Sue Colucci
Area Specialized Agent, Commercial Horticulture
November 18, 2009
Hops Diseases and Insect Pests
Diseases are one of the main reasons why hops production moved to the Pacific Northwest (PNW)
We are unsure what are major diseases will be in western North Carolina so it is important to work together.
History and problems in the PNW tells us that our major disease problems will probably be powdery mildew, downy mildew, viruses and viroids. Other diseases that may pose a problem in our warm, humid environment are Fusarium canker and cone tip blight, Vertcillium wilt, Alternaria Cone Disorder (Alternaria alternata), Red Crown Rot (Phacidiopycnis sp.) and Black Root Rot (Phytophthora citricola).
Our major insects will most likely be two-spotted spider mites and hop aphids. Other insect problems may include caterpillars, Japanese beetles and garden symphylans.
Powdery Mildew
•Fungus: Podosphaera macularis
•Survival: infected buds and overwintering structures (chasmothecia)
•Spread: airborne spores, leaf/buds of infected planting materials, crop debris (sexual spores)
•Yield loss potential: 100% due to crop rejection
•Occurs on all green tissues of the bine
•Different symptoms on different cultivars, weather conditions and fertility
•Obligate parasite (needs host to survive)
•Specific to hops
Management
•Reduce disease pressure throughout season to DELAY and REDUCE RATE of epidemic
•Early control measures key
–Thorough pruning, timely first spray
•Select resistant varieties, if possible
–Susceptible: ‘Glacier’, ‘Perle’, ‘Saaz’
–Resistant/Tolerant: ‘Nugget’, ‘Cascade’
•Avoid excessive nitrogen fertility
•Mid- and late-season basal growth control
•Fungicide applications during flowering and cone development helpful to minimize cone infection
•Early harvest can minimize crop loss when powdery mildew is present
•Intensive preventative fungicide program
•Boscalid, mycobutanil, pyraclostrobin, quinoxyfen, azoxystrobin, sulfur, tebuconazole
•Various petroleum and plant oils, soaps, sodium bicarbonate have direct activity
•Copper and phosphite salts reduce plant susceptibility to infection
Downy Mildew
•Fungus-like: Pseudoperonospora humuli
•Survival: infected rootstock, perhaps sexual spores in soil (oospores)
•Spread: airborne spores, infected planting materials, soil/crop debris?
•Yield loss potential: 100% due to plant death and/or crop rejection
Management
•Early control measures key
•Thorough pruning, late pruning, timely first spray
•Plant disease-free rootstock
•Avoid highly susceptible varieties
–Susceptible: ‘Nugget’, ‘Centennial’, ‘Hallertauer Mittlefrue’
–Tolerant: ‘Perle’, ‘Fuggle’, ‘Willamette’
•Remove severely diseased plants
•Promote air movement in canopy
•Remove basal growth. Strip leaves from bines at a height of 4’ soon after training to reduce the spread of downy mildew up the canopy
•Manage irrigation to avoid prolonged wetness on leaves and cones
•Avoid overhead irrigation, especially during and after burr development
•Destroy escaped hop vines near or in hop yards.
•Prune crown before growth starts in the spring or burn back green tissue before training. Complete removal of green tissue or pruning of entire hill is necessary for most effective disease management
•Fungicide applications during disease conducive weather, particularly wet weather > about 42F
•Effective fungicides include, Copper, Cymoxanil (Curzate), Famoxadone + Cymoxanil (Tanos), Phosphorous acid, Aliette (an aluminum tris-phosphonate product), Mefenoxam (Ridomil Gold) – resistance exists in PNW
Hops Stunt Viroid
•Viroid: Hops stunt viroid
•Survival: infected plantmaterials, possible leaves, possible even by plant-to-plant contact
•Spread: infected plantingmaterials, mechanically ininfected sap, root grafting
•Yield loss potential: variesbetween varieties and years;50-80% yield loss reported in‘Willamette’ and ‘Glacier’
•Plant height is reduced, number of cones is reduced, cone size is reduced by 50-75%.
•Alpha acids may be reduced by one-half to one-third of that in healthy cones.
•Not well known. Known to infect ‘Omega’, ‘Glacier’, ‘Mt.Hood’, ‘Nugget’, ‘Sterling’ and ‘Willamette’.
Management
•Viroid-tested stock certified to be free of viroids
•Avoid planting where hops have already been, kill and eliminate volunteer hop plantsbefore replanting a hop yard
•Sanitation
–Work infected yards last
–Clean equipment (5-10% bleach solution)
–Destroy severely infected plants
–Remove infected plants.
