Potential ascospore production and resulting blossom blight byMonilinia fructicolain unsprayed peach trees

Abstract

Apothecial emergence of Monilinia fructicola Wint. (Honey), potential ascospore dose (PAD), infection periods (IPs), and blossom blight (BB) were quantified in a peach (Prunus persica) block in 1996 during bloom. Flowering began on 21 August, with full bloom 1–7 September and last flowers on 20 September. First apothecia appeared on 25 August after 15 mm rain on 18–20 August, but shrivelled after a few days of drying weather. Main apothecial emergence began, peaked, and declined on 4, 7, and 13 September respectively. Apothecial density from 4 to 13 September was 4.4 per m² of ground but 90% of these were confined to a 15 m wheel rut in the observation plot. Apothecia on bare ground were short‐lived compared with those protected by overhanging grass. Total hymenial surface area within the observation plot was 58.7, 88.6, 18.2, and 5.7 cm² on 4, 7, 10, and 13 September respectively. PAD on each date was 9.35, 14.14, 2.85, and 0.92 × 10⁶/m², or 27.26 × 10⁶/m² for the whole period. From 4 to 6 September, airborne ascospores trapped by a cyclone spore trap during daylight ranged from 0 to 39/litre air per min. Percentage of flowers contaminated by spores on 3, 5, 12, and 20 September was 10, 65, 68, and 92% respectively. Moderate IPs occurred on 2, 3, 5, 11–12 September, marginal on 16 September, and light on 19 September, resulting in blossom infections of 3, 12, 29, 29, 5, and 50% respectively. This corresponded to natural BB levels of 25% on 7 September and 62% on 27 September. A gradient of increasing BB from the windward to leeward end of the plot occurred. Primary inoculum for BB up to 15 September was primarily ascospores, and thereafter conidia. Sources of conidia were initially twig cankers, then diseased blossoms.