Powdery mildew of Hops

Hosts and Symptoms. The pathogen that causes powdery mildew of hops was once considered to be Sphaerotheca macularis, which is capable of infecting many plants; however, in recent years, the pathogen that causes powdery mildew of hops has been taxonomically classified as Podosphaera macularis. This ascomycete is only pathogenic on hop plants, including both ornamental and wild hops. The host range of many Podosphaera macularis strains is restricted by the existence of resistant hop varieties, such as the “Nugget” variety of Washington state and Oregon, although in recent years, resistance within this hop variety has been overcome in the laboratory. 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. Occasionally, chleistothecia are visible as small, black dots on the undersides of leaves.
Disease Cycle. 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. 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.