Michael Bradshaw unveils the beauty and diversity of plant-pathogenic fungi.

Hidden amid the vast and diverse array of plants in botanical gardens, a world of thousands of plant diseases thrives. Amidst the vibrant blooms and lush foliage lurk plant pathogens that are ready to infect a diversity of hosts. These organisms take many forms, from leaf-devouring fungi to viral assailants capable of transforming a healthy plant into a yellow, withering mass. The garden in essence becomes a living petri dish, nurturing and sheltering pathogens. Gardeners and horticulturists often go to great lengths to shield these diseases from visitors and scientists alike. However, these unsightly and potentially deadly afflictions can only remain concealed for so long; one infamous plant pathogen, Phytophthora infestans, is credited with causing the devastating Irish Potato Famine, claiming over a million lives. Closer to home, the chestnut blight caused by the fungal agent Cryphonectria parasitica forever reshaped American forests. As a plant pathologist, I find these tiny organisms possess a captivating allure despite their destructive nature. Under the microscope, delicate threads of fungal hyphae weave intricate patterns upon infected leaves. To the trained eye, these are akin to nature’s abstract art; the vibrant hues of rusts, smuts, and leaf spots present an unexpected visual appeal.

Fungi are among the least studied organisms in terms of their diversity, and this is especially true for powdery mildews, among the world’s most common plant pathogens. Powdery mildews, which should not be mistaken for the mold or mildew growing in your shower, are a group of some one thousand species of fungi that infect over ten thousand species of flowering plants, including such favorites as hops, marijuana, and wine grapes. Achieving effective control of powdery mildews through organic methods poses a formidable challenge. What’s worse, these pesky, pervasive fungi tend to evolve resistance to the synthetic pesticides available. In a garden, it’s often more practical to find ways to coexist with these pathogens, or to choose plant varieties that are naturally resistant to powdery mildew. Through millions of years of evolution, they have have honed their ability to efficiently target plant organisms. At the same time, as biotrophic parasites, powdery mildews have undergone adaptations that prioritize the preservation of their food source. One of the evolutionary traits they possess is the development of a specialized feeding organ called the haustorium, which allows them to extract nutrients from their hosts while avoiding lethal damage. However, while they spare the host’s life, they can still wreak havoc in other ways. For instance, they can lower crop yields in agriculture and damage the leaves and flowers of ornamental plants, making them unsuitable for sale.

One fascinating aspect of many fungi, including powdery mildews, is their ability to exist in both asexual and sexual forms during different parts of their life cycle. Powdery mildews exemplify this by undergoing a striking transformation from their asexual form, sometimes seen as a chain of spores resembling white, translucent pillars on leaves, to their sexual form, which takes on the appearance of alien-like spheres with long tentacles. These unique structures serve as the fungi’s fruiting bodies, allowing them to survive the winter and infect new hosts in the following spring. While they may be unsightly, powdery mildews possess a captivating beauty and complexity when viewed through the lens of a microscope.

When it comes to plant-pathogenic fungi, the biodiversity of the Arnold Arboretum is as uncharted as the Amazon rainforest.

The past decade of my research has been dedicated to these tiny fungi, and they have become a special passion. My work revolves around identifying and naming the various species found in North America. To effectively manage the diseases they cause, it’s crucial to accurately identify plant pathogens. Different pathogens may require specific control measures, from targeted fungicides to the cultivation of resistant plant varieties. Precise identification also enables us to track and monitor pathogen populations, allowing for the early detection of newly introduced pathogens and undertaking intervention in a timely manner. Interestingly, species of powdery mildew are narrowly specific to their host plants. So, if you have powdery mildew on your peony, don’t worry—it won’t infect your lovely chestnut or even your roses (although another species of powdery mildew might!).

As a passionate powdery mildew hunter, knowing the best hunting grounds is key to success. It’s no surprise that botanical gardens, with their diversity of plants, hold a treasure trove of powdery mildew species. To boost my collection efforts, I recently contacted horticulturists and gardeners in botanical gardens across North America. In just a few weeks, we discovered a whopping 30 new species of powdery mildews! As renowned mycologist Amy Rossman once remarked, “(f)or some types of organisms, such as microfungi, New York state’s forests are almost as unexplored as the tropical forests.” Taking it a step further, I’d say that, when it comes to plant-pathogenic fungi, the biodiversity of the Arnold Arboretum is as uncharted as the Amazon rainforest. Just as we strive to protect tropical forests, recognizing the importance of microfungi and exploring their habitats can deepen our knowledge, address global challenges, and contribute to their conservation.

If you’re interested in delving into the world of powdery mildew (or other fungus or plant-pathogen) collecting, it’s important to familiarize yourself with the fungi and their host plants. Start by identifying common host plants like roses, cucumbers, or sunflowers, then learn to recognize the signs of infection. Time your collections to capture both the asexual stage in spring/summer and the sexual stage in fall. When selecting specimens, opt for plants showing distinct powdery patches on leaves or the black dots of the sexual stage, and check both sides of the leaves. Handle the samples carefully, placing them in labeled paper envelopes or bags. Ensure the valuable specimens find their way to a local fungarium or research institution, where they can contribute to scientific endeavors and expand our understanding of powdery mildew diversity.

Exploring plant pathogens within botanical gardens offers a fascinating glimpse into the intricate dance between plants and their microscopic foes. The vast array of powdery mildew species discovered in these environments highlights their remarkable adaptability and resilience. Just as “plant blindness” refers to the inability to appreciate the significance of plants, I hope this article can cure your “powdery mildew blindness.” Once cured, you’ll no longer overlook the incredible diversity and captivating beauty of these common plant pathogens. So, whether you’re strolling through the Arnold Arboretum or simply wandering around your own garden, be sure to take a moment to admire these charming and striking fungi.

Michael Bradshaw completed his Ph.D. at the University of Washington in Plant Pathology and is currently a Harvard University Herbaria fellow studying powdery mildews and their hosts.

From “free” to “friend”…

Established in 1911 as the Bulletin of Popular Information, Arnoldia has long been a definitive forum for conversations about temperate woody plants and their landscapes. In 2022, we rolled out a new vision for the magazine as a vigorous forum for tales of plant exploration, behind-the-scenes glimpses of botanical research, and deep dives into the history of gardens, landscapes, and science. The new Arnoldia includes poetry, visual art, and literary essays, following the human imagination wherever it entangles with trees.

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