Atmospheric concentrations of Sphaerotheca macularis conidia were monitored for 2 years on a strawberry crop in Huelva (southwestern Spain). The presence of airborne conidia was determined to assess the role of weather conditions on conidial release. The relationship between airborne conidia and incidence of powdery mildew on fruit was also studied. Concentrations of conidia were estimated with a Burkard volumetric spore sampler. The presence of conidia was related to temperature, relative humidity, and rainfall, with a positive correlation for the first factor and a negative correlation with the other two. The presence of conidia in the air was positively correlated with disease incidence. A diurnal pattern of conidia release was observed.

Abstract:The early stages of the infection of attached leaves and petioles of strawberry (Fragaria ×ananassa Duch. 'Camarosa') by Colletotrichum acutatum Simmonds were studied using scanning and transmission electron microscopy. Pre-penetration events of these tissues were similar, but the production of secondary conidia (microcyclic conidiation) was detected only on leaves. At the ultrastructural level, different stages of maturation of appressoria were observed and described. In young appressoria, the cell wall was composed of two layers and the plasma membrane displayed a wavy appearance. In the following stage, the appressorium developed a third electron-transparent layer between the cell wall and the plasma membrane. This new electron-transparent material was especially visible in the region of the appressorium near the cuticle. The plasma membrane of this appressorium showed a smooth appearance. Afterwards, a penetration peg emerged through the pore penetrating the cuticle and reached the epidermal wall where it enlarged to form an intramural infection vesicle. Both structures of infection, the penetration peg and the intramural infection vesicle, produced during the early phases of infection of strawbery tissues by C. acutatum, have not been previously reported and confirm that its invasion strategy is that of a subcuticular intramural pathogen. Once the infection was well established, abundant subcuticular and intramural hyphae were produced on petioles, causing severe degradation of the host cell walls. Occasionally, the cuticle appeared disrupted in those regions where the host walls were very degraded and dilated. Differences between colonization of petioles and leaves were observed.Key words: Colletotrichum acutatum, Fragaria ×ananassa, infection vesicle, invasion strategy, penetration peg.Résumé : À l'aide de la microscopie électronique par balayage et par transmission, les auteurs ont étudié les premiers stades de l'infection des feuilles attachées et des pétioles du fraisier (Fragaria ×ananassa Duch. 'Camarosa'), par le Colletotrichum acutatum Simmonds. Les évènements de pré-pénétration de ces tissus sont similaires, mais la production de conidies secondaires (conidiation microcyclique) ne s'observe que sur les feuilles. Les auteurs ont observé et décrivent les ultrastructures, aux différents stades de maturation des appressoriums. Chez les jeunes appressoriums, la paroi cellulaire est composée de deux couches et la membrane plasmique montre une apparence ondulée. Au stade suivant, l'appressorium développe une troisième couche transparente aux électrons, entre la paroi cellulaire et la plasmalemme. Ce nouveau matériel transparent aux électrons est particulièrement visible dans la région de l'appressorium voisin de la cuticule. La plasmalemme de cet appressorium est d'apparence lisse. Par après, l'hyphe de pénétration émerge à travers le pore et pénètre la cuticule pour atteindre la paroi épidermique, où il s'élargit pour former une vési-cule d'infection intramurale. Les deux structures d'infection...

Field experiments were conducted in southwest Spain for three consecutive years from 2000 to 2003 to evaluate the effectiveness of solarization and Trichoderma spp., alone and combined, in reducing Phytophthora cactorum soil populations and consequently leather rot on fruit of strawberry plants. Plots (12.5 by 3.3 m), never treated with methyl bromide, were naturally infested by P. cactorum. Solarization was conducted during the summer, using clear 50-µm low-density polyethylene mulch. Trichoderma spp. were applied via drip and dip, adding to the soil 7 days before planting (108 conidia/m2), and strawberry roots were dipped in a suspension of Trichoderma spp. (106 conidia/ml) prior to planting. Solarization reduced the soil P. cactorum population 100% in year 1, 47% in year 2, and 55% in year 3 relative to the untreated control. Trichoderma spp. applications reduced soil populations of P. cactorum and reduced leather rot incidence 76.6% in year 1 and 33.8% in year 2 compared with the untreated control. The combination of solarization and Trichoderma spp. reduced P. cactorum soil population the most each year, 88.9% in January 2001, 97.6% in 2002, and 99.0% in 2003. The very promising effect of Trichoderma spp. and solarization against P. cactorum indicates that there may be future alternatives to traditional chemicals for disease control.

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