by Tamara Scully
“We learned the hard way that powdery mildew can do very well in Michigan,” Erin Lizotte, of Michigan State University, said after several growers lost their hop crops to the disease last season. “It’s a difficult disease to control, especially on susceptible varieties.”
Lizotte was the featured speaker during the May 2018 session of the Hop Power Hour, a program of the University of Vermont Extension, and available via Youtube.
Podosphaera macularis, the fungus that causes powdery mildew, can cause complete crop loss, particularly if the pathogen is present in young plants or emerges at the beginning of the growing season. It overwinters as mycelia inside of the buds, and emerges from the crown of the hop plant as a flag shoot.
The contaminated shoot will have a white substance with raised blisters, which are the spore masses, or conidia. Flag shoots, even in heavily infected hop yards, are rare, making up less than one percent of all shoots, and are easy to overlook.
In Michigan last season, “everybody had these late spring infections but not everyone recognized the issue,” Lizotte said, as it can be difficult to scout these spores, which are part of the fungus’s asexual life cycle. This asexual form of the fungus is an obligate parasite, so it can only survive in living plant tissue.
If left untreated, the conidia will spread to healthy plant tissue, infecting leaves and cones, and will spread new infections to healthy plants.
The later in the season a hop plant becomes infected, the less symptomatic it will be. Leaf infections begin as lesions which pucker and blister, then the white, fuzzy conidial spores become visible. Infected burrs and cones can show some reddish discoloration, have visible white fuzz or be noticeably stunted, depending on the severity of the infection.
Plants infected early in their development can have cones that are visibly diseased, while cones from plants infected later in their life cycle can appear relatively normal. But even cones with little visible changes will have significantly reduced alpha acid levels, and reduced quality. Delaying infection in the plant is key to limiting cone damage.
Controlling infection
If growers can slow down the rate of initial infection, the fungus has less opportunity to complete its life cycle during the season. Fewer cycles of asexual reproduction means fewer infective agents. Powdery mildew is species specific, so it does not jump from one crop type to the next. The only inoculant, in most situations, is from the fungal bodies overwintering in the hop plant tissues or introduced via new plantings.
Crowning, or removing the initial buds from the crown as they emerge in the spring, of mature plants will eliminate the majority of the pathogen lurking within the bud and awaiting plant growth. By reducing the initial inoculum and following up with approved fungicides, growers can drastically reduce the effects of powdery mildew. Scratching, which is a less intensive rubbing off the crown buds, is also effective, Lizotte said.
The later this pruning can be completed, the better. Pruning as late as possible can delay the regrowth of any flag shoots, so any remaining inoculum will not cause early season infections. Where the onset of disease is delayed into June, the rates of infection in the hop yard have been shown to be significantly reduced. Early infections resulted in high rates of disease by the end of the growing season.
Crowning will “also help synchronize growth for optimal yield, with cones all ready for harvest at the same time,” Lizotte said.
By mechanically pruning later, less chemical pruning applications are needed, saving money and labor. Equipment used for mechanical crowning can, however, spread viruses, so caution must be used.
Growing conditions
Powdery mildew thrives in temperatures between 46 and 82° F, with its ideal zone being between 64 and 70 degrees. Low light, due to cloud coverage, dense canopies, high soil fertility and increased soil moisture all contribute to the pathogen’s growth. Unlike downy mildew, powdery mildew does not thrive in wet conditions per se, so dew, rain or overhead irrigation aren’t causative agents.
It’s also important to note that fungicides labeled for downy mildew – which is a gray color and resides on the bottom surface of leaves and causes yellow, stunted shoots – are not effective on powdery mildew. Downy mildew, or Pseudoperonospora humuli, thrives in temperatures between 60 and 68° F, under wet conditions. Infected cones will be dark brown and shriveled. Downy mildew, a water mold, is one of the most severe hop pathogens. Crowning can help control it as well.
Increasing the air flow around the crown of the hop plant can be done via a chemical desiccant. The lower three feet of the plant are kept free of leaves. The bine must have enough leave growth to support photosynthesis, be tall enough so that the growing point isn’t affected, and the stem should be hardening.
Fungicide applications aren’t needed immediately if pruning is done at about six inches of initial shoot growth, as removing the infected material will remove the fungal agent. After this, fungicides for powdery mildew are applied at three time periods; emergence to June, mid-June to bloom, and bloom to pre-harvest. Earlier applications can be made with oils and copper sprays, while later applications utilize products with greater efficacy. Fungicide resistance is a serious concern, however.
Harvesting the cones as soon as possible reduces the risk of problems if any powdery mildew inoculum is present. Leaving cones hanging can enhance any disease concerns.
Sexual reproduction
There is another way for P. macularis to infect a hop yard: sexual propagation. The fungus requires two mating types, which can then reproduce via ascospores. These resting spores do not require living plant tissue to survive. They can overwinter in plant debris or soil, and survive for long periods without a host plant.
If sexual reproduction is occurring, or if two mating types are found in close proximity, growers will have to manage powdery mildew differently. The spores formed from sexual reproduction are not obligate parasites, so equipment, soil, plant debris and other modes of transportation can move these spores from field to field. When they encounter a living hop plant, they can then form conidia and spread their infection. Sexual infections are identified by white fungal colonies complete with black specks – the resting ascospores – present.
“We haven’t yet found two mating types within a single hop yard in Michigan,” Lizotte said, although feral hop stands in the state do have mating and sexual reproduction occurring.
Clean plants
Beginning with clean plant material is the only way to ensure that powdery mildew is not being introduced into the hop yard. Because hop powdery mildew is more prevalent in greenhouses than in the field, growers should order a small batch of plants and have them tested for pathogens prior to placing a large order and planting those out in the fields. In Michigan, all of the powdery mildew seen has been introduced via planting material. Selecting resistant hop varieties when possible is also a good practice.
“It’s really critical to avoid introducing the pathogen,” Lizotte concluded.