| Guignardia bidwellii (Ellis) Viala & Ravaz (anamorph Phyllosticta ampelicida (Englem.) van der Aa) is the cause of black rot of grape, a destructive disease that has caused significant losses to grape (Vitis spp.) production, in the northeastern United States on the subgenus Euvitis (Vitis vinifera L., V. labrusca L. and Vitis interspecific hybrids).;The effects of temperature and surface wetness duration on the development of black rot of grape shoots by G. bidwellii conidiospores was evaluated on three cultivars, ‘Niagara’ (V. labrusca ), ‘Chardonnay’ (V. vinifera) and ‘Aurore’ (V. labrusca x V. vinifera). Plants were inoculated at combinations of seven temperature levels (12°C to 30°C in 3°C intervals) and six shoot wetness durations (10 to 30 hours in 5 hour intervals) and evaluated for black rot shoot symptom incidence, DS, and PD. Symptoms did not develop at 12 and 30°C at wetness durations less than 15 hours. All other temperature treatments promoted disease development on shoot tissue with wetness durations ≥10 hours. Analysis of variance (ANOVA) and regression analyses were used to determine linear, quadratic, or cubic functional relationships between temperature and shoot wetness duration and disease severity (%) and pycnidium density (pycnidia/cm2). Significant terms were incorporated into a regression model to describe the relationship.;Square root transformed DS was significantly (p<0.0001) influenced by wetness duration, temperature and the interaction term. The adjusted coefficients of determination (R2) for the polynomial regressions were all low (0.173).;Square root transformed pycnidium density was significantly (p<0.05) influenced by cultivar, temperature, and the temperature with wetness interaction. Each cultivar was examined separately due to the significant cultivar response. The response of ‘Aurore’ and ‘Chardonnay’ was significantly influenced by temperature. At 24°C significantly more pycnidia were produced than at any other temperature. Wetness duration was significant for ‘Niagara’, 20 hours of wetness produced a significantly greater number of pycnidia than any other wetness period.;Consistent development of black rot shoot symptoms by artificial methods has been difficult. A small humidity chamber was developed that promoted shoot infection and symptom development. Application of conidia at concentrations of 1x104 and 1x105 spores/ml promoted significantly (p<0.05) greater disease severity. Pycnidium production was significantly higher at an inoculum concentration of 1x105 spores/ml on Vitis shoots, regression analysis of the instantaneous growth rate calculated from daily internode measurements (GRI) immediately after inoculation was significant (p=0.0350, R2=0.026) for the square root (SQRT) of pycnidia density (PD-- pycnidia/cm 2) but not for symptom severity (DS) in the absence of the concentration terms in the analysis.;A refinement of the infection requirements was conducted on V. vinifera cv. ‘Chardonnay’ grapes, a minimum of 7.5 hours of shoot wetness at 21°C was suitable for infection. Disease severity and pycnidium density increased as leaf wetness increased at a given temperature.;DS and PD were both significantly (p<0.0001) influenced by temperature, wetness duration, the average growth rate immediately after inoculation (GRA 2) and the wetness-temperature interaction. The growth four to five days after inoculation (GRA5) also significantly influenced shoot DS (p=0.003) and PD (p=0.0005).;Two experiments looked at the susceptibility of V. labrusca, V. vinifera, interspecific hybrids, and V. riparia cultivars. Eighteen cultivars were screened for DS and PD from shoot infections by conidia. Cultivar significantly influenced DS and PD (p=0.0001). Individual runs differed in the order of the cultivars, but generally the V. vinifera cultivars, ‘Riesling’, ‘Cabernet Sauvignon’, and Chardonnay’, V. labrusca ‘Niagara’, and the interspecific hybrid ‘Aurore’ were significantly higher in disease severity than the other cultivars (p<0.05). Pycnidium density (PD) was generally highest among the V. vinifera cultivars, the V. labrusca cultivar ‘Niagara’ and the American hybrid cultivar ‘Cayuga White’. V. labrusca cultivar ‘Concord, recognized as susceptible to black rot leaf and fruit infections, was not among the most susceptible cultivars for shoot infections.;The influence of post infection relative humidity level (RH) on the development of shoot symptoms after inoculation and subsequent drying of foliage with G. bidwellii conidia was investigated. There was a significant effect of relative humidity treatment for PD (p=0.038) but not for DS (p=0.3840). PD was significantly greater when RH=86.1±3.7% (High), as opposed to RH=36.7±12.3% (Low) for the first five days after inoculation, RH conditions five days after inoculation had no effect on PD. This study is the first investigation into the conditions that can influence black rot symptom severity and pycnidium development on grape shoots. (Abstract shortened by UMI.). |
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