes and sporulation are slowed by several days to one week. In this way, further spread of the infection into the crop and the surroundings can be reduced. After this, the foliage burning will probably have to repeated one or more times to sufficiently slow the epidemic.
In addition, in 2009 several agents were tested on downy mildew