–In Japan, if >10% of plants infected,replanting yard suggested
–Clean equipment and knives betweenfields
–Chemical instead of mechanical pruningmay reduce rate of spread
Apple Mosaic Virus
•Virus: Apple mosaic virus
•Survival: infected planting materials
•Spread: infected planting materials, mechanically in infected sap, root grafting
•Yield loss potential: varies between varieties and years, 20-30% commonly reported; poor rooting
Testing
Commercial tests available, e.g., AgDia, university plant clinics
Management
•Plant virus free rootstock
•Correct zinc deficiencies with foliar applications to suppress symptom expression
•Chemical pruning instead of mechanical pruning may reduce rate of spread of Apple mosaic virus
Fusarium Wilt
•A fungal disease, Fusarium sambucinum
•Survives in soil and on diseased plants
•Confirmed on ‘Chinook’, ‘Fuggles’, ‘Galena’, ‘Glacier’, ‘Mt. Hood’, ‘Nugget’, ‘Sterling’, ‘Willamette’ and ‘Zeus’ in PNW
•Sporadic
•More severe in wet season
•Poor drainage
•Survival: soil, crop debris, diseased crowns
•Spread: pathogen appears to colonize wounds caused by insect feeding or mechanical injury
•Yield loss potential: not quantified well, but generally minor
Management
•Avoid injury
•Reduce crown wetness
–Remove suckers that shade crown
–Reduce mulch
•Line to increase soil pH above 7
–Use nitrate-based fertilizer rather than ammonium-based nitrogen
Verticillium Canker (Wilt)
•Fungus: Verticillium albo-atrum and V. dahliae
•Survival: overwintering structures in soil and crop debris
•Spread: infested soil, plant debris, infected planting material
•Yield loss potential: Up to 100% with certain strains; typically less than 10% in PNW
•The virulent form that occurs in Europe has not been confirmed in US
–‘Bullion’ and ‘Brewers Gold’ are resistant to the mild form
–‘Fuggle’, ‘Cascade’, ‘Willamette’ and ‘Columbia’ sometimes get the mild form
Management
•Obtain roots from disease-free sources
•Plant resistant/less susceptible cultivars (e.g., ‘Cascade’, ‘Brewers Gold’, ‘Bullion’, ‘Chinook’, ‘Crystal’, ‘Olympic’)
•Supply sufficient nitrogen, avoid excess
•Long rotations (5 yrs+) to nonhosts such as corn or wheat
–Verticillium infects many common weeds and cultivated plants (300+ species) such as tomato, potato, peppers, strawberries, tulip trees…
•Control weed hosts
•Compost crop debris and do not return compost to hop yards
•Avoid excess irrigation in spring
Two-Spotted Spider Mite
•Mite: Tetranychus urticae
•Overwintering: Mated females in association with crowns or other protected areas
•Damage: Yield loss to due to leaf feeding and cone quality reduction
•Yield loss potential: 100% due to crop rejection
•Feed on greater than 180 plant species
•Development varies with temperature
–Can reproduce every 7-10 days in summer
•Thrive in hot, dry, dusty conditions
•Outbreaks favored by:
–Dusty conditions
–High N fertility
–Use of certain pesticides (pyrethoids, broad spectrum miticides, sulfur)
–Disruption of predators
–Certain cultural practices (basal foliage removal)
Management
•Apply adequate but not excessive N and irrigation
–Avoid large applications of N (>~20 pounds)
•Avoid use of broad-spectrum pesticides to conserve predators
–Pyrethroids, organophosphate insecticides, abamectin, sulfur
•Minimize dust in and around hop yards
–Cover cropping, dust control on roads
•Miticides often are needed—thresholds vary widely among growers (0-10+ mites/leaf)
–Abamectin
–Etoxazole (Zeal)
–Savey
–Bifenazate (Acramite)
–Sulfur and oils are moderately effective, suppress mites and diminish predatory mites
Hop Aphid
•Insect: Phorodon humuli
•Overwintering: Eggs laid on Prunus spp. (plum, sloe, damson)
•Damage: Direct yield losses due to defoliation; cone quality due to sooty mold
•Yield loss potential: 100% due to crop rejection
•Complex lifecycle
•Winged females disperse from overwintering host to hop in May
•Up to 10 generations per year in PNW
•Sensitive to high temperatures
•Populations decline rapidly when temperatures exceed 85F for extended periods of time
•Honeydew can lead to black sooty mold on leaves and cones that diminish crop quality
•Outbreaks favored by:
–Moderate temperatures, high N fertility, lack of predators
Management
•Apply adequate but not excessive N
–avoid large applications of N
•Avoid use of broad-spectrum pesticides to conserve predators
–Pyrethroids, organophosphate insecticides, abamectin
•Insecticides often needed
• Thresholds vary widely: first detection to >100 per leaf; aphids often not tolerated late season
•Imidacloprid (Admire or Provado)
•Spirotetramat (Movento)
•Fulfill (pymetrozine) and Beleaf (flonicamid) also work well
–Less toxicity
–Newly registered
–Efficacy depends on population
Scouting for Diseases and Insects
•Why scout?
–Identify disease problems during the season
–To change practices this year
–For next year
•Scout your crops on a regular basis (calendar)
•Scouting supplies include:
–Plastic bags
–Marker
–Pen or pencil and notebook or calendar
–Hand lens
–Field guide
Collecting Samples to Submit to the NCSU Plant Disease and Insect Clinic
•Collect fresh, don’t send over weekend.
•Send several examples
•Crush proof container
•Provide lots of information
•Don’t pull plants-do bag roots
•Don’t get soil on foliage
•Press leaves
•Contact lab
•Use your extension agent for help
•Collect as much information as possible
•Submitting a sample: