Downy mildew is capable of causing damage to a hop plant in two ways:
- Above-ground foliar infection, which can ultimately spread to cones and turn them dark brown.
- Systemic spread into the crown of the hop plant, causing crown rot and a decrease in plant vigor year after year.
Interpreting the graph:
Horizontal axis: Percentage of shoots infected with downy mildew, a measure of resistance to foliar infection. The greater the percentage of shoots infected, the more susceptible the variety to foliar infection.
Vertical axis: Number of shoots produced, a measure of resistance to downy mildew crown infection. The fewer shoots produced, the more susceptible to the grown rot phase the variety might be.
Section 1: These are varieties expected to possess the greatest resistance to both foliar infection and tolerance to crown infection. These are the most preferred varieties for downy mildew resistance, and will require the fewest number of spray applications to control the pathogen. Historically, a light spray program involves 0-4 well ntimes sprays. It is important to recognize that no variety is completely immune to downy mildew.
Section 2: These varieties are highly suseptible to foliar infection, but maintained high shoot vigor over 3 years of downy mildew infection, indicatinf some resistance to crown rot. it is difficult tio make a general reccomendation for the group because the foliar ssuceptibiliy and crown rot susceptibility are antagonistic. A regular spray program will generally be required for this group, but this may vary by variety based on geography and how favorable a given season is for downy mildew.
Section 3: These varieties produced few infected shoots, but had poor vigor over the 3 year study. Depending on the variety, this poor vigor could be caused by susceptibility to downy mildew crown rot, or could ne due generally to poor vigor of a variety. Similar to section 2, these varieties will generally require a regular spray program, but it is improper to make a broad reccomendation for this group based on what is currently known.
Section 4: These varieties are highly susceptible to both foliar infection during the season, and reduced vigor from season to season downy mildew exposure. These varieties are generally highly susceptible to reductions in both yield and vigor due to downy, and will require a regular to heavy spray program to manage the disease. Intensity of the spray program will vary based on past disease pressure and amount of rainfall in the current season.
Authors: Bill Weldon, David M. Gadoury, David H. Gent
Citation:
Woods, J., and Gent, D.H. 2016. Susceptibility of Hop Cultivars to Downy Mildew: Associations with Chemical Characteristics of Region of Orgin. Plant Health Progress 17:42-48.