Are microconidia infectious principles in Neonectria ditissima?

Abstract

Comparative studies of different isolates of Neonectria ditissima obtained from canker lesions and rotten fruit showed that both five-septate macroconidia and aseptate microconidia were capable of germination by germ-tube formation, but that growth commenced earlier and proceeded faster from the former than the latter type of spore. Further, following wound inoculation of apple fruit with different numbers of conidia (50, 500 or 5000 per wound) the resulting rot lesions were always significantly larger with macroconidia than microconidia, and in both conidial types lesion size increased with higher inoculum loads. These data confirm that microconidia are capable of causing infections, but indicate that their contribution to the success of the pathogen in the field is probably negligible.

Detecting the asymptomatic colonization of apple branches by Neonectria ditissima, causing European canker of apple

“…Scheper et al (2015) showed that an inoculation with an isolate producing 1% macroconidia started to develop symptoms after 7 weeks compared to < 5 weeks for isolates with over 60% macroconidia and that the isolate was significantly less aggressive. These and our observations are also in line with Wesche and Weber's (2022) findings of a slower germination and elongation of microconidia compared to macroconidia and symptom development on fruit. Another explanation could be that suboptimal weather conditions may have slowed down the colonization.…”

“…Based on our current findings pruning practices should focus on cutting the whole branch rather than a part of the branch as was previously concluded (Olivieri et al, 2021;Xu et al, 2021). This study demonstrated that the TaqMan PCR assay could detect N. ditissima at very low and unlikely amounts in the host (Wesche & Weber, 2022) and proved therefore potential to further assess the internal colonization over time and further refine pruning practices. Future studies using isolates producing high numbers of macroconidia and assessing wood at further distances from an inoculation site over a longer period of time are suggested to further establish our understanding of the colonization pattern of N. ditissima representative for the infections in nature.…”

“…Using the semi-selective PDA, we detected N. ditissima in 25 out of 120 samples taken at 2-6 mm over 8 weeks. Microconidia are less active than macroconidia (Scheper et al, 2015;Wesche & Weber, 2022), therefore the lower numbers of detection in our study could have been caused by the used inoculum with 100 microconidia per wound versus the 10000 macroconidia per wound used by Olivieri et al (2021). Using inoculum of isolates with a higher number of macroconidia would be closer to field inoculum (Scheper et al, 2010) and could have resulted in a higher detection rate in culturing as well as by the TaqMan PCR assay.…”

“…Systemic spread (colonization inside the tree under the bark) of N. ditissima in the wood causing new cankers at different spots of the tree has been proposed (Crowdy, 1949;Weber, 2014) and may be an explanation for certain observations on spatial disease patterns such as the positive correlation in canker development between neighboring leaf scars in the same shoot (Xu et al, 2021). N. ditissima in symptomatic plant material, such as necrotic canker lesions, dead and stunted buds and fruit rot, can be detected through several methods including microbiological culturing methods (Amponsah et al, 2014;Amponsah et al, 2015;McCracken et al, 2003;Olivieri et al, 2021;Wesche & Weber, 2022), immunolocalization (Dewey et al, 1995) and molecular tools such as PCR and qPCR (Ghasemkhani et al, 2016;Langrell, 2002;Langrell & Barbara, 2001). Asymptomatic presence of N. ditissima in wood tissue after inoculations has been confirmed using culturing (Olivieri et al, 2021;Walter et al, 2016;Xu et al, 2021) where detection was achieved at a maximum depth of 5-10 mm from the inoculation site after 2 weeks (Olivieri et al, 2021).…”

Methods for quantifying rain-splash dispersal of Neonectria ditissima conidia in apple canopies

“…The seasonality of spore types varies with region, and in New Zealand apple orchards, lesions producing conidia are found more frequently than those producing perithecia (Campbell et al, 2016). The focus of this publication is on macroconidia because of their importance to apple canker epidemiology (Wesche and Weber, 2023) and therefore ascospore and microconidia dispersal were not explicitly investigated.…”