Ascospore germination and comparative monocycle of neonectria fruit rot on apple and pear

Abstract

Neonectria fruit rot (NFR), an apple disease caused by the fungus Neonectria ditissima, is common in Brazilian orchards, but several aspects of its epidemiology are not well elucidated. The objectives of this study were to: i) test in vitro germination of ascospores at different temperatures and wetness periods and compare with conidia germination, ii) evaluate and compare the monocyclic components of NFR caused by conidia and ascospore infection in wounded and unwounded ripe detached ‘Gala’ apple and iii) evaluate and compare the monocyclic components of NFR caused by conidia infection in ‘Rocha’ pear fruit under different temperatures. Germination of conidia and ascospores occurred in the range of 12–28 °C. At 35 °C no germination occurred, for both ascospores and conidia, regardless of the wetness period (3–60 h). With the same wetness period, conidia germination was always higher than ascospores at any temperature. For ascospores, the optimal temperature for germination ranged from 18–21 °C, lower than the estimated for conidia, which ranged from 21 to 24 °C. In wounded detached apple fruit, the incubation period varied from 35 to 49 days after ascospore inoculation, and for conidia from 14 to 42 days. When comparing the same temperatures, mean lesion growth rate was always higher for fruits inoculated with conidia than with ascospores. Unwounded fruits inoculated in the calyx-end did not present Neonectria fruit rot symptoms. The disease on ‘Rocha’ pear fruit has shorter incubation and latent periods than those of ‘Gala’ apple and higher AUDPC at 25 °C. The fast monocycle development represents a risk if the disease spread to pear orchards but also sets pear fruit as a good option for future NFR control studies.