MSU Extension

So far this season, the Benton Harbor Enviro-weather station has accumulated 1,661 GDD50 with 0.41 inches of rain over the past week; the Clarksville Enviro-weather station has recorded 1,503 GDD50 and 0.38 inches of rain this past week; and the Northwest Michigan Horticultural Research Center accumulated 1,390 GDD50 with 0.96 inches of rain over the last week.

Hop development on July 29, 2013, in northwest
Michigan. Photo credit: Erin Lizotte, MSU Extension

Downy mildew infections have been reigned-in for the most part, but should remain a concern for growers with early initial infections continuing to fuel significant inoculum potential in some hopyards. Downy mildew is caused by Pseudoperonospora humuli and can cause significant yield and quality losses, depending on variety and when infection becomes established. It is important that growers do not mistake downy mildew for powdery mildew (see photos below for clarification) as the effective pesticide classes are completely different. According to Michigan State University Extension, powdery mildew has not been confirmed in Michigan and is caused by Podosphaera macularis, a completely different pathogen than what causes downy mildew.

Left, The variety Centennial with downy mildew infections sporulating on the underside of the leaf.
Right, Powdery mildew on hop. Photo credits: (Left image) Erin Lizotte, MSU Extension, and (Right image) David Gent, USDA Agricultural Research Service, Bugwood.org

Typically, downy mildew appears early in the season on emerging basal spikes. Spikes then appear stunted, brittle and distorted. Asexual spore masses appear fuzzy and black on the underside of infected leaves. As bines continue to expand, new tissue becomes infected and bines fail to climb the string. Growers can retrain new shoots but often incur yield loss as a result.

It takes a multipronged approach to manage successfully for downy mildew. Growers should maintain a protectant fungicide management strategy to mitigate the risks of early and severe infections. Keep in mind that varieties vary widely in their susceptibility to downy mildew and select the more tolerant varieties when possible. Clean planting materials should be selected when establishing new hop yards since this disease is readily spread via nursery stock.

Cultural practices alone are not enough to manage downy mildew. Protectant fungicide strategies are particularly important during the year of planting to minimize crown infection and limit disease levels in the future. Fungicide applications should be made in response to environmental conditions that favor disease. Fungicides containing copper, boscalid, pyraclostrobin, phosphorous acids and a number of biopesticides have varying activity against downy mildew (see Table 1 below). For organic growers, OMRI-approved copper formulations are the most effective. Sulfur is not an effective downy mildew material.

Table 1. Fungicides registered for hop production with activity against downy mildew

¹These product labels are OMRI-approved for organic production.

Japanese beetle populations persist in some areas. Japanese beetle adults are a generalist pest and affect many crops found on or near grassy areas, particularly irrigated turf. Japanese beetle grubs feed on grass roots in early spring and again in the fall and can cause significant damage to turf. Larvae prefer moist soil conditions and do not survive prolonged periods of drought.

Adult Japanese beetles emerge in early July and feed on the top surface of leaves, skeletonizing the tissue. If populations are high, they can remove all of the green leaf material from a plant. Japanese beetle adults measure 0.375 to 0.5 inches long with a green thorax and copper-colored wing covers. There are five tufts of white hairs on both sides of the abdomen and a pair of tufts on the end of the abdomen that can help distinguish the Japanese beetle from other look-alike species. Visual observation of adults or feeding damage is an effective scouting technique. Because of their aggregating behavior, they tend to be found in larger groups and are typically relatively easy to spot.

Adult Japanese beetle. Photo credit: David Cappaert, Michigan State University, Bugwood.org

There are no established treatment thresholds or data on how much Japanese beetle damage a hop plant can sustain, but growers should consider that well-established and vigorous bines will likely not require 100 percent protection. If treatment is necessary, growers will find a number of products labeled for Japanese beetles (Table 2). Organophosphates take longer to take effect (up to three days), but provide 10 to 14 days of residual control. Pyrethroids have good knockdown activity and seven to 10 days of residual control, but can be a concern in hopyards where mites are a problem. Pyrethroid use has been shown to flare mite populations as a result of its toxicity to beneficial predatory mites. Neonicitinoids act initially as a contact poison for two to five days, and then have a longer residual period of plant protection during which they have anti-feedant effects on adult beetles. OMRI-approved organic options include neem-based products (azadirachtin) which have a one- to two-day residual and good knockdown activity, as well as Surround (kaolin clay) which has had good results in blueberries and grapes and acts as a physical barrier and irritant.

Table 2. Insecticides labeled for hop production with activity against Japanese beetles, 2013

¹Products containing these active ingredients are classified as a restricted use pesticide and require the applicator to retain a pesticide applicator license.
²These product labels are OMRI-approved for organic production.

