When you are putting in a new crop or expanding the plant diversity of your backyard, it can be daunting to read about all the potential diseases that will afflict the crop for which you have such high hopes. It may seem like you and your plants don’t stand a chance. But while it is true that growing hops in the East is going to be challenging, especially if you are trying to avoid reliance on routine application of chemical sprays that may be harmful to the environment, it can be and is being done.
Traumatic or not, reading about the various diseases you may encounter is critical. And it’s something you need to do before you plant, not after, because the main way to manage disease in your hop yard using sustainable practices will be through actions you take in the early stages. As discussed in previous chapters, planting the disease-resistant varieties that have been developed since hop production vanished from the East will be your first line of defense. Locating your hop yard to optimize drainage and exposure to sun and wind is also key, as is preparing the soil and suppressing the weeds prior to planting. Once the hops are growing, sharp eyes, quick action, and ongoing vigilant weed control will help you keep diseases in check when they arrive in your hop yard.
The major diseases that appear in hops in the East are either fungal or viral. A fungal disease is caused by a fungal parasitic organism that lives off a host. Some of the major diseases of hops are fungal in nature.
Downy Mildew
Downy mildew is the number-one enemy of hop farmers around the world, and the wetter it gets in the East, the better the conditions are for it to flourish. That’s why agronomist Heather Darby, from UVM’s Extension agency, focuses intently on the disease at the experimental hop yard she oversees in northern Vermont, on the Canadian border. We met up with her there one day to talk about disease prevention, and her efforts to test organic approaches. Darby walked through the hop yard, turning over leaves to search out signs of disease. “Everything bad in the hop yard first appears as little yellow spots,” she exclaimed. “First you tell yourself it’s just this or it’s just that, then one day you see it full blown and say, ‘No! It can’t be!’ But by then it is usually too late to stop it.” She didn’t have to look far. She soon came across a cluster of blighted leaves where downy mildew had struck.
Downy mildew is a sort of generic term for a group of funguslike parasites that specialize in moving through water to penetrate the surface of plants, then sucking the life out of them. In actuality, it is an oomycete, hailing from a group of microorganisms sometimes called water molds. So it is not exactly a fungus, but it acts like one, hence the name. Different types of downy mildew affect different species of plants. The scientific name for the type of downy mildew that goes after hops is Pseudoperonospora humuli. It is closely related to another type of downy mildew, Pseudoperonospora cubensis, that afflicts farm crops such as cucumbers and squash. Although P. humuli and P. cubensis attacks are species specific, researchers are looking into whether it is possible for them to cross species.
Heather Darby, agronomist for UVM’s Extension agency, examines hop leaves at UVM’s experimental organic hop yard at Borderview Research Farm in Alburgh, Vermont.
Where P. humuli came from is anyone’s guess, but it infects hop yards around the world, with the sole exception of Australia, New Zealand, and South Africa. It made its first appearance in Japan in 1905, then the following year popped up randomly on the other side of the world in Wisconsin. It hit England in 1920 and spread through Europe. In 1928 P. humuli arrived in New York, playing a major role in the ultimate demise of commercial hop farming in that state, and ten years later established itself in the hop-growing regions of the Pacific Northwest.
Downy mildew lives in hop rhizomes and spreads when rhizomes harvested from an infected hop yard are replanted somewhere else. The spores can also travel on the wind as well as on the clothes, shoes, and hands of people who handle infected plant material or visit an infected hop yard, then enter an uninfected hop yard.
Downy mildew first appears as a diseased shoot called a basal spike emerging from the ground.
Downy mildew advances from the roots into the plants, becoming systemic. Photographs by D. R. Smith, Compendium of Hop Diseases and Pests, American Phytopathological Society
Systemic downy mildew causes real damage when it reaches the cones, causing extreme browning and destroying the lupulin. Photographs by D. R. Smith, Compendium of Hop Diseases and Pests, American Phytopathological Society
What’s the damage? A lot. Downy mildew stunts the growth of the hop bines, reducing the number of cones they can produce. It also impacts yield by damaging the cones themselves and in some cases can outright kill the entire plant. We had always grown single plants here and there on our farm and never had downy mildew. Then we put in our pilot hop yard and were free of downy mildew for the first two years. In the third year that yard was full of it and it began making a sporadic appearance in the 1-acre (0.4 hectare) yard. So we joined the ranks of hop farmers fighting downy mildew. You can and should try to stay free of downy mildew for as long as you can, but sooner or later you will get it, and from that point on it is a question of management and control. As a matter of fact, control of downy mildew is the principle on which many of the standard best management practices in the hop yard are based. To control downy mildew, first you must understand it. It is a diabolical pathogen that, like all other living creatures on this Earth, is working a number of different angles to perpetuate its existence. Its end goal is to reproduce itself as much as possible.
LIFE CYCLE
The disease is so hard to get rid of because it over-winters underground in the crown of the hop plant. Infected crowns have reddish-brown to black flecks and streaks in the white crown tissue next to the bark. The infection infiltrates developing buds, causing them to send up diseased shoots, called primary basal spikes. These spikes appear stunted, with yellowed leaves that are brittle and curl downward. Sporangia (spore sacs) develop on the undersides of these leaves as well as on the spike at night, when the temperature and moisture levels are right. Between midmorning and early afternoon, especially when it is wet and rainy, the sporangia release spores into the hop yard. The production of spores from the diseased spike and leaves continue throughout the conducive period, spreading the disease throughout the hop plant and between plants. These zoospores, essentially little swimmers with tails, really like it when there is standing water on hop leaves that are in close proximity to the basal spike producing the spores because they can swim through the water to enter the plant through one of the tiny openings in the plant’s surface, called stomata—pores through which the hop breathes. This is how downy mildew spreads through the hop plant.
When secondary spikes (which look much like the primary spikes) emerge from the top of the plant or its side shoots, the infection is no longer on the surface of the plant but has gone systemic. As the disease progresses, it moves up through the plant. Aerial spikes develop on the lateral branches and trained bines in late spring or midsummer. Infected trained bines will stop growing and simply fall from the string they were previously climbing so vigorously. Lesions develop on infected leaves. Developing flower clusters turn brown and shrivel. If the infection didn’t take hold until later in the season, only the bracts will turn brown and the cone will look striped. In severe cases, cones turn completely brown and harden. In some cases the plant dies of reduced energy reserves caused by disease.
Remember the disease triangle? When it comes to the pathogen known as downy mildew, the host is hops and the environment is wet and warmish. The question is how wet and how warm, and for how long. In general, if downy mildew is present in your hop yard the infection can spread significantly if conditions remain wet and humid at a moderate temperature of 65 to 76 degrees Fahrenheit (18.3 to 24.4 degrees Celsius) for four to eight hours. Sounds like spring in the Northeast, right? Right.
The first time I was reading all the stats about the conditions under which downy mildew spread, it was early June and it was raining, and warm. It rained and stayed warm all week, day and night. When it finally stopped raining I went out on the deck and checked the rain gauge. We had gotten 3 inches (7.6 centimeters) of rain in four days. In our small hop yard, downy mildew had taken over and had started to appear in the larger yard, too. We didn’t know where it came from, but it had undoubtedly arrived. I was really starting to understand why hop farmers are so neurotic about water. Hops are really thirsty plants, but they do not want to go swimming. What’s a hop farmer to do?
I thought about the disease triangle some more. We could not eliminate the host. The host is our crop. Some of the varieties we had planted were tolerant of downy mildew and some were susceptible. We couldn’t exactly pull the susceptible ones up—at least not yet. And unfortunately we cannot change the environment (although we seem to have already changed it for the worse). The only thing we can control at this point is the pathogen, so that is what we set out to do.
PREVENTION AND CONTROL
The first step in controlling downy mildew is to try to keep it out of your hop yard in the first place. If you’ve acquired disease-free rhizomes through the National Clean Plant Network, that’s a good first step, but don’t forget to think beyond your crops. Downy mildew can infect wild hops growing near your hop yard. Monitor these wild hops as closely as you monitor your own hop yard. If there are signs of infection it is a good idea to eliminate the wild hops, if possible.
Although you can’t prevent downy mildew spores from blowing in on the wind, you can avoid introducing them into your hop yard yourself. If you visit other hop yards or go out to collect wild hop cones for a home brew, change your clothes and shoes and wash your hands and tools before you enter your own hop yard. Think twice about who enters your hop yard and if they might inadvertently be carriers of downy mildew spores. Although hop yards are relatively far and few between in the eastern United States, it doesn’t hurt to check with people entering your hop yard to make sure they have not just come from another hop yard. Extension workers aiding with activities such as soil testing and pest scouting should know about precautions against the spread of disease, but field workers or contractors installing trellising may not be aware of the issue.
Also think twice about what varieties you plant, choosing hop varieties that are bred to have a natural resistance to downy mildew. No hop is completely resistant, but some varieties are much more susceptible than others. We can only hope that brewers in the East will care enough about brewing beer made with local ingredients that they will develop recipes that make good beer out of varieties that the region’s farmers can successfully grow.
