Plant diseases are common and damaging in both natural and agricultural systems. Fungal pathogens are drivers of interactions in natural ecosystems and major sources of economic losses for agricultural fruit crops domestically and around the world. Monilinia vaccinii-corymbosi, or mummy-berry disease, is the most threatening fungal pathogen to both wild and cultivated highbush blueberry (Vaccinium corymbosum). This fungus employs a two-stage infection that first creates ‘pseudoflowers’ to promote the fungal life cycle, and a second, insect-vectored stage wherein fungal spores mimicking pollen grains are deposited on floral stigmatic surfaces. Similar to pollen tube growth, hyphal growth extends down the stylar canal from the spore to the ovary, causing secondary infection. Infected flowers develop inedible berries that drop, overwinter, and produce spores that begin a new cycle of infection. Resistance varies greatly within highbush blueberry cultivar, and with a hand inoculation and pollination experiment across 25 cultivars with known levels of resistance, we answer the following questions: 1) Do cultivars differ in conidia and pollen germination? 2) Does conidia germination correspond with published resistance rankings for cultivars, suggesting a mechanism of resistance? 3) Is there a trade-off such that cultivars with lower conidia germination also have lower pollen germination?
Results/Conclusions
Our results show that germination of fungal spores varied significantly by cultivar. However, spore germination did not correlate with established resistance levels for secondary infection in blueberry cultivars, nor did it correspond to earlier findings using fewer cultivars. No trade-offs were observed between pollen and conidia germination, suggesting that disease resistance does not have a negative effect on pollen germination. The lack of trade-offs between spore germination (resistance) and pollen germination, however, have several ecological implications. While many plants must balance the need for pollination with adequate disease resistance, such a problem does not seem evident in this system. Stigma diameter, pistil length, and time that the stigma is receptive may be independently or synergistically important to resistance. Although spore germination may be only one facet of susceptibility to secondary infection, this may have positive implications for breeders desiring to favor resistant cultivars since no negative effect on pollen germination was observed.