Growers should continue to monitor their hops for potato leafhopper populations as significant outbreaks are possible yet this season. Potato leafhoppers move in all directions when disturbed, unlike some leafhoppers that have a distinct pattern of movement. Right now the adults and nymphs appear a fluorescent green color. Some very small nymphs are actually clear, but have the characteristic shape of the larger nymphs when viewed using a hand lens.

Although hop plants are susceptible to potato leafhoppers, they can tolerate some level of feeding and growers should be conservative in the application of insecticides. Potato leafhoppers causes what growers have termed “hopper burn,” which causes necrosis of the leaf margin in a v-shaped pattern and may cause a yellowed or stunted appearance as well. The easiest way to observe potato leafhoppers is by flipping the shoots or leaves over and looking for adults and nymphs on the underside of leaves.

Multiple nymph stages of potato leafhopper and the associated
hopper burn symptoms around the leaf margin. Note that
leafhoppers can be very small and clear at some leaf stages.
Photo credit: Erin Lizotte, MSU Extension

Growers needing to treat for potato leafhoppers can utilize products containing neonicitinoids, pyrethroids, organophosphates or spinosyns. Organic growers can utilize Entrust (spinosad) or Pyganic (pyrethrin) formulations that are OMRI-approved for potato leafhopper management.

As temperatures rise and the weather dries out in the coming days, growers should remain vigilant in scouting for two-spotted spider mites with populations currently holding relatively steady due to the cool, wet weather. Two-spotted spider mites are a significant pest of hops in Michigan and can cause complete economic crop loss when high numbers occur by decreasing the photosynthetic ability of the leaves and causing direct mechanical damage to the hop cones.

Two-spotted spider mites feed on the liquid in plant cells eventually causing visible symptoms. Leaves take on a white appearance and will eventually defoliate under high pressure conditions. Intense infestations weaken the plant and reduce yield and quality. Infested cones develop a reddish discoloration, do not hold up to the drying process, and commonly have lower alpha levels and shorter storage potential. Additionally, the mites themselves act as a contaminate issue for brewers.

Two-spotted spider mites thrive under hot conditions with the pace of development increasing until an upper threshold around 100 degrees Fahrenheit is reached. Conversely, cold and wet weather is not conducive to development. Two-spotted spider mites are very small, but can be observed on the underside of leaves using a hand lens. The eggs look like tiny, clear spheres and are most commonly found in close proximity to adults and larvae. The larvae themselves are small, translucent versions of the adults which begin the season with a distinctly orange hue that changes over to translucent, yellow or green as they feed. Adults also have two dark spots. When you are observing the underside of leaves, keep an eye out for beneficial, predatory mites that actually feed on two-spotted spider mites. Predatory mites are often translucent, larger than two-spotted spider mites and move at a much faster speed across the leaf surface. Predatory mites play an important role in balancing the two-spotted spider mite population and should be protected when possible.

Adult two-spotted spider mites, nymphs (immatures) and eggs.
Photo credit: David Cappaert, MSU

Growers should be scouting for mites now and remember that only when mites reach an economically significant level should cultural and chemical intervention be considered. Use a hand lens to evaluate two leaves from 20 plants per yard. Thresholds developed in the Pacific Northwest have established that more than two adult mites per leaf in June indicate the need to implement a pest management strategy. By mid-July, the threshold increases to five to 10 mites per leaf. Remember that if cones are not infested, hop plants can tolerate a good deal of damage from mites.

Hop aphids continue to be observed at levels well below the eight to 10 per leaf threshold established in the eastern United States. Hop aphids can reduce plant productivity and excrete “honeydew” which makes an excellent growth medium for sooty mold and can greatly reduce the quality and salability of a crop. Symptoms of hop aphid feeding include leaf cupping and the appearance of honeydew (a sugary frass) and the associated black sooty mold.

Hop aphids overwinter as eggs on Prunus species. In early spring, eggs hatch into stem mothers that give birth to wingless females that feed on the Prunus host. In May, winged females are produced and travel to hop plants where additional generations of wingless females are produced. As cold weather approaches, winged females and males are produced, move back onto a Prunus host, mate and lay eggs before winter.

Wingless hop aphid on the underside of a hop leaf.
Photo credit: Erin Lizotte, MSU Extension

Control before the flowering stage is important to protect crop quality when populations are high. Insecticides containing neem (some of which are organic), neonicitinoids including products containing imidacloprid or thiamethoxam, flonicamid (labeled as Beleaf) or spirotetramat (labeled as Movento) all have activity against hop aphid.