Taisha continues with her series on mimicry and deception; she writes:
Today’s image features Vaccinium angustifolium, or the low-bush blueberry. It was chosen from the Botany Photo of the Day Flickr Pool and is courtesy of the late James Gaither (aka J.G. in S.F@Flickr). He photographed these flowers at U.C. Berkeley Botanical Garden in mid-April of 2012. We continue to be grateful for James’ contributions.
You may be wondering why I’ve included a blueberry bush in a series about mimicry. Truthfully, it’s not Vaccinium angustifolium I am going to highlight, but rather an associated fungal species known as Monilinia vaccinii-corymbosi. This fungus causes mummy-berry disease in many species of Vaccinium (including Vaccinium angustifolium). It employs several mimetic techniques to infect its hosts. Unfortunately, we’ve not been able to track down a high-resolution image freely available for BPotD use, but the link in this paragraph contains many images scanned from slides for your perusal.
Monilinia vaccinii-corymbosi infects species of Vaccinium in two specifically timed stages (link contains illustration of life cycle). The primary stage of infection takes place when the blueberry is in bud-break. The mummy-berry fungus infects emerging leaves and shoots with wind-dispersed ascospores. The secondary stage of infection is where some mimicry takes place.
The ascospore-produced fungal blight that develops on the leaves and some flower clusters generates conidia (asexual, non-motile spores), developed more or less synchronously with the blooming flowers of the blueberry plant. In order for the conidia to disperse from the leaves to the stigma (the initial site of the secondary infection), the pathogen uses host mimicry and deception in order to take advantage of the host’s typical pollinators. By producing a sweet odour and reflecting ultraviolet light at an analogous wavelength to that of the blueberry flower calyces, pollinators are lured to the conidia-laden blighted leaf tissue. The conidia are transferred to the pollinator, and then transported to the stigmatic surface when the pollinator next visits a flower.
Once on the stigma, secondary infection and a second round of mimicry begins when the spores germinate on the stigmatic surface. From Ngugi, HK and Scherm, H. Pollen mimicry during infection of blueberry flowers by conidia of Monilinia vaccinii-corymbosi. Physiological and Molecular Plant Pathology. 64(3):113-123, the authors state in the abstract: “Similar to blueberry pollen tubes, conidial germ tubes of Monilinia vaccinii-corymbosi adhered selectively to imprints of stylar transmitting tract tissue on nitrocellulose membrane, with adhesion in both cases occurring at the tips of the tubes. By contrast, hyphae of the related Monilinia fructicola, which is nonpathogenic on blueberry and does not cause gynoecial infection, adhered indiscriminately to the entire membrane”. In other words, the conidia of Monilinia vaccinii-corymbosi mimic the pollen of the species of Vaccinium, through a “match” with the stigmatic surface. Continuing on, the hyphae growing from the conidia will then navigate the same path as the pollen tubes through the stylar canal and into the ovary. The thallus or body of the fungus then grows within the developing fruit, resulting in “mummification”. This mummified fruit later falls to the ground and serves as an overwintering structure for the fungus.