Tag Archives: plant diseases

Friday Fellow: Pear Rust

by Piter Kehoma Boll

Beautiful and deadly, today’s fellow appears during spring as gelatinous orange projections coming out of juniper trees in Europe and North America. Its name is Gymnosporangium sabinae, commonly known as the pear rust.


The jelly-like horns of the pear rust on a juniper tree. Photo by Mark Sadowski.*

The pear rust is a basidiomycete, i.e., a fungus of the phylum Basidiomycota, therefore related to the common mushrooms, but belonging to a different class, the Puccioniomycetes.

During winter, the pear rust remains in a resting state inside branches and twigs of juniper trees. After wet days in spring, the fungus sprouts and appears as horn-like growths covered by an orange gelatinous mass, which are called telia. The telia produce wind borne spores called teliospores that can infect pear trees.


The pear rust on pear leaves. Photo by Jan Homann.

Once reaching the pear tree, the teliospores germinate and infect the leaves of the new host. The infection appears in summer as rust-colored spots on the leaves, hence the name pear rust. In heavily infected plants, the effects of the pear rust can be severe, sometimes causing the plant to lose all its leaves.

In pear trees, the fungus produce reproductive structures known as aecia. They come out from the underside of pear leaves and produce spores called aeciospores, which are able to infect new juniper trees.


The aecia coming out of the rust on a pear tree. Photo by H. Krisp.**

Due to the economic importance of pear trees to humans, the pear rust is a species of great concern. Some countries have policies intended to reduce the spread of the disease, such as preventing transportation of juniper trees from areas known to have the fungus to areas in which it is unknow. In areas where the fungus exist, the solutions to reduce the damage include the use of chemical fungicides, the removal of infected branches in juniper trees and sometimes the removal of any juniper tree around the areas where pear trees are cultivated.

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Fraiture, A.; Vanderweyen, A. (2011) Gymnosporangium sabinae: such a beautifiul disease. Scripta Botanica Belgica 11: 193–194.

Ormrod, D. J.; O’Reilly, H. J.; van der Kamp, B. J,; Borno, C. (1984) Epidemiology, cultivar susceptibility, and chemical control of Gymnosporangium fuscum in British Columbia. Canadian Journal of Plant Pahology6: 63–70.

Wikipedia. Gymnosporangium sabinae. Available at < https://en.wikipedia.org/wiki/Gymnosporangium_sabinae >. Access on April 27, 2018.

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Friday Fellow: Brown spot of maize

by Piter Kehoma Boll

I’ll continue the parasite trend from last week, but this time shifting from human parasite to maize parasite, and from a prokaryotic to a eukaryotic parasite. So let’s talk about Physoderma maydis, commonly known as the brown spot of maize or brown spot of corn.

The Brown spot of maize is a fungus of the division Blastocladiomycota that infects corn (or maize) plants. Its common name comes from the fact that it causes a series of brown spots on the leaves of an infected plant.


The brown spots seen on this corn leaf are due to an infection by Physoderma maydis. Credits of the photo to Clemson University – USDA Cooperative Extension Slide Series.*

The life cycle of the brown spot of maize is as complex as that of many fungi. The infection of the plants occur through spores that remain in the soil during winter and are carried to the host by the wind, germinating in the rainy season. The germinated spores produce zoospores, flagellated spores able to swim. Swiming through the maize leaf, the zoospores infect single cells and produce zoosporangia at the surface of the leaf. The zoosporangia release new zoospores that infect new cells. In late spring and summer, the zoospores produce a thallus growing deep inside the maize leaf that infects many cells and produces thick-walled sporangia. After the plants dies and the leaves become dry and broke, the sporangia are released and reach the soil, where they wait for the next spring to restart the cycle.

The brown spot of maize is a considerable problem for maize crops in countries with abundant rainfall. Heavy infections may kill the maize plant or severely reduce its fitness before the ears are ready to be harvested. Although fungicides may help in slowing down the infectio throughout the crops, one of the most efficient ways to reduce the damage is to destroy, usually by fire, the remains of the last harvest.

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Olson, L. W.; Lange, L. (1978) The meiospore of Physoderma maydis. The causal agent of Physoderma disease of maize. Protoplasma 97: 275–290. https://dx.doi.org/10.1007/BF01276699

Plantwise Knowledge Bank. Brown spot of corn (Physoderma maydis). Available at: < http://www.plantwise.org/KnowledgeBank/Datasheet.aspx?dsid=40770&gt;. Access on Agust 7, 2017.

Robertson, A. E. (2015) Physoderma brown spot and stalk rot. Integrated Crop Management News: 679. http://lib.dr.iastate.edu/cropnews/679/

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Friday Fellow: Downy Mildew

by Piter Kehoma Boll

Last week I introduced a serious plant pathogen, the gray mold, that attacks many crops and has a special role as either a bad or a good guy in wine grapes. But a plant that is never happy with an infection by the gray mold is certainly the lettuce. And in this case our juicy vegetable has an enemy that makes it susceptible to the mold, and I’m bringing it to you today.

Named Bremia lactucae, this organism is a oomycete, thus belonging to a group of organisms that was formerly classified as a fungus, but that currently is known to be more closely related to brown and golden algae. This species attacks lettuces and closely related plants, causing a disease called downy mildew.


A lettuce leaf with downy mildew. Photo by Gerald Holmes.*

The downy mildew is the most important disease affecting lettuce worldwide. The disease itself is not the main problem, although it decreases the quality of the crop. Its main problem is that it makes the vegetable more vulnerable to other infections, such as those by the gray mold, and also increases the risk of contamination by human pathogens, such as intestinal parasites.


A branch of the downy mildew under the microscope. Photo by Bruce Watt.*

The usual forms of controling the spread of the downy mildew is by using fungicides and developing mildew-resistant lettuces by hybridization with wild and naturally resistant varieties. However, as usual, the downy mildew eventually adapts to this, giving rise to fungicide-resistant strains, as well as strains able to neutralize the resistance of lettuce lineages. It’s one more evolutionary arms race.

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Beharav, A., Ochoa, O., & Michelmore, R. (2013). Resistance in natural populations of three wild Lactuca species from Israel to highly virulent Californian isolates of Bremia lactucae Genetic Resources and Crop Evolution, 61 (3), 603-609 DOI: 10.1007/s10722-013-0062-5

Parra, L., Maisonneuve, B., Lebeda, A., Schut, J., Christopoulou, M., Jeuken, M., McHale, L., Truco, M., Crute, I., & Michelmore, R. (2016). Rationalization of genes for resistance to Bremia lactucae in lettuce Euphytica, 210 (3), 309-326 DOI: 10.1007/s10681-016-1687-1

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