MSU Extension

So far this season, the Benton Harbor Enviro-weather station has accumulated 1,234 GDD50 with 0.05 inches rain over the past week; the Clarksville Enviro-weather station has recorded 1,114 GDD50 and 0.32 inches of rain this past week; and the Northwest Michigan Horticultural Research Center accumulated 996 GDD50 with 0.57 inches of rain over the last week.

Symptoms of leafhopper burn on Summit in northwest
Michigan on July 10, 2013. Photo credit: Erin Lizotte, MSU Extension

Downy mildew remains the major concern for Michigan growers right now, with early initial infections continuing to fuel significant outbreaks 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. 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.
Photo credit: Erin Lizotte, MSU Extension. Right, Powdery mildew on hop. Photo credit: David Gent, USDA Agricultural Research Service, Bugwood.org

Typically, downy mildew appears early in the season on the 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.

This season, symptoms have appeared more readily on expanded leaves as small, angular lesions that are yellow and chlorotic in appearance. These small lesions expand over time and eventually sporulate on the underside of leaves when warm and moist conditions occur. According to “A Field Guide for Integrated Pest Management in Hops,” infection is favored by mild to warm temperatures (60 to 70 degrees Fahrenheit) when free moisture is present for at least 1.5 hours, although leaf infection can occur at temperatures as low as 41 F when wetness persists for 24 hours or longer. At this point in the season, we are also seeing stunting and wilt of terminal portions of the bine.

It takes a multipronged approach to manage for downy mildew. Growers should utilize 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. It is also recommended that growers pull all basal foliage during spring pruning. Pruning should be performed as late as possible and all green materials should be removed from the hopyard and covered up or burned.

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. Well-timed fungicide applications just after the first spikes emerge and before pruning have been shown to significantly improve infection levels season long. Subsequent fungicide applications should be made in response to environmental conditions that favor disease (temperatures above 41 F and wetting events). Fungicides containing copper, boscalid, pyraclostrobin, phosphorous acids and a number of biopesticides have varying activity against downy mildew.

For organic growers, OMRI-approved copper formulations are the most effective. Sulfur is not an effective downy mildew material. Again, we have not documented powdery mildew in Michigan yet, but given the experiences of hop growers around the United States, growers should keep an eye out for this potentially significant pathogen. Look for an upcoming Michigan State University Extension article on the difference between downy and powdery mildew.

If you already have downy mildew established in your hopyard, cultural practices will be very important in regaining ground. According to David Gent, a hop specialist at Oregon State University, diseased shoots on the string should be removed by hand and healthy shoots retrained in their place. Remove superfluous basal foliage and lower leaves to promote air movement in the canopy and to reduce the duration of wetting periods. If there is a cover crop, it should be mowed close to the ground. If yards have no cover crop, cultivation can help dry the soil and minimize humidity. Keep nitrogen applications moderate.

Growers should also carefully monitor their hops for potato leafhopper populations as significant populations are being observed. 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.

Potato leafhoppers can’t survive Michigan’s winter and survive in the Gulf States until adults migrate north in the spring on storm systems. 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. At this time, there is no set economic threshold for potato leafhoppers in hops; however, some hopyards are seeing significant damage from potato leafhoppers at this time.

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 remain warm and the weather dries out for the summer, growers should remain vigilant in scouting for two-spotted spider mites which are being reported at densities of around two mites per leaf in southern hopyards, but have yet to be observed in significant numbers in the north. 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 mite like it hot, with the pace of development increasing until an upper threshold around 100 F is reached. Conversely, cold and wet weather is not conducive to development which may explain the low pressure thus far this season.

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.

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. Scouts should take leaf samples from 3 to 6 feet up the bine; as the season progresses, samples should be taken from higher on the bine as the mites migrate upward. 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 are also being 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.

Rose chafer adult activity has finally subsided, but Japanese beetles have begun to arrive and feed at low levels in southern Michigan hopyards. Look for more information regarding Japanese beetle in next week’s report.