You may take all these precautions and still end up with downy mildew—a reality that is driving downy mildew research in Oregon and Europe, where breeding varieties of hops that are tolerant of and resistant to downy mildew has been a huge priority for researchers for quite some time. Heather Darby at UVM is researching organic means of controlling downy mildew. In addition to choosing resistant varieties, there are management strategies that can help control downy mildew in the hop yard.
TABLE 8.1. Downy Mildew Resistant Hop Varieties
|
Variety |
Resistance Level |
Type |
|
Brewer’s Gold |
moderately resistant |
bittering |
|
Cascade |
moderately resistant |
aroma |
|
Chinook |
moderately resistant |
dual purpose |
|
Columbia |
moderately resistant |
aroma |
|
Fuggle |
resistant |
aroma |
|
Hallertauer Gold |
resistant |
aroma |
|
Hallertauer Magnum |
resistant |
bittering |
|
Hallertauer Traditional |
resistant |
aroma |
|
Liberty |
moderately resistant |
aroma |
|
Newport |
resistant |
bittering |
|
Opal |
resistant |
dual purpose |
|
Perle |
resistant |
dual purpose |
|
Spalt |
resistant |
aroma |
|
Willamette |
moderately resistant |
aroma |
In fact, many cultivation practices standard in the hop yard have their roots in downy mildew control, including the spring activities of crowning and pruning. The most important thing you can do to control downy mildew is scout your hop yard frequently, keeping a vigilant lookout for primary basal spikes and, as the season advances, the secondary spikes that may appear higher up on the plant. Wherever they appear they must be cut down, removed from the hop yard, and destroyed. Do not compost downy mildew–afflicted vegetation; it is too risky. If the temperature of your compost does not get high enough and you put the compost in the hop yard, you will recontaminate your hops. The best way to destroy it is to burn it.
If you know heading into the season that you already have downy mildew lurking belowground, you can get a step ahead of it by crowning the plant. When crowning, a mechanical spinning blade attached to a motor in an arrangement similar to a weed trimmer removes the top 3/4 to 2 inches (1.9 to 5.1 centimeters) of the crown. This intense underground pruning process helps prevent the hop from sending up an early crop of diseased shoots. This can be accomplished by hand with a garden knife. Larger hop yards crown their plants routinely to remove the previous year’s growth as well as cutting back early shoots, which are not generally the strongest.
In addition to controlling downy mildew below-ground, in the plant’s root base, there is also much you can do at ground level—including pruning any diseased shoots that emerge from the ground and keeping the base of the bine weed free. Allowing weeds to grow around the base of the bine creates a moist, shaded environment perfect for harboring the downy mildew spores that are making their way up from ground level. Pulling all weeds allows sun in and improves air circulation, keeping the area dry and unfavorable to the spread of downy mildew. In addition, once you have selected and trained your bines of choice onto the string, it is important to remove all subsequent shoots the hop sends up, again to thwart any buildup of vegetation on the ground.
Once the trained bines have reached a height of 4 feet (1.2 meters) or so, start to strip the leaves off the base of the hop bine itself. As the bine grows higher, continue to strip leaves—working your way up until the bine is without leaves from the ground to a height of about 4 feet. Like removing vegetation on the ground, this process keeps downy mildew from progressing from the ground up into the plant. This is a lot of work, but it can all be done by hand in smaller hop yards. Most large hop yards use chemicals to kill the weeds and even to strip the leaves from the hops themselves.
Hop farmers have also experimented with bringing sheep into hop yards to eat the lower leaves from the hop bines and the weeds at the base. The long period of strict confinement it would take to get sheep to eat weeds down to bare dirt would probably put the hop bine at risk of being eaten as well, but sheep will do a pretty good job of eating the bottom leaves off the hop plant and shouldn’t cause much damage as long as there is plenty of vegetation on the ground for them, too. The advantage of sheep over goats is that sheep don’t tend to stand on their hind legs to browse what they cannot reach with four feet on the ground, which limits the height up to which they will eat the hop vegetation. That being said, you can’t totally count on sheep to stick to lower heights: I have certainly seen sheep on their hind legs eating out-of-reach apples from trees on our farm. Another major concern when using sheep in the hop yard is that copper, a common spray used to control downy mildew, is highly toxic to sheep. Once the decision is made to spray copper in the hop yard, sheep must be removed from the hop yard for the remainder of the season.
Once the vegetation is cleaned out, put down a layer of mulch and rake it into a hill around the base of each plant. This will discourage the emergence of any further spikes. Although keeping the base of the hop plants clean of vegetation helps prevent disease, this practice also can have a negative impact because it deprives beneficial insects of a good environment to live in while they do their work of preying on insect pests in the hop yard. As with everything else in farming, the trick is to achieve a balance. Assess what your greatest risk is and do what you have to do to minimize it.
