What Is It? | Facts in Depth | For the Professional Diagnostician
Hop Diseases | Downy Mildew
Hops Downy Mildew
Identification
Typical symptoms of the disease are angular, chlorotic lesions visible on the upper side of the leaf. Purple–grey or black growth, which includes the sporangia of the pathogen, may appear on the bottom of the leaf or spike.
One of the most characteristic downy mildew symptoms is the presence of systemically infected shoots, called “spikes”. Primary spikes emerge from the crown, while secondary spikes are diseased shoots that appear from the growing point of an infected plant. Spikes are stunted, with short internodes, and appear chlorotic with yellow green, downward curling leaves.
The plant tissue below the infection point looks normal in appearance with a secondary spike. Infected cones may exhibit dark brown discoloration. Infected crowns of hops may show reddish-brown to black flecks and streaks due to downy mildew infection.
Pathogen Biology
The hops downy mildew pathogen, Pseudoperonospora humuli, is an obligate biotroph and can only survive in infected plant materials, such as leaves, shoots, cones, and even dormant buds and crowns. When environmental conditions are favorable in the early spring, the mycelia grow and and start to infect healthy tissues. This is especially apparent in the buds once dormancy breaks, resulting in the characteristic “spikes” from new shoot growth described in the identification section. Spore-bearing structures, known as sporangia, are produced on the undersides of leaves of infected plants as a blackish-purple mold. The sporangia produce numerous motile zoospores that enter the host through stomata via free water. The foliage and shoots are further infected and the cycle repeats until the growing season is over.
Favorable Environmental Conditions
This pathogen thrives in moist environments with moderate temperatures. The pathogen produces sporangia when relative humidity is > 70%. Leaf wetness duration is also critical for leaf and shoot infection (at least 4-8 hours at moderate temperatures).
Often Confused With
- Frost damage – Causes similar symptoms to those caused by hops downy mildew. The foliage becomes very chlorotic and stunted and necrosis forms in new leaves and shoots of the plant, which resembles the “spikes” produced from downy mildew infection.
- Two-spotted spider mites
Scouting Notes
The crop should be monitored regularly to maximize chances of finding the disease, as well as to determine disease severity and pathogen spread in the hop yard. Some characteristic symptoms of downy mildew on hops includes purple-grey or black mold on the undersides of leaves and the production of “spikes” from new infected shoots. Monitoring the weather conditions required for downy mildew infection may help to predict disease infection time.
Thresholds
There have been a few studies predicting the risk index for infection, but there are no concrete threshold models as of now.
Management Notes
- Avoid Excessive Wetness on Foliage – Avoiding long periods of leaf and cone wetness with an appropriate irrigation system is critical to downy mildew management since the pathogen needs excessive wetness in order to germinate and thrive.
- Variety Selection – Many cultivars of hops have been developed that are resistant or partially resistant to downy mildew. Further information on highly or partially resistant hop cultivars can be found in the cooperative publication produced by Oregon State University, University of Idaho, USDA-Agricultural Research Service, and Washington State University.
- Use Disease-Free Plants – Planting disease-free hop rhizomes or plugs is very important. NCPN-Hops was added to the National Clean Plant Network in 2010. Currently there is one center in the network, the Clean Plant Center Northwest (CPCNW) at the Washington State University research and extension center.
- Use Sound Sanitation Practices – Severely diseased plants must be removed from the hop yard. Late winter or early spring pruning can significantly reduce primary inoculum and spread of downy mildew. The timing and quality (based on the amount of foliage remaining on plants following pruning) of spring pruning are important considerations for managing downy mildew. Hop plants must be cleaned of leaves and laterals from the lower few feet of the bine (stripping). This encourages airflow, decreases chances of disease development and directs nutrients to the upper portion of the bines where most of the cones will develop.
- Use Fungicides – When preventive and cultural methods for downy mildew control are insufficient to manage the disease, chemical and biorational products can be applied. There are several preventative fungicide groups that can be applied pre-harvest that have proven effective in controlling downy mildew, including copper, metalaxyl, cymoxanil, mandipropamid. Some biorational products that can be applied are Actinovate (Streptomyces lydicus), Sonata (QST strain 2808 Bacillus pumilus), and Regalia (extract of Reynoutria sachalinensis). More information on these biorational products can be found at: http://attra.ncat.org/attra-pub/biorationals/.