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Evaluation of Inoculation Methods to Reproduce Sunflower Premature Ripening Caused by Phoma macdonaldii

Three inoculation methods were evaluated for effectiveness to cause sunflower premature ripening (PR). Evaluations were conducted on a sunflower (Helianthus annuus) cultivar susceptible to PR in replicated, multilocation experiments under greenhouse conditions. Plants were inoculated with Phoma macdonaldii, either with mycelium, conidia, or infected residues at the stem base or with buried residues. Disease severity (DS) was measured by percent girdling necrosis at the stem base and percent final PR; the infection spread was assessed using the area under the disease progress curve (AUDPC). Inoculation with mycelia or 1 × 10⁶ spores/ml caused significantly more DS and PR than lower spore concentrations or infected residues (P < 0.05). Amending soil with residues induced root necrosis but no PR. P. macdonaldii was mainly isolated at the stem base and above but rarely on root systems. Microscopic evaluations showed that hyphae colonized mainly the cortex and vascular stem tissues. The overall results demonstrated a clear role of aerial infection in PR compared with soilborne inoculum, and that inoculation at the stem base with a spore suspension could be used for screening genotypes for resistance to PR.