Another strategy for controlling downy mildew is to control moisture levels. Although it is clearly impossible to control rain and humidity, you can choose how you water your hops. Because of the threat of downy mildew and its preference for standing water on leaves, hop farmers avoid overhead, sprinkler-type irrigation. Drip irrigation that runs water directly onto the ground but can be lifted for crowning is the standard in many hop yards (see Chapter 5 for more on irrigation design).
FUNGICIDES
If you think you are looking at a bad outbreak of downy mildew, you’ll need to keep up with the cultivation practices outlined above, but it also may become necessary to spray a fungicide. There are not a wide range of fungicides permitted for use on hops in the East, and most only work when applied preventively, before the disease strikes or right after it makes its first appearance. These nonsystemic fungicides treat the surface of the plant and work best at preventing downy mildew from taking hold before it gets bad. Others, known as systemic fungicides, are better for killing downy mildew once it gets into the plant’s system; there are far fewer of these. Only a few types of fungicides are considered organic. The primary fungicide used in organic hop yards is copper.
The benefit of fungicide for downy mildew is clear. It kills the arch enemy of hops. However, fungicides have distinct disadvantages as well. One of these disadvantages is that fungicides also kill beneficial insects. You can spray fungicide and beat back the downy mildew but only end up jumping from the frying pan into the fire when the weather changes and a new pest comes to town. The health of your beneficial insects as well as human, environmental, and economic issues are more good reasons to choose to plant hop varieties bred to tolerate downy mildew. The more tolerant your hop varieties are, the less likely they are to contract a severe case of downy mildew and, if they do, the less you will have to spray them to bring the infection under control.
There are a couple of important things to remember if you are going to spray fungicide. Since fung icides tend to be most effective on downy mildew in hops if they are sprayed as a preventive, you’ll need to monitor the weather. If you know downy mildew is present in your hop yard and you see a stretch of weather on the horizon that is going to be conducive to downy mildew reproduction, you should spray fungicide before that weather hits. The other really important thing to remember is that downy mildew is quick to develop resistance to fungicides, some more so than others. If you just keep on spraying the same fungicide over and over, it won’t be long before it has no effect. You have to keep rotating fungicides.
Remember, spraying pesticides—whether fungicides, insecticides, or herbicides—is complicated and can be dangerous. There are plenty of people out there trained to help you with this, from the manufacturers to Cooperative Extension agents. Don’t wing it.
Powdery Mildew
Powdery mildew is a major problem for hop growers worldwide. It tends to afflict areas that are drier; therefore, it is less of a problem in the humid eastern region. As of this writing it has not yet impacted fledgling twenty-first century hop growers in the East. The disease is a challenge to fruit and vegetable growers in the region, but it is species specific. It used to be thought that the form of powdery mildew that affects hops went by the scientific name of Podosphaera macularis and also infected chamomile, strawberries, and hemp, all of which grow wild and are cultivated in the East. But new research has shown that Podosphaera macularis only affects hops and cannabis. Some research indicates that powdery mildew afflicting the genus Cucurbita—which includes crops such as summer and winter squash, pumpkins, and cucumbers—can jump to hops and has done so in North Carolina, Michigan, and Washington. This type of powdery mildew is called Podosphaera fusca. This is not good news for our farm since pumpkins and winter squash are grown in great numbers on the apple farm our land is carved from. Late in the season the great, broad leaves on the pumpkin and squash plants often appear grayish white and blasted as powdery mildew takes its toll.
True to its name, powdery mildew looks like talcum powder on the surface of the leaf, first appearing as white spots, then covering the entire leaf. Photograph by W. F. Mahaffee, Compendium of Hop Diseases and Pests, American Phytopathological Society
Once established, powdery mildew will move up the bine, covering the entire plant. Photographs by D. H. Gent, Compendium of Hop Diseases and Pests, American Phytopathological Society
Despite strict quarantines, powdery mildew finally made its appearance in greenhouses in Washington in 1996. This prolific fungus, which causes extreme reductions in crop yield and quality, was spotted in a Yakima Valley hop yard in June of 1997 and by the end of July had rampaged through every hop yard in the state, destroying $10 million worth of hops on 2,000 acres (810 hectares). The following year it took over in Oregon and Idaho. Needless to say, the hop industry in the Northwest is very focused on fighting powdery mildew. In fact, Paul Matthews’s lab at Hopsteiner, in the City of Yakima’s industrial zone, has a disease challenge greenhouse, where he and his staff put hop seedlings on trial by introducing “a hop plant furry with powdery mildew” into the greenhouse. “We throw out the seedlings that get powdery mildew,” explains Matthews. “The ones that don’t get cleaned up and go out to the field. We do this at the greenhouse here in town because no grower wants a bunch of mildew-induced hops anywhere near his hop yard. It just feels bad.”
