May 2023: Jewels in the Crown

There was a fair amount of pomp and circumstance earlier this month surrounding the coronation of King Charles III of England. As I was reading the news articles, I began to imagine how plant diseases might fit into a coronation ceremony. I doubt that British royals would find any of the diseases that I deal with particularly appealing. However, in typcal Dr. Death fashion, I found three diseases that, in my eccentric world view, could fit into a ceremony to crown a plant pathologist king or queen. I hope you enjoy my selections.

Crown Rots: The most destructive of the “crown” diseases are crown rots. The fungi and fungi-like water molds involved in these diseases often infect though a plant’s roots and eventually destroy a plant’s crown (i.e., the part of the plant where the roots and above-ground plant parts converge). Plant death is a common result. Crown rot pathogens prefer wet conditions; thus, crown rots tend to be more prevalent in heavier (e.g., clay) soils, in low areas, and in wet growing seasons. Management of crown rots involves improving soil drainage by adding organic matter (e.g., compost, leaf litter) to heavier soils and/or creating raised beds. Proper mulching (not more than two inches on heavier soils) can help prevent these diseases as well. Finally, for plants of high economic or sentimental value, fungicide treatments are a possibility. For treatments to be effective however, proper identification of crown rot pathogen(s) is critical, as some crown rot fungicides target fungi, others water molds.

Crown Gall: I got excited recently when I was removing leaf litter from my parents’ wintercreeper and caught sight of greenish-white, tumor-like blobs (galls) on the main trunk of the shrub. These blobs are typical of crown gall, a disease caused by the soil-borne bacterium Agrobacterium tumefaciens. This bacterium has a very wide host range; I commonly see crown gall on wintercreeper, rose shrubs, and apple trees. Often tumors form in the crowns of affected plants, but root galls are also common. Management of crown gall involves removing and destroying infected plants and avoiding planting susceptible plant species in areas where the disease has been observed.

Interestingly, when Agrobacterium tumefaciens infects a plant, it injects a small piece of its DNA (i.e., genetic material) into plant cells, where this DNA inserts into plant chromosomes. This bacterial DNA codes for enzymes that produce a variety of interesting chemical compounds. These compounds hijack plant cell growth and cause cells to grow fast and divide like crazy, leading to characteristic crown gall tumors. Other enzymes make opines, a class of chemicals that are a favorite food of the bacterium. Scientists eventually discovered that they could manipulate Agrobacterium tumefaciens DNA and incorporate plant genes into it. With these plant genes in place, the bacterium then could be used to infect a different type of plant, thus moving plant genes from one plant to another. Via this process, genetically engineered/modified plants were first produced.

Crown Rust: This is perhaps the most fitting plant disease for this month’s article, as the name of the causal fungus is Puccinia coronata. The species designation (coronata) refers to projections on the fungus’ club-shaped reproductive structures that give the clubs a crown-like appearance. Home gardeners most commonly see crown rust on turf; if you have ever walked through your lawn and ended up with orange shoes, you’ve encountered this disease. The orange powder is sporulation that allows the fungus to reinfect turfgrass. Interestingly, Puccinia coronata needs two host plants to complete its life cycle. The second host plant for Puccinia coronata is buckthorn, the invasive tree that takes over understories of wooded areas. Puccinia coronata produces a second type of spore in the spring that drifts from turfgrass to buckthorn, infecting leaves and green branch tissue of the tree. These infections lead to yellow leaf spots and yellow, distorted branch growth. These yellow areas produce yet another spore that drifts back to turfgrass, completing the fungus’ life cycle. Management of crown rust involves eradicating buckthorn, as well as regular mowing and optimal nitrogen fertilization of lawns to help remove the fungus and help turfgrass outgrow the disease.

Can you think of any other plant diseases that are fit for a plant disease king or queen? If so, let me know. Also, if you have questions about the diseases discussed above and/or how to submit samples to the clinic, feel free to give me a shout. As always, you can reach me at pddc@wisc.edu or (608) 262-2863. Long live plant diseases!!