Elisabetta Pattori

Growth and sporulation of Stemphylium vesicarium, the causal agent of brown spot of pear, on herb plants of orchard lawns

The inoculum sources of ascospores of Pleospora allii and of conidia of its anamorph Stemphylium vesicarium were investigated in relation to the brown spot disease epidemiology on pear. Dead and living leaves of three pear varieties (Abate Fétel, Conference and William), seven grasses (Poa pratensis, Festuca rubra, Festuca ovina, Lolium perenne, Digitaria sanguinalis and Setaria glauca) and Trifolium repens, which are used in pear orchard lawns, were inoculated with conidia of Stemphylium vesicarium virulent on pear and incubated under controlled-environment. Stemphylium vesicarium was always re-isolated from dead leaves of the considered plants, but not from symptomless green or yellowish living leaves. The fungus was occasionally re-isolated from leaf segments showing unspecific necrosis. Inoculation of pear leaves with isolates from grasses demonstrated that the fungus did not lose pathogenicity. Pseudothecia, ascospores and conidia were produced on all the dead inoculated leaves; differences between specimens were found for phenology of pseudothecia, their density and size, and for the number of conidia produced. Pseudothecia were produced faster in the lawn species than in pear leaves, and their density was higher, especially for S. glauca, L. perenne and P. pratensis. Ascospore maturation and ejection was more concentrated for the pseudothecia developed on pear leaves than for those on F. ovina and S. glauca. All the lawn species produced more conidia than pear leaves.

Sources and seasonal dynamics of inoculum for brown spot disease of pear

The dynamics of the production of Stemphylium vesicarium conidia and Pleospora allii ascospores from different inoculum sources on the ground were compared in a model system of a wildflower meadow mainly composed of yellow foxtail, creeping cinquefoil and white clover. The meadow was either inoculated (each October) or not inoculated with a virulent strain of S. vesicarium, and either covered or not covered with a litter of inoculated pear leaves. Spore traps positioned a few centimetres above the ground were exposed for 170 7-day periods between October 2003 and December 2006. Ascospores and conidia were trapped in 46 and 25% of samples, respectively. Ascospore numbers trapped from the pear leaf litter were about five times higher than those from the meadow, while conidial numbers were similar from the different inoculum sources. The ascosporic season was very long, with two main trapping periods: December–April, and August–October; the former was most important for the leaf litter, the latter for the meadow. The conidial season lasted from April to November, with 92% of conidia caught between July and September. The fungus persistently colonized the meadow: the meadow inoculated in early October 2003 produced spores until autumn 2006. The present work demonstrates that orchard ground is an important source of inoculum for brown spot of pear. Thus, it is important to reduce inoculum by managing the orchard ground all year long.

Control of brown spot of pear by reducing the overwintering inoculum through sanitation

Stemphylium vesicarium, the causal agent of brown spot of pear, overwinters in the leaf residues of pear and herbaceous plants of the orchard floor. Pseudothecia of the teleomorph, Pleospora allii, are formed on these residues where they produce ascospores. New methods were tested aimed at reducing this overwintering inoculum and increasing the efficacy of control of brown spot of pear. Sanitation methods were evaluated in nine trials in Girona (Spain) and Ferrara (Italy) over a 4-year period. The sanitation methods were leaf litter removal in December to February, and application of biological control agents (commercial formulates of Trichoderma spp.) to the orchard ground cover from February to May. Fungicides were also applied to the trees during the pear-growing season, scheduled according to the BSPcast model. The different methods were tested as stand-alone applications or in combination. All methods consistently reduced the disease incidence at harvest on fruit with an efficacy between 30 to 60% for leaf litter removal and more than 60% for the combination of leaf litter removal and biological control. Efficacy of sanitation alone (leaf litter removal and biological control) in reducing the brown spot level on fruit was similar in most of the trials to the efficacy obtained when fungicides were applied alone. However, integration of sanitation methods and fungicides did not improve the efficacy of disease control over the level provided by fungicides alone.

Predicting the dynamics of ascospore maturation of Venturia pirina based on environmental factors.

Airborne ascospores of Venturia pirina were trapped at two sites in northern Italy in 2002 to 2008. The cumulative proportion of ascospores trapped at each discharge was regressed against the physiological time. The best fit (R(2) = 0.90, standard error of estimates [SEest] = 0.11) was obtained using a Gompertz equation and the degree-days (>0 degrees C) accumulated after the day on which the first ascospore of the season was trapped (biofix day), but only for the days with > or =0.2 mm rain or < or =4 hPa vapor pressure deficit (DDwet). This Italian model performed better than the models developed in Oregon, United States (R(2) = 0.69, SEest = 0.16) or Victoria, Australia (R(2) = 0.74, SEest = 0.18), which consider only the effect of temperature. When the Italian model was evaluated against data not used in its elaboration, it accurately predicted ascospore maturation (R(2) = 0.92, SEest = 0.10). A logistic regression model was also developed to estimate the biofix for initiating the accumulation of degree-days (biofix model). The probability of the first ascospore discharge of the season increased as DDwet (calculated from 1 January) increased. Based on this model, there is low probability of the first ascospore discharge when DDwet < or =268.5 (P = 0.03) and high probability (P = 0.83) of discharge on the first day with >0.2 mm rain after such a DDwet threshold.