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Up to 2005 the sensitivity of Stemphylium vesicarium (Wallr.) Simm., the causal agent of pear brown spot, to the strobilurin fungicides kresoximmethyl, trifloxystrobin and pyraclostrobin was still comparable with baseline values associated with good efficacy in the field. During 2006, the first resistant isolates were detected in two commercial pear orchards in the Emilia-Romagna region (Italy), one of which was affected by considerable control failure linked to strobilurin treatments as demonstrated in a field trial. In vitro sensitivity tests with 0.5 mg l−1 of kresoxim-methyl, trifloxystrobin and pyraclostrobin showed that in the population collected in the orchard with control failure the conidial germination was greater than 90% compared to an untreated control both in 2006 and in 2007, i.e. 1 year after the suspension of strobilurin applications. In the other orchard, where only a few symptomatic fruits were found and the strobilurins were still in use, the conidial germination was lower, about 50% in 2006 and 25% in 2007. The molecular analysis of mitochondrial cytochrome b gene of some monospore isolates with different levels of sensitivity confirmed the presence of the mutation causing G143A substitution in all the resistant isolates. In conclusion, both in vitro tests and molecular analysis confirmed the first occurrence of Stemphylium vesicarium resistance to all strobilurin fungicides tested. more...

In Italy one of the most important concerns for pear growers is the control of pear brown spot, caused by Stemphylium vesicarium, also present in some other European countries. Up-to-date knowledge of pathogen sensitivity to the most used fungicides is fundamental for the success of pear brown spot control, especially after the occurrence in northern Italy of a widespread resistance to dicarboximides in the early 1990s and recently of the first case of control failure caused by strobilurin resistance (2006). After several years of in vitro sensitivity monitoring we can state that the resistance to dicarboximides is stable or regressing while the resistance to strobilurins is increasing. In a very few cases a cross resistance between dicarboximides and fludioxonil was found whereas a good activity was recorded for dithiocarbamates, despite intense use for about 30 years. After some years of applications, S. vesicarium sensitivity to boscalid is still comparable to baseline values. more...

Dicarboximide-resistant isolates of Stemphylium vesicarium have been detected in Italian pear orchards since the early 1990s. Two different resistant phenotypes were observed during a 9 year monitoring study. R1 phenotypes showed Resistance Factor (RF)> 100 towards procymidone and between 3 and 100 to the other dicarboximides (iprodione, vinclozolin and chlozolinate) whereas R2 phenotypes showed RF>100 towards all fungicides of this group. Various morphological and physiological characteristics of the different resistant isolates, such as mycelial colour, mycelial growth rates, sporulation, germination rates and pathogenicity on detached leaves, were compared to those of sensitive isolates. No significant differences could be noted among the phenotypes. Competitive abilities of sensitive and R1 resistant isolates were then evaluated in vitro on unamended medium through mixed inoculations of R/S conidia in 75:25, 50:50 and 25:75 ratios. R1 resistant isolates did not seem to be less competitive in vitro than sensitive ones in the absence of dicarboximide fungicides. more...

The Ascomycete Valsa ceratosperma (Tode:Fr.) Maire [anamorph Cytospora sacculus (Schwein.) Gvritischvili = C. vitis] is a new causal agent of bark canker recently reported on pear growing areas in the Emilia Romagna (Italy). The Regional Plant Protection Service isolated the fungus in 2001 and since then reports have greatly increased in all pear growing areas of the region. This is the first occurrence of Valsa ceratosperma on pear in Europe while Valsa canker is one of the most important diseases of apple orchards in China, Japan and Korea; in these countries the fungus was only occasionally found on pear and quince. The poor knowledge about the disease on pear led us to undertake various studies on the epidemiological and biological aspects. Observations of the disease evolution were carried out in affected orchards, and laboratory and greenhouse assays made it possible to clarify the preferential ways of fungus penetration and the optimal climatic conditions for spore release. It was also investigated whether phloridzin (a dominant component distributed in leaves, stems, fruits and roots of apple tree) plays an important role in Valsa canker on pear as well as that reported on apple in Japanese studies. Preliminary results confirmed that the fungus infects pear trees through wounds such as the pruning ends; the pathogen can be isolated for a distance of about 5 cm beyond visible lesions and cankers develop rapidly in spring and early summer. Cankers with pycnidia appear in a short time only by the inclusion of a small piece of mycelium in a wound created in the trunk of pear in pot. The studies also showed that pycnidia release spores from February to November, conidia ooze gradually from the different pycnidia inside the cankers and the most important factor for fungus sporulation is relative humidity rather than temperature. Neither perithecia formation nor ascospore release were observed on Valsa cankers. more...

Brown spot, caused by Stemphylium vesicarium, is the most important pear fungal disease in Northern Italy and its management relies on frequent preventative applications of fungicides. Dicarboximides (mainly procymidone) are the fungicides most effective against this pathogen but the occurrence of field resistant isolates in the early 1990s led to a reconsideration of other products. S. vesicarium sensitivity to SBIs, dithiocarbamates, anilinopyrimidines, strobilurins, phenylpyrroles and sulphamides was evaluated through inhibition tests of conidial germination, mycelial growth and hyphal elongation. Fenbuconazole, penconazole, ziram and strobilurins showed good efficacy on conidial germination also in isolates resistant to dicarboximides, while the best activity on mycelial growth was obtained with anilinopyrimidines, tebuconazole, flutriafol, difenoconazole and propiconazole. The best hyphal growth inhibition was observed with ziram and fenbuconazole. more...