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Code: OIV 451

Name: Inflorescence: degree of resistance to downy mildew

Category: , GROUP OF CHARACTERISTICS, ORGAN

Subcategory: DOWNY MILDEW, BIOTIC RESISTANCE - FUNGUS, INFLORESCENCE

Description

Due to the wide genetic variability of the downy mildew pathogen (Plasmopara viticola) worldwide, the choice of inoculum source is no guarantee of universal judgment on the disease levels in open field conditions. The lab tests must be conducted on samples of inflorescences with tight or separated flowers (stages BBCH 55 – 57), taken from vines grown in greenhouses or in the field and not sprayed with phytosanitary products, including resistance inducers and biostimulants. Operational methodology Observation during the pre-flowering (flower button formation = E-L stage 17) time on vines not treated with chemicals, grown in field or better under greenhouse conditions (it is possible to employ agronomic technique enabling flowering anticipation such as fruiting cutting production). a) Inoculum maintenance: as in OIV 452-1. b) Propagation protocol: as in OIV 452-1. c) Inoculation protocol: soak detached inflorescences in a spore suspension (10,000 spores/ml) for 2 h, then place samples in sterile plates with 1% agar covered with sterile wet filter paper. Insert the peduncle in the agar (this step is essential to keep inflorescences alive) and incubate plates at 21 °C in complete darkness (with aluminium coat) for 2 days. Then place them under light conditions with a photoperiod of 16/8 h (light/dark) for supplementary 5 days. Remark that if the inoculum remains on the inflorescence too long, rotting can be produced. Therefore, 24 hours after inoculation, the spore suspension must be removed by drying the inflorescences for 40-45 minutes under biological laminar flow hood. d) Symptom annotation: observe extension and density of sporulated area on the total length of the sample. Distribution of the disease on the total length of the inflorescence: 1 = Highly abundant sporangiophores involving the whole organ and the majority of the inflorescences. 3 = Abundant sporulation localized in large patches. 5 = Widespread sparse sporangiophores. 7 = Sporadic sporulation. 9 = Absence of sporulation. Optional additional parameters SD: sporulation density of Plasmopara viticola on the single flower cluster: 1 = Extremely dense and overlapping sporangiophores. 3 = Relatively dense, but confined sporangiophores. 5 = Sparse sporangiophores. 7 = Sare sporulation at single sporangiophore level. 9 = Absence of sporulation. Nspor: spore count. Put each sample in a 50 ml tube, add 1-5 ml of distilled water and vortex. Discard plant tissue and centrifuge the spore solution at 3,000 rpm for 10 minutes. Eliminate 0.7-3.5 ml solution paying attention not to touch the bottom of the tube. Gently agitate the solution to resuspend spores and count them on a counting chamber.

Level expression