Watch your hops plantings for hop cyst nematodes
Hop cyst nematode, the most common nematode associated with hops, is a pest of which you should be aware.
Hop cyst nematode (Heterodera humuli) is a little known pest in Michigan, but one that you should still be aware of. It is the most common plant parasitic nematode found on hops. It is known to occur in the United States, Canada, Europe, New Zealand, Australia and other locations. This tiny roundworm is found in the soil and in and on hop roots. There are one to two generations per year. Don’t expect to be able to see this pest easily. The cysts are brown to black, lemon-shaped and smaller than the head of a pin. Males are transparent and less than 1/25th of an inch long. Females are cream-colored and lemon-shaped and about 1/50th of an inch long.
The adult female hop cyst nematode produces up to several hundred eggs inside her body, then dies and creates a protective cyst around the egg mass that stays attached to the roots. Thus protected, the eggs can remain viable for several years. As soil temperatures increase and hop roots begin to grow in spring, the juveniles (J1) molt within the egg to form the second stage juveniles (J2) that invade the roots. The J2 remain in the plant, feeding on cell sap and, after several more molts, become either males or females. The wormlike males migrate to mate with the sedentary, lemon-shaped females who remain attached to the roots. After mating, the females continue to feed, produce eggs and then die, leaving the eggs behind in a protective cyst to start the cycle anew.
Hop plants are generally quite vigorous and can tolerate some nematode feeding. However, high levels of this nematode can cause symptoms such as yellowing of leaves and reduced length of bines. Hop cyst nematodes can also interact with a soilborne fungus (Verticillium albo-atrum) that causes a disease called Verticillium wilt to intensify the severity of wilt symptoms. It appears that there may be differences in the tolerance of different hop cultivars to hop cyst nematodes, but not much research has been done.
Hop cyst nematodes are not a quarantine pest in the United States. Of the three major hop-producing states in the west, Washington has a hop disease quarantine for Verticillium wilt, powdery mildew, hop stunt viroid, Arabis mosaic viruses and Ilar viruses, but not for hop cyst nematodes. Oregon has a quarantine against powdery mildew on hops. Idaho has quarantines for Verticillium wilt and powdery mildew on hops. All states have some sort of nursery certification requirement stating that nursery stock shipped from that state must be free of pests, diseases and noxious weeds, and be accompanied by a shipping certificate issued by the Department of Agriculture from the state of origin, but that is not the same thing as a quarantine.
The best protection against getting a “gift with purchase” such as unwanted pests accompanying your hop plants or rhizomes is to know the source you are purchasing from and make sure that it is reputable, or purchase a certified hop plant from the National Clean Plant Network and propagate it yourself. There have been several recent detections of hop cyst nematodes from unthrifty plants in Michigan hop yards.
If you are a hop producer and are purchasing plants or rhizomes from the three states known to have H. humuli (Washington, Oregon and Idaho), you may want to have them tested for hop cyst nematodes at MSU Diagnostic Services before ever putting them in the ground. Cyst nematodes persist for long periods of time, so the main control strategy is avoidance. There are no post-plant chemical control options for nematodes on hops.
Related MSU Extension article: Purchasing hops for planting