Viruses and Viroids Infecting Hop: Significance, Epidemiology, and Management.

Abstract

The hop (Humulus lupulus) is a hardy, climbing, dioecious, perennial plant native to Europe, Asia, and North America (172). The genus Humulus belongs to the family Cannabaceae and contains three species: H. japonicus, H. lupulus, and H. yunnanensis (32,117). Hops are grown predominantly for their cones (strobiles), which contain glands producing resins, essential oils, and polyphenols (Fig. 1). These compounds are used primarily to add bitterness and aroma to beer. The most important of these compounds for brewing are the alpha acids. Alpha acids are acylsubstituted phloroglucinols, differing from each other only in the nature of the acyl R side chain. They can be separated into humulone (R = isovaleryl), cohumulone (R = isobutryl), adhumulone (R = alphamethyl butryl), prehumulone, and posthumulone (127). The main properties of alpha acids in relation to beer production are improved foam stability, suppression of gushing, and contributions to bacteriological stability (48). The bitterness of beer is related to stereoisomer formation of each major alpha acid in the brewing process (124). Beta acids are predominantly separated into lupulone, colupulone, and adlupulone. They have limited bittering power but are particularly important because of their bactericidal properties (194). More than 200 other essential oils also occur within the cones, including hydrocarbons and oxygenated and sulfur-containing compounds, which are responsible for the aroma and flavor of the final product (170). Hops are also grown for their medicinal and soporific effects, as ornamental plants, and as edible delicacies (146). The areas in which hop can be grown are limited by strict day length and temperature requirements for flowering and hence cone production. Production is generally restricted around 35° latitudes in both hemispheres (32,117). Supplementary artificial lighting has been used to produce hops in areas of lower latitudes, such as in South Africa (196). In 2005, the most significant regions of hop production (i.e., countries with >1,000 ha) were Germany, the Czech Republic, Poland, Slovenia, Ukraine, the United Kingdom, the northwestern states of the United States (Idaho, Washington, and Oregon), and China. Lesser quantities were grown in 14 other European countries. A few other countries grow 500 ha or less, most notably Japan, Argentina, Australia, South Africa, New Zealand, and India (26). The largest area of production was in Germany (17,161 ha producing 34,466.8 metric tons), while 11,956 ha were grown in the United States, producing 24,002 metric tons (26). Viruses and viroids pose significant constraints to the production of high yields of hop cultivars worldwide. In some countries, such as Australia and New Zealand, these pathogens are considered the only significant pathological problems. This is due to the absence of severe fungal diseases, such as powdery mildew (154) and downy mildew (155). Infections by five viruses and two viroids are or have been widespread and important in commercial hop yards. A further 12 viruses (of which two are poorly characterized) and 1 viroid have been reported in hop; these either have limited distributions or occur only sporadically and are not considered important. The viruses considered generally important are the three carlaviruses, Hop mosaic virus (HpMV) (3,20,94,156), Hop latent virus (HpLV) (4,21,55,150,180), and American hop latent virus (AHLV) (5,22,149); the ilarvirus Apple mosaic virus (ApMV) (27,28,46,64); and the nepovirus Arabis mosaic virus (ArMV) (6,9,50,82). The important viroids infecting hops are Hop latent viroid (HpLVd) (10,14,24,25,151) and Hop stunt viroid (HpSVd) (162–164,201). Although the problems posed by viruses and viroids for the production of hops are not unique among perennial crops, they are particularly challenging because the rates of spread are often much higher than in other crops such as top fruit. As such, the control measures used with hops present a good case study for the control of these pathogens in crops where long life (hop yards are typically kept for up to 20 years and may last 50 years) means they are repeatedly exposed to infection. The reasons for rapid spread relative to tree fruit crops are unclear, but one might speculate that the very rapid and massive annual growth (the entire aboveground part of the plants is replaced every year, and main stems generally grow more than 5 meters in only 3 to 4 months) in closely spaced plantings favors both mechanical transmission (for HpSVd, HpLVd, and ApMV) and heavy infestations with aphids (for the carlaviruses). The study of hop viruses and viroids also emphasizes the need to fully understand and quantify the effects of these pathogens on yield and/or quality of crops, which are highly dependent on cultivar, pathogen strain, and environmental conditions.