Downy Mildew (Pseudoperonospora humuli)

     This is the most troublesome disease of hop growers along the entire east coast. You cannot grow hops in this part of the country outdoors and not contract Downy Mildew (DM). No matter how certified disease free your plants are when you purchase them, it has been proven that after planting, they will get DM. It could show up the first year or maybe the second year. But it will show up. However, it does not mean that you can not grow and produce hops in this area. Like all pests, DM is all about control and management, not eradication. Do not get scared away from growing hops because of all the things you might have read or heard about DM.   


     This pathogen overwinters in dormant buds and crowns and can lead to basil spikes in the spring (Fig. 1). These spikes often indicate a systemic infection which means that the DM pathogen is inside the plant and crown tissue. As the plant grows, some lateral sidearms may become stunted from a systemic DM infection or from the DM pathogen entering the tip of the growing shoots, known as the apical meristem (Fig. 2) Non-systemic DM (Fig 3) appears as wet-looking spots on the undersides of leaves and eventually turn the leaf necrotic in those spots. This pathogen can also attack the cones through both systemic and non-systemic vectors.   The disease sporulates from the infected tissue when temperatures are greater than 43 degrees Fahrenheit and humidity is above 90%. Infection is most favored by mild to warm temperatures of 60 to 70 degrees Fahrenheit and free moisture (such as after or during a rain or morning fog and dew event) are present for at least 1.5 to 2 hours. This allows the swimming zoospores created by the DM sporanogia to easily infect new plant tissue. 


(Fig 1)  Notice the down curling stunted leaves and shortened intenodes


                                                                               (Fig 2)  Stunted apical meristem

(Fig 3)  From left to right: DM "wet-looking spots", DM after fungicide application, non-systemic DM on leaves and cone

     Even though DM is found everywhere in the mid-Atlantic to include the soil, the wind, and on us, it does not mean the end. Just because a plant may have a systemic infection of DM does not mean it is doomed. Not all of the shoots that emerge in the spring will be stunted. In fact, some cultivars can be infected heavily for years before the infection level starts to decrease plant health and weaken carbohydrate reserves to a critical point. Furthermore, a particular cultivar or individual plants may lose their resistance to DM. This can be a result of pathogen mutation and/or plant age and its reduced ability to fight disease. For this reason, many commercial hop growers monitor their hop yards and replace plants as necessary based on observations. This is a standard practice in the large commercial yards of the Pacific Northwest and is factored into the cost of production.  

     Controls for DM include cultural, mechanical, and spray treatment. Cultural practices include growing varieties that are more tolerant of DM (resistance), and keeping the hop yard mowed and hops trimmed properly to reduce hop yard humidity levels and moisture retention zones. Also, planting the correct types of ground cover between and around the hop yard will facilitate keeping hop yard humidity levels lower. Clover is pretty, but it can retain moisture very well and increase the humidity in a hop yard. Planting hops in raised beds or rows will help facilitate water runoff.   Mechanical controls include removing all basal spikes in the spring and removing aerial sideshoot spikes as they appear. This is one of the reasons spring pruning, or burn back of initial growth, is performed. In larger yards, they typically use a desiccant spray to kill back the spring growth rather than spend many labor hours hand trimming. This is to remove basil spikes and keep the ground area dry at the base of the plants.               

     As a side note: using mowers and hand trimming, without proper sanitizing/disinfecting between every crown, can lead to spreading the DM pathogen very easily from an infected plant to non-infected plant tissue. The cost and time required to clean the mowing/trimming equipment between every crown is prohibitive in commercial yards; thus the risk of spreading DM throughout the yard is factored in. Furthermore, DM is not as problematic in the high arid areas of the Pacific Northwest like it is here in the mid-Atlantic.                 

     Finally, there are spray options. These come in the form of synthetic and organic, as well as powder and liquid. Organic methods include Copper, Ramnolipids, Neem oil, Oxidate and Mineral Oil. Most all organics are not systemic, meaning that they do not enter the plant tissue to help kill the pathogen or pest, but rather kill or smother on contact on the leaf or tissue surface. Most synthetic fungicides are systemic and range in modes of action. These synthetics are typically very costly, but last longer because they require less product per gallon of water. Two things to remember when using fungicides; one is to ALWAYS rotate the products you use to prevent resistance buildup in the DM pathogen. Secondly, pay close attention to the pre-harvest interval (PHI) of the fungicide you are using so that you do not harvest cones that are still containing harmful traces of the fungicide.