As most growers are aware, downy mildew is extremely active in Ontario hops this year due to the ideal conditions for development of the disease. For new growers, an article on identifying symptoms of this disease can be found in this blog post (https://onspecialtycrops.wordpress.com/2014/06/03/identifying-downy-mildew-in-hops/). Although some areas of the province have dried up somewhat, it is still important to continue sprays of preventative fungicides, as the disease can easily spread to the cones. Given the high levels of the disease across the province it is important to continue with sprays to protect developing bracts before cones close up.
For conventional growers, rotations of Zampro (ametoctradin + dimethomorph – Group 40/45), Torrent (cyazofamid – Group 21) , Revus (mandipropamid – Group 40) and Copper 53W (copper sulphate – Group M) should be continuing on a 7-10 day interval. In areas of the province that are drier and have less active infection, it may be possible to stretch the application interval longer, Organic growers have more limited options for control of downy mildew. Copper 53 W is likely the most effective organic product, but is limited to 4 applications per season. This can be rotated with Regalia Maxx and Serenade Opti, however both of these products are labelled for partial suppression only of downy mildew.
Because the downy mildew fungicide toolbox in Canada is much more limited than in the United States, it is very important to rotate products to avoid the development of resistance. Do not make sequential applications of the same product and always follow an application of a group 40 product with one from a different fungicide group (so Zampro and Revus applications should always be separated with an application of Torrent or Copper).
Pre-harvest intervals for downy mildew products range from 0 days (Serenade and Regalia), to 2 days (Copper 53 W) to 3 days (Torrent), to 7 days (Zampro and Revus). Zampro, however, is a bit confusing. While Zampro has a 7 day pre-harvest interval, it has an 18 day restricted entry interval hand harvesting, stripping or training. This means that growers planning on hand harvesting hops cannot apply Zampro within 18 days of harvest, even though the PHI is 7 days.
With powdery mildew also active in Ontario hops, many growers are adding powdery mildew fungicides to their spray rotations. It is important to be aware that most powdery mildew products do not control downy mildew and should not be considered as replacements for typical downy mildew products.
We have had a lot of questions about Pristine (boscalid + pyraclostrobin). Pristine is a good powdery mildew product which is also labelled for suppression of downy mildew. However, this product has limited effect on hops downy mildew and should always be applied to target powdery mildew. If Pristine is applied for powdery mildew control during periods that are not conducive to downy mildew (e.g. dry periods with little to no active downy infections), then the application of Pristine may allow growers to stretch out the period between sprays of downy-specific products. However if conditions become rainy or there are high levels of downy mildew infection in the hop yard, growers should go in with a fungicide that is stronger against downy mildew.