Will powdery mildew on hops make its way to the East? Is it already here living on other plants and the occasional wild hop? Will it jump from our pumpkins into our hop yard? The answer to at least the first, if not all, of the above questions is likely yes. So what do we do?
Powdery mildew in the hop yard can (to a certain extent) be prevented, at least for a while, and it can definitely be controlled; however, it cannot be eliminated. So, much like downy mildew, once it turns up in the hop yard, it becomes a management issue, and to manage it you need to understand it.
LIFE CYCLE
Like downy mildew, powdery mildew thrives in a moist environment; unlike downy mildew it can also develop and spread during spells of dry weather because it does not need direct contact with water to reproduce or infiltrate the plant. Between 64 and 70 degrees Fahrenheit (17.8 and 21.1 degrees Celsius) is powdery mildew’s ideal temperature, but it can reproduce when the temperature is in the 46 to 82 degree Fahrenheit (7.8 to 27.8 degrees Celsius) range.
Its fungal spores, called conidia, blow into a hop yard on the wind and overwinter underground on the buds on the rhizomes, in the soil, and on plant litter on the ground around the hills. In the early spring it makes its appearance on emerging shoots. Infected shoots, called flag shoots, are stunted, pale yellow, and look as if they have been dusted with white talc. This white powdery substance comprises the fungus’s spore-producing masses. When the conditions are right, spores are released and spread through the hop yard—again by the wind but also by water splash and people and equipment moving through the hop yard. Conditions most conducive to spore release are cloud cover, too much moisture in the soil, an over-abundance of nitrogen, dense vegetation, and temperatures between 64 and 70 degrees Fahrenheit. Powdery mildew particularly likes it when the daytime and nighttime temperatures are similar and range between 50 and 68 degrees Fahrenheit (10 and 20 degrees Celsius). Powdery mildew can be hard to detect because the flag shoots are only produced intermittently and may be quickly overgrown by other emerging hop shoots as well as weeds.
As with downy mildew, if left unchecked, powdery mildew will ultimately destroy the hop cone. Photographs by W. F. Mahaffee, Compendium of Hop Diseases and Pests, American Phytopathological Society
Later in the season powdery mildew appears as small, pale, yellowish spots on the tops of the leaves. This discoloration, referred to in plant science lingo as chlorotic, indicates that the plant is lacking in nutrients. Although this can sometimes indicate a problem with the soil, in the case of a fungus such as powdery mildew, which is essentially a parasite, it means that something else is eating the nutrients that should be going to the plant. Without those nutrients, the plant is unable to produce sufficient chlorophyll, which is what makes it green—hence the pale spots on the leaves. The lack of chlorophyll also reduces the plant’s ability to feed itself with the carbohydrates it produces through photosynthesizing sunlight. Over time this slows the plant’s growth and kills off leaves.
As the powdery mildew takes hold, it will actually become visible on the leaves as powdery white spots, similar to the white dusting on the flag shoot. These white spots will first appear on the undersides of leaves but will also become visible on the tops of leaves as well as on other parts of the plant, including stems, buds, and flowers. These white powdery spots produce spores. Over time the spots get bigger and more spots appear on other leaves, producing more spores, and so it goes.
Powdery mildew is unlikely to kill the plant itself, but it significantly damages the crop by reducing both the yield and the quality of the cones. If the infection gets into the burrs or the young cones, it will either kill off the cone or deform it. Sometimes the white powdery fungal growth will be visible on the cone, but sometimes it is only visible under the bracts and bracteoles and sometimes only with magnification. Damaged cones can appear reddish-brown, but sometimes they look normal at harvest, then turn brown after kilning.
PREVENTION AND CONTROL
As with downy mildew the first line of defense is to plant disease-resistant varieties. Wouldn’t it be great if the same varieties that are resistant to downy mildew were resistant to powdery mildew? Yes, it would, but that is not the case. Cascade, Liberty, Newport, and Nugget are tolerant. Note that Cascade is the only variety tolerant of downy mildew that is also tolerant of powdery mildew; hence, its popularity with growers. Fortunately it is also popular with brewers because it is dual purpose, with bittering qualities as well as a big hoppy aroma.
