Podosphaera macularis (formerly Sphaerotheca macularis) is an ascomycete fungus that causes powdery mildew of hops. It is only known to be pathogenic on hop plants, including both ornamental and wild hops, and hemp.
Taxonomy The pathogen that causes powdery mildew of hops was once considered to be Sphaerotheca macularis, which is capable of infecting many plants; in 2000, the pathogen that causes powdery mildew of hops was taxonomically classified as Podosphaera macularis.
Distribution P. macularis can be found in most hop growing regions in the northern hemisphere. Powdery mildew is one of the oldest fungal diseases known on cultivated hop, dating back to reports in England from the 1700s, and has been recorded in eastern North America since at least the early 19th century. It was partly responsible for the decline of the hop industry in New York state. The fungus was first reported in the Pacific Northwest of the United States in June 1997 and powdery mildew is now epidemic annually in this region, where 98% of US hops are grown. As of 2020, only the MAT1-1 mating type was known in the Pacific Northwest suggesting there has only been one introduction there, whereas in Europe and eastern North America both mating types are present. This is important as in the latter case the fungus can overwinter as chasmothecia (fruiting bodies) on dead leaves and cones; if only one mating type is present it must overwinter as mycelium on living host tissue such as crown buds that lead to heavily-infected “flag shoots” in the following season, although these are rare (<1% of plants).
Symptoms When disease does occur, early symptoms include chlorotic spots on the leaves of hop plants. Spots may fade to gray or white as the season progresses. Signs include white clusters of hyphae, which are often present on the leaves, and in some cases can infect the cone itself. If this infection occurs, a brown, necrotic lesion may develop. When both mating types exist within a population, chleistothecia can form and are visible as small, black dots on the undersides of leaves.
Disease cycle In order for pathogenesis to occur, a viable pathogen, susceptible host, and conducive environment must simultaneously be present. Podosphaera macularis overwinters on the soil surface in debris as fungal survival structures (chasmothecia) or as mycelia in plant buds. These chasmothecia are formed closer to the end of the growing season. The characteristic morphology of chasmothecia of Hop Powdery Mildew are spherical black structures with spiked appendages. When favorable conditions are encountered during early spring, the asci (sac-like structures) within chasmothecia will rupture and ascospores will be discharged. Specifically, the favorable conditions for ascospore release include low light, excess fertility, and high soil moisture. Additionally, optimal infection is observed when the temperature is between 18 and 25 °C. Furthermore, the ascospores act as the primary inoculum and are dispersed passively by wind. Upon encountering a susceptible host plant, the ascospores will germinate and cause infection. The germ tube of P. macularis plays an important role in determining the pathogen's viability, because it can penetrate its host in approximately 15 hours. The germ tube begins branching, leading to as many as three potentially conidia-forming germ tubes. As the pathogen invades host tissue, it establishes a haustorium to facilitate the collection of nutrients from the host cells. Following infection, masses of asexual spores (conidia) will be produced during the season. It is these masses of conidia that contribute to the characteristic white, powdery appearance of infected plants. The lower leaves are the most affected, but the disease can appear on any part of the plant that is above the ground. These conidia are dispersed through wind. Thus, Podosphaera macularis is a polycyclic pathogen as conidia are produced/dispersed during the growing season and can further infect additional host plants. Particularly, the disease will be noticeable on infected plants as soon as the hop shoots start to emerge with the latent period being approximately 10 days at 12 and 15 °C compared to 5 days at 18-27 °C. These spore-covered shoots that emerge from infected buds are called “flag shoots” and will be stunted with distorted leaves. Periods of rapid plant growth are the most favorable for infection. In addition, the period in which lateral branch development takes place within the plants is also very vulnerable to the development of the disease. Due to Podosphaera macularis causing local infection, only the location of the host plant tissue where spores have landed will develop the disease.
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