Germination and Establishment of Infection on Grape Berries by Single Airborne Conidia ofBotrytis cinerea

Abstract

Table grapes (cv. Dauphine) at different phenological stages were dusted in a settling tower with dry conidia of Botrytis cinerea. The berries were incubated for periods of 3 to 96 h at high relative humidity (RH; ±93% RH, moist berries), or were covered with a film of water (wet berries). Germination of the solitary conidia, appressorium formation, stilbene and suberin induction by germlings, and germling viability were examined by fluorescence microscopy after each incubation period. Isolation and freezing studies were conducted to determine surface colonization (berries left unsterile) and penetration (surface-disinfested berries). Symptoms were determined on berries incubated at a specific wetness regime, kept dry for 10 days, and then incubated for 4 days at high RH. Microscopic observations indicated that germination was delayed on immature berries, but proceeded at a high rate on mature berries. Growth was invariably restricted on moist berries. Attempted penetration was always direct. Stilbene and suberin were generally induced early and were intense on berries at the pea-size and bunch closure stages. Dieback of conidia and germlings occurred at a significantly higher rate on wet than moist berries, and was more pronounced on immature than on mature berries. The segment isolation and freezing studies showed that infections in grape berry cheeks established by this infection mode were few. Extended incubation periods did not lead to substantially higher rates of surface colonization and skin penetration. Disease symptoms did not develop during the 14-day period on the berries transferred to dry perspex chambers, irrespective of phenological stage, incubation period, or wetness regime. According to these findings, this mode of infection should not contribute to a gradual build-up of secondary inoculum in the vineyard, and to B. cinerea epiphytotics.

Effect of Fungicide Spray Cover on Botrytis Cinerea Infection in Grape Bunches

“…Since B. cinerea incidence on paraquat medium was the result of infection after the termination of host resistance by the herbicide (Grindat & Pezet, 1994), infection levels would therefore provide a clear indication of the fungicide action in the absence of active host resistance. When studying B. cinerea infection on Kerssies medium, fungicidal action was masked by host resistance (Coertze & Holz, 1999;Coertze et al, 2001), which might have contributed to the variability of incidence data.…”

“…The different isolation treatments provided conditions that facilitated the development of B. cinerea by conidia on the surface of bunch tissue, or by mycelia in the tissue, during the period of incubation. Previous studies (Coertze & Holz, 1999;Coertze et al, 2001;Gütschow, 2001) with grape bunch tissue on Kerssies medium showed that segments retained their active defense abilities and no superficial mycelial growth developed on the segments during the first five days of incubation. Hyphal growth usually occurred from cells underlying the cuticle into the medium after five days, which indicated direct penetration by conidia on the surface, and the development of mycelia from the host tissue during the incubation period.…”

“…Conidia and germlings can survive for extended periods on structural bunch parts but do not survive for long on the surface of immature berry skins when latent infection occurs in the skin . B. cinerea symptom expression is furthermore predominantly associated with pedicels and the bases of berries under South African conditions (Coertze et al, 2001;Coertze & Holz, 2002;Holz et al, 2003). This is also true for other fungi (Penicillium, Aspergillus, Alternaria, Mucor and Rhizopus spp.…”

“…B. cinerea development and colony formation on segments therefore gave an indication of infection at that site as influenced by the amount of surface conidia and mycelia confronted by active defense. Paraquat terminated host resistance in the cells of the cuticular membrane without damaging host tissue (Grindrat & Pezet, 1994), and allowed the development of conidia and mycelia on the surface, and mycelia in the tissue in the absence of active defense (Coertze & Holz, 1999;Coertze et al, 2001).…”

Resistance to Botryticides

“…Infections of berries occur at temperature interval between 20 and 25 C and are accomplished by conidia. Germlings that developed from conidia enter grape berries through different pathways, namely through stigmata, pedicels, natural openings and wounds, or by direct penetration of the cuticle (Coertze et al, 2001;Holz et al, 2003). Conidia are deposited on berry surphace by air, rain or insects.…”