6.6
Leaf test system for assessing the tolerance of hops to downy mildew (Pseudoperonospora humuli)
Project Management: Dr. E. Seigner, A. Lutz Team:
B. Forster
Collaboration:
Th. Eckl, IPZ 1e (biometrics)
Objectives A standardized test system for leaf evaluations, known as “detached leaf assay,” was established in the laboratory. It allows for the collection of reliable data about a breeding line’s Peronospora tolerance. These tolerance tests rely entirely on so-called secondary infections, that is on the degree to which a hop is resistant or susceptible to fungal zoosporangia that are administered to the leaf surface. At high air humidity, the sporangia release the zoospores, which then penetrate through openings in the leaf surfaces into the interior of the leaf, where they grow into a fungal mycelium unless the hop mounts a defense reaction. Vulnerable hops, therefore, develop fungal mycelium sporulating on the leaf as a typical infection symptom. Method The first step is to spray the underside of the hop leaves with a Peronospora sporangia suspension. Five to 14 days later, the reactions of the leaves (no visible symptoms, chlorosis, necrosis, sporulation) are assessed visually. The evaluation is rated on a scale from 0 to 5, with a focus on sporulation: 0 (highly tolerant) = no symptoms, 1 (tolerant) = 1-10% of the leaf surface affected, 2 (medium) = 11-30%; 3 (vulnerable) = 31-60%; 4 (highly vulnerable) = 61-80%; 5 (extremely vulnerable) = 81-100%. The disease index was calculated according to Townsend and Heuberger and evaluated statistically using this rating data. On the leaves of hops that are more susceptible or less tolerant, chlorotic leaf spots with clear signs of sporulation appear on the underside of the leaves a few days after inoculation. Strong, early sporulation is a clear indication of a strong susceptibility to the fungus. Dark brown necrosis spots become visible at a later stage. These leaf responses vary depending on leaf age. Young leaves in the growth phase show clearer symptoms than do older ones. Tolerant hops, on the other hand, suppress sporulation completely or they show smaller necrosis spots on the leaves as a defense reaction, especially in the early stage of infection (hypersensitive reaction of the host cells).
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