You should also be careful not to overdo it with the nitrogen. Even though hops need a huge amount of nitrogen, there is such a thing as too much. If you overload the hop yard, the plant growth will be spongy and succulent, which is conducive to the development of powdery mildew.
Since there are not yet very many hop yards in the East, and no current powdery mildew outbreak in those that do exist, you may think that means you are safe from powdery mildew. But remember, there are wild hops growing in the region, some of which may be native, but most of which escaped from cultivated hop yards during the 1800s. Most hop growers have a soft spot in their hearts for wild hops. But remember, if there are wild hops growing in the vicinity of your hop yard and they become infected with powdery mildew, they will spread it to your hop plants. So once again, some growers recommend eradicating any wild hops growing near your hop yard. If you can’t bear to do this, at least monitor the wild hops carefully so that if there is an outbreak of powdery mildew or something else, you are aware of it. Remember, most feral hops have the same genetics as Cluster, which is the variety many colonists brought with them from Europe, and Cluster is notoriously susceptible to both downy and powdery mildew.
Sunshine and air circulation are enemies of powdery mildew. Although spores can arrive on the wind, good exposure to wind and sunlight will keep them from taking hold on the plant’s surface. When you lay out your hop yard, don’t skimp on space in between plants. If, as they grow, the plants become crowded together, the thick vegetation will limit air circulation and sunlight exposure, and powdery mildew will proliferate.
Powdery Mildew Home Remedies the key to success is the interaction of compounds within the milk and the ultraviolet rays of the sun.
Powdery mildew is a fungus that afflicts numerous garden plants. As a result the gardening community has come up with a number of home remedies to treat it. Currently there is no research into whether or not these remedies work on hops, but if you only have a few hop plants it might be worth experimenting.
MILK
Although the exact reasons for why it is effective are still being debated, it has been proven that spraying milk diluted in water on afflicted vegetation controls powdery mildew. The same effect has also been attributed to whey. The best ratio of milk to water for fighting powdery mildew is not yet known, but people have had success with ratios as low as 10 percent milk to 90 percent water and as high as 40 percent milk to 60 percent water. The main thing is to spray the milk onto the plant in full sun, as it seems that
BAKING SODA
Baking soda, sodium bicarbonate, is also effective against powdery mildew. Combine 1 tablespoon (15 milliliters) of baking soda with 1 gallon (3.8 liters) of water and mix with 21/2 tablespoons (37 milliliters) of horticultural oil. Put this solution in the spray attachment on your hose and spray. Test a small area a few days prior to spraying an entire hop plant to make sure the mixture does not cause phytotoxicity.
To make horticultural oil, mix 21/2 to 3 tablespoons (37 to 45 milliliters) of seed oil (such as canola, cottonseed, or soybean) with 1 gallon of water and 1/4 teaspoon (1 milliliter) of liquid soap, such as an all-natural Castile soap, which is vegetable-oil based.
For the same reasons it is also important to remove vegetation from the base of the plant, including both weeds and hop shoots that you are not training. Not only will this increase sunlight and air circulation, it will make it easier for you to spot those diseased flag shoots when they come up. These flag shoots must be removed immediately, taken from the hop yard, and destroyed. Understand that once you see a flag shoot or white patches on the leaves of the plant it is too late. You have powdery mildew in your hop yard. Your purpose at this point will not be to eliminate it but to minimize it. Once it occurs, removing the plant growth on the ground at the base of the trained bines keeps the powdery mildew from spreading upward. As with downy mildew, it also makes sense, once the plant has reached a height of at least 6 feet (1.8 meters), to begin stripping the leaves off the bottom of the bine to a height of about 4 feet (1.2 meters) off the ground. Keeping the area on the ground at the base of the bine free of vegetation also ensures that if you are forced to spray a fungicide the area will be open and the spray will be able to reach the hop plant itself.
FUNGICIDES
All commercial hop yards infected with powdery mildew find it necessary to combine cultivation practices such as those described above with a fungicide spray program. As with downy mildew the most effective spray program is a preventive one. If you know you have powdery mildew, start your spray program as soon as the shoots emerge—then watch the weather. When weather conditions conducive to the spread of powdery mildew begin to line up, it is time to spray. What you choose to spray will be decided by several factors. The first factor will be whether you are committed to organic practices or are using conventional practices. Even if you are using conventional practices you may want to start with organic fungicides. Most organic fungicides are best suited to preventing the spread of powdery mildew rather than killing it, and they can be less harmful to the environment. You will then need to consider what fungicides are legal in your state and which of these is labeled for use on hops. As with downy mildew, powdery mildew is quick to develop resistance to fungicides so it is important to rotate the fungicides you use. Not to do so is irresponsible. By fostering fungicide resistance in a pathogen that destroys a commercial crop, you are not only harming yourself but also harming others who are trying to make their living growing hops.
