Downy Mildew on Hops Present in WNC

This week I have been visiting with our WNC hops growers. To my dismay, I found the dreaded downy mildew, Pseduoperonospora humuli, in two of the yards.

We saw the beginnings of this problem late last season on the hop cones. See my post from September 2008 on downy mildew of hops.

When determining if you have downy mildew in your hops yard, look for pale green to yellow newly emerged spikes that appear stunted.

Note the pale green, stunted spikes of this hops plant.

A closer look at the pale green to yellow new spike. Note the shorten internodes and miniature leaves.

Another shot of the infected, newly emerged shoot. Take note of the curled and brown lower leaves.

A close up of the browning, curled leaves on the hops shoot. The leaf looks dirty, but those are actually millions of downy mildew sporangia!

A hops leaf exhibiting foliar symptoms of downy mildew.

The underside of the downy mildew infected leaf. Note the fuzzy spores.

The worst downy mildew infection that I noted was on 'Nugget'. 'Nugget' is considered to be susceptible to downy mildew. Interestingly, in the hops yard where downy mildew was found on 'Nugget', it appeared to be the most vigorous variety.
**Update May 21**
Some sources report that 'Nugget' is resistant to downy mildew, while others report it is susceptible.

Downy mildew was also found to be prevalent on 'Centennial' which is considered to be tolerant to downy mildew.

One of the best management strategies for the control of downy mildew on hops is to remove primary basal spikes and to heavily prune and strip leaves. This is important in order to prevent downy mildew from moving up the bines and eventually infecting the cones. Pruning and thinning also helps to reduce moisture and humidity in the lower part of the canopy which is favorable for disease development.

Hops yard with lower leaves stripped up to 3 feet.

Because we are expecting wet weather in the next few days here in WNC, action needs to be taken quickly to prevent the spread of disease to other bines. The first step is to prune and thin the bines.

In addition, the application of fungicides is recommended as soon as the wet weather is over or before if product is rainfast. For a complete list of fungicides (conventional and organic) used to control downy mildew on hops, visit the Oregon State University Downy Mildew Fact Sheet. Before you apply any pesticide make sure you follow the specific label directions and make sure that the product is labeled for downy mildew on hops in NC.

*UPDATE*

Organic Fungicides Labeled for Management of Downy Mildew of Hops in NC:

• Sporatec (rosemary oil, clove oil and thyme oil)
• Copper products (look for OMRI label)
• Sonata (Bacillus pumulis strain QST 2808)
  

Conventional Fungicides Labeled for Management of Downy Mildew of Hops in NC (not a complete list):

• Curzate (cymoxanil) - must be tank mixed with a fungicide of different mode of action, such as copper
  
• Tanos (cymoxanil + famoxadone) - must be tank mixed with a fungicide of different mode of action, such as copper
• Acrobat and Forum (dimethomorph) - must be tank mixed with a fungicide of different mode of action
• Fosphite and other phosphorous acid products
• Legion (Aluminum tris)
  

The best approach would be to incorporate a few of these products in order to slow down the possibility of fungicide resistance. Again, always follow the directions on the label - it's the law!