|Contour map showing numbers of NADP sites testing positive for P. pachyrhizi in the center and east of US in 2006|
Phakopsora pachyrhizi is an obligate biotrophic pathogen that causes Asian soybean rust. Phakopsora pachyrhizi is able to affect up to 31 different plant species that belong to 17 different genera under natural conditions. Experiments in laboratories were able to use Phakopsora pachyrhizi to infect 60 more plant species. The main hosts are Glycine max (soybean), Glycine soja (wild soybean), and Pachyrhizus erosus (Jicama).
*Preferred hosts. Other hosts were minor or determined experimentally under artificial conditions.
The disease forms tan to dark-brown or reddish-brown lesions with one to many prominent, globe-like orifices. Urediniospores form from these pores. At initial stages, small yellow spots are formed on the surface of the leaf. These spots may be better observed using assistance of a light source. As the disease progresses, lesions start to form on the leaves, stems, pod, and petioles. Lesions are initially small, turning from gray to tan or brown as they increase in size and the disease gets more severe. Soon volcano-shaped marks are noticed in the lesions.
Phakopsora pachyrhizi is a fungus which has a spore moved by wind, called urediniospore. These spores are quite different from others as they don't need an open stomata or natural openings in the leaves. Urediniospores are able to penetrate the leaf. Pustules are visible after 10 days and they can produce spores for three weeks. The disease reaches its climax when the crop begins flowering. The cycle of the pathogen continues until the crop is defoliated or until the environment becomes unfavorable to the pathogen.
The Asian soybean rust is a polycyclic disease: within the disease cycle, the asexual urediniospores keep infecting the same plant. Teliospores (sexual spores) are the survival spores that overwinter in the soil. Basidiospores are the spores that are able to contaminate an alternative host. The urediniospores need a minimum of six hours to infect leaves at a favorable temperature (between 15 and 24 °C).
The favorable conditions for the disease to progress are related to temperature, humidity, and wind. The appropriate temperature for the pathogen to be active is 12 to 29 °C (more efficient between 18 and 26.5 °C). The humidity must be high, about 90% or more, for more than 12 hours. A significant amount of wind is also important for the pathogen to move from one plant to the other. Currently, in the United States, infected plants can be found in Florida, Georgia, Louisiana, and Texas.
Phakospsora pachyrhizi is a pathogen that acts quickly in contaminating the host. The plant can be severely contaminated in as short a period as 10 days. This makes it difficult to control the disease, as it does not just spread quickly, but its progression is also fast. That is why it is important to implement control techniques as soon as possible.
The disease may be controlled by using genetic resistance, but this has not exhibited great results and has not been durable because the soybean genome almost entirely lacks potential genes for ASR resistance.[Kawashima et al 2016 1]
A second form of management that can work is using fungicides, but this is only efficient at early stages of the disease. The disease spreads fast and it is complicated to control after certain stages, so it is important to act with care around contaminated plants, as the spores can be attached to clothing and other materials and infect other plants.
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- Kawashima, Cintia G; Guimarães, Gustavo Augusto; Nogueira, Sônia Regina; MacLean, Dan; Cook, Doug R; Steuernagel, Burkhard; Baek, Jongmin; Bouyioukos, Costas; Melo, Bernardo do V A; Tristão, Gustavo; de Oliveira, Jamile Camargos; Rauscher, Gilda; Mittal, Shipra; Panichelli, Lisa; Bacot, Karen; Johnson, Ebony; Iyer, Geeta; Tabor, Girma; Wulff, Brande B H; Ward, Eric; Rairdan, Gregory J; Broglie, Karen E; Wu, Gusui; van Esse, H Peter; Jones, Jonathan D G; Brommonschenkel, Sérgio H (2016-04-25). "A pigeonpea gene confers resistance to Asian soybean rust in soybean". Nature Biotechnology. Springer Science and Business Media LLC. 34 (6): 661–665. doi:10.1038/nbt.3554. ISSN 1087-0156.
- p. 664, "Previous work did not identify novel soybean germplasm that displayed immunity to ASR and identified only 33 accessions with moderate RB type resistance, and thus revealed that the number of ASR resistance genes in soybean germplasm is limited. Resistance genes that provide immunity to ASR are a valuable resource."
- p. 664, "In conclusion, we have identified and cloned a gene from C. cajan that confers resistance to P. pachyrhizi when expressed in soybean."
- p. 664, "Thus, the significance of this work is the demonstration that it is possible to effectively transfer a dominant resistance gene from a related legume into soybean."
- p. 664, "The Fabaceae (Leguminosae) is a large and diverse plant family, with around 700 genera and 20,000 species. Our results suggest that this tremendous natural resource can be used to identify additional resistance genes against ASR that are absent from the soybean gene pool."
- p. 664, "These legume resistance genes could be used to develop durable and environmentally sustainable ASR control strategies."
- p. 664, "Finally, although we have not been able to identify P. pachyrhizi isolates that can overcome CcRpp1, P. pachyrhizi has demonstrated that it can rapidly overcome resistance genes that are deployed individually. With 30 million hectares of soybean under cultivation in Brazil, it would be prudent to only deploy CcRpp1 in soybean together with additional resistance genes that have different specificity or different mechanism of ASR resistance, to increase the durability of these resources."