Verticillium Wilt
Verticillium wilt is yet another disease of hops caused by a fungus. This disease affects a broad range of cultivated and wild plants worldwide ranging from alfalfa to watermelon. There are two species of the fungi: Verticillium albo-atrum and Verticillium dahliae. The symptoms of the disease range from mild to severe depending on the virulence of the strain and the susceptibility of the plant. It normally strikes fairly late in the season when the cones are in development, causing yellowing and wilting of the plant moving from the ground up. In severe cases the plant will die. The disease can be dormant in the soil or be brought into the hop yard from new infected rootstock or plants as well as by people who have been in a contaminated hop yard.
LIFE CYCLE
Some types of fungi can live in the soil in what is referred to as a “survival structure” for several years. The verticillium wilt mode of survival structure is mycelium, a vegetative part of the fungus that stays dormant in the soil waiting for the root of a potential host plant, such as the hop, to come meandering by. Verticillium albo-atrum can lie dormant in the soil for four years. Verticillium dahliae can hang around in its survival structure for as long as fifteen years. Sensing the proximity of the root, the fungus comes to life and enters it, usually through a minor wound the root sustained while digging through stony dirt. It travels through the root into the plant.
The first visible symptoms generally come on at first flowering but can occur when the plant is stressed. The first thing you will see is the yellowing of the plant’s lower leaves. Once the fungus infiltrates the plant’s vascular system, it interferes with and in severe cases eventually blocks off the circulation of water and nutrients through the plant tissue, killing the plant. The yellowing starts on the sections of the leaves that lie between the leaf veins and gives the leaves a streaked appearance. Eventually the edges of the leaves begin to curl upward. The bine itself becomes swollen, and if you cut into it you will see that the interior of the bine is streaked with brown. Eventually the leaves become entirely brown and fall from the bine. The cones themselves literally wither on the bine. When the plant dies, the fungus, presumably well nourished on all the resources that should have been going into the plant, retreats back into its survival structure, where it waits for its next victim.
Verticillium wilt appears as severely wilted, curled leaves that quickly die. Photograph by S. Radisek, Compendium of Hop Diseases and Pests, American Phytopathological Society
A hop bine suffering from verticillium wilt appears swollen. Photograph by S. Radisek, Compendium of Hop Diseases and Pests, American Phytopathological Society
PREVENTION AND CONTROL
If you believe you have verticillium wilt in your soil, the best option is to avoid planting hops there; choose another location for your crop. There is nothing that can be sprayed on hop plants to kill verticillium wilt once it has gotten into a plant. Soil fumigation will kill it in the soil, but soil fumigation uses highly toxic chemicals that are not only bad for people and the environment but are bad for the soil itself.
One very sustainable means of controlling verticillium wilt is crop rotation. By planting crops that are not potential hosts for verticillium wilt over a period of years you can use the land productively while depriving verticillium wilt of a host until you can be sure it has died out. But hops are perennial plants and do not fit into a crop rotation model. You’d have to rotate other nonsusceptible crops for more than four or fifteen years, depending on the verticillium species, before planting hops.
Once it strikes a hop yard, verticillium wilt is very difficult to eradicate. The best recourse is to relocate the hop yard. Photograph by S. Radisek, Compendium of Hop Diseases and Pests, American Phytopathological Society
As a safeguard against the eventual appearance of verticillium wilt in the hop yard, the best and really only option you have is to plant tolerant varieties of hops. Tolerant varieties include Cascade and Perle. Fuggle is particularly susceptible.
The fungus spreads when roots from an infected plant touch the roots of a neighboring uninfected plant. It can also move into different locations in the hop yard through the cultivation of the soil. In some cases, its spores travel on the wind. In addition, common weeds such as pigweed and lamb’s-quarter can contract verticillium wilt and spread it through the hop yard. To make matters even trickier some plants, including hops and weeds, can have verticillium wilt and not show it. They can, however, spread it. So if you think you have verticillium wilt in the hop yard it is best to remove weeds and hop vegetation left behind after harvest from the hop yard.
Hops pull a lot of nitrogen from the soil, and because much of this nitrogen remains in the vegetation of the plant after harvest, hop farmers often return the vegetation to the field and let it decompose. This is not a good idea if you have verticillium wilt in your hop yard as it will just reintroduce the pathogen into the soil. Research has shown that small amounts of verticillium wilt spores can even survive composting, so returning even composted hop vegetation that has been exposed to verticillium wilt to the yard should be avoided.
