Hop Powdery Mildew (Podosphaera macularis) Spore Cryopreservation

Hop powdery mildew (HPM), Podosphaera macularis, an important disease organism for hops, is an obligate parasite, requiring constant culture on living plant tissue for strain maintenance.

This study determined the parameters required to successfully cryopreserve HPM spores for the first time and reduce the need for constant culture.

Spores of an Oregon HPM strain, OSU C-100 were desiccated over silica gel for 2-10 h to determine the spore moisture content (MC). Regrowth of the hyphae before and after drying and liquid nitrogen exposure was determined on glass slides and leaf discs of several susceptible hop cultivars. A second mixture of strains was later tested with the protocol.

Desiccation to an optimal 2-3 percent MC produced hyphal growth on slides and infection of leaf discs. The OSU C-100 HPM spore strain required 8-10 h desiccation to reach 2-3 percent MC while the mixed strains required 6-8 h due to slightly different MC when collected.

HPM strains should be placed in cryovials, dried to 2-3 percent MC over silica gel, cryopreserved by direct immersion in liquid nitrogen. They can be rewarmed for 1 min each in 45C and 20C water and the viability tested on isolated leaf discs.