VIRUSES
A virus is a microscopic particle that only becomes active when inside its host of choice. Hop viruses only come to life and begin to reproduce when they are present in the plant itself.
APPLE MOSAIC VIRUS
Apple mosaic virus is the most significant viral disease impacting hops around the world. It also affects apples, pears, and roses but does not naturally transmit from one species to another. Plant propagation is the main way this disease spreads, and the only way to make sure you’ll keep it out of your hop yard is by purchasing only certified healthy rhizomes and plants. This is worth doing because if unleashed, the destructive virus is capable of reducing your yield of cones as well as the production of valuable alpha acids by as much as 50 percent. Sometimes plants can be infected with apple mosaic and hop mosaic viruses at the same time, and the loss becomes even more severe. Its primary symptom is a yellowing of leaves in an oak-leaf-like pattern. It becomes most severe when the temperature remains under 80 degrees Fahrenheit (26.7 degrees Celsius) for a period of time, then becomes hotter. Insects do not spread this disease through the hop yard—but people do on their hands, tools, and equipment.
CARLAVIRUS COMPLEX
The carlavirus complex is a family of viruses that includes hop mosaic virus, hop latent virus, and American hop latent virus. Carlavirus is short for “carnation latent virus.” A latent virus is one that has the ability to lie dormant within a plant for a period of time, then suddenly begin to reproduce at a high rate. When the virus is dormant the host does not show any symptoms but can still pass the virus onto a new host. This is a great trick. The three hop viruses included in the carlavirus complex can occur in the hop yard in various combinations. The hop latent and hop mosaic viruses are found worldwide, whereas the American hop latent virus is only found in North America.
The most common way the carlaviruses enter the hop yard is through infected plants and rhizomes. Once in the hop yard the virus can be spread by aphids, so control of aphids can reduce the virus’s spread through the yard. The viruses are also spread through mechanical cultivation. While hop latent virus and American hop latent virus do not cause obvious symptoms, hop mosaic virus causes a pattern of yellow mottling on the leaves and weakened plants with reduced bine growth. In some cases the bines will not even have the strength to wind around the string.
Apple mosaic virus first appears as a yellowing of the leaves and can ultimately cut the yield of alpha acids in half. Photograph by D. H. Ghent, Compendium of Hop Diseases and Pests, American Phytopathological Society
Symptoms of the carlavirus complex include yellow spots on the leaves and weak growth. Photograph courtesy of David Gent, USDA Agricultural Research Service, Bugwood.org
Golding, and varieties descended from it, seem to be most susceptible to hop mosaic virus. Infection with the carlaviruses slows plant growth, making it hard to establish new plantings, and reduces yield. As with all viruses, the best way to protect your hop yard is to only obtain new plant material from certified healthy plant sources. If you find a plant in your hop yard stricken with one of these viruses the best thing to do is to remove the plant as well as any neighboring plants. Since the disease can be transmitted from plant to plant by mechanical activities such as pruning, it is important to clean tools (and hands or gloves) in between plants.
HOP STUNT VIROID
Hop stunt viroid is a serious disease that afflicted hops in Japan and South Korea before being discovered in North America in 2004. It stunts the plant as well as the cones, dramatically reducing the alpha acid. Like the carlaviruses, the hop stunt viroid is a latent viroid. A hop plant can be infected with the viroid, and spread it to other plants, for several years before it shows any symptoms. Although the plant may appear healthy it will produce less alpha acids, even before other symptoms strike. Once it shows symptoms, the plant itself will appear stunted, with curling, yellowed leaves, and produce small cones. The bine often has trouble climbing because it does not develop trichomes. The viroid is present in the sap of the plant and is spread through the hop yard as workers prune through the sap that gets on the tools. The viroid usually enters the hop yard on diseased plants and rhizomes. If the hop stunt viroid shows up, the best thing to do is to remove the diseased plants. Unfortunately, already infected plants could still appear healthy while continuing to spread the disease.
It may seem like everything is out to get your hops, but remember, the best defense against disease is a well-nourished, carefully tended plant grown from healthy stock. Maintaining a healthy soil ecology rich in organic matter and populated by beneficial microbes will go a long way toward fending off disease. Scouting your hop yard regularly and keeping a sharp lookout for the telltale signs of disease will enable you to act quickly to control an outbreak and keep it in check.
A hop plant can be infected with hop stunt viroid without showing symptoms and still be contagious for several years. When symptoms do start to show, the plant will exhibit curling, yellowed leaves and produce small cones. Photograph courtesy of David Gent, USDA Agricultural Research Service, Bugwood.org