The 2008 growing season was one of extremes. Heavy rains early in the year led to flooded field conditions and anxiety about diseases caused by Pythium and Phytophthora. Then the rains stopped, fields dried out, and drought-like conditions occurred throughout much of the state. In spite of these dry conditions, by early August reports started coming in to county extension offices and the University of Wisconsin of soybean fields with symptoms similar to Phytophthora root rot (PRR). Because many of these fields were planted to varieties containing the Rps 1k gene, serious concern arose over the breakdown of resistance to PRR conferred by this gene.

 

Figure 1. Symptoms observed in Wisconsin fields during the 2008 growing season. A, open areas interspersed with symptomatic plants. B, Dark patches within fields contain symptomatic plants. C, Petiole and leaf retention at harvest.

 

 

 

 

 From mid-August to early October fields were sampled throughout Wisconsin where soybean varieties containing the Rps 1k gene were expressing symptoms characteristic of PRR. Although efforts were made to isolate and observed Phytopthtora sojae, the cause of PRR, P. sojae was neither observed or isolates from any of our samples. Instead, isolates of Diaporthe phaseolorum var. caulivora (DPC), D. phaseolorum var. sojae (DPS), and Macrophomina phaseolina were obtained. Northern stem canker and pod and stem blight are diseases caused by DPC and DPS, respectively. M. phaseolina is the fungus responsible for the disease charcoal rot.

 

Figure 2. A, Classic symptoms of Phytophthora root rot. B, Symptoms observed in Wisconsin fields in 2008.

 

 

Based on both field observations and data obtained from collected plants, the symptoms observed in soybean fields across Wisconsin do not appear to be the result of infection by P. sojae or the breakdown of the Rps 1k gene. Alternatively, infection by DPC, DPS, and M. phaseolina are believed to be the cause of these symptoms with northern stem canker and charcoal rot the diseases that developed.

Northern stem canker was a significant disease of soybeans during the 1950’s but was brought under control by the use of resistant soybean varieties. Since 2003, incidence of stem canker has been increasing throughout the state. Although the exact reason(s) are unknown, changes in the genetics of soybean varieties, cultural practices, the pathogen, and/or environment may be responsible for the re-emergence of stem canker. Symptoms of stem canker are usually seen around the time of flowering beginning as a reddish-brown lesion on the lower stem near the base of petioles. As the lesion expands, it can become dark-brown to black and may girdle the stem. Stem tissue bordering the lesion remains green until the plant prematurely dies or matures. Foliar symptoms include necrosis and chlorosis of the leave around the veins, wilting, and leaf and petiole retention after plant death. These symptoms can easily be mistaken for PRR although stem canker does not cause a root rot. Infection of a soybean plant by DPC occurs early in the season however, symptoms do not typically appear until the plant undergoes stress. Thus, symptoms can develop from flowering until maturity and DPC can be isolated from healthy-looking tissues.

 

Figure 3. Symptoms of stem canker. A, Reddish-brown lesion surrounded by green tissue. B, Absence of root rot. C, Striations in tissues just below the outer tissues.It is unclear whether this symptom is caused by DPC, DPS, or both.  

 

 

Charcoal rot is a disease often seen in states to our south. Although M. phaseolina is found throughout Wisconsin, environmental conditions have not typically favored disease development. Symptoms of charcoal rot are generally observed around flowering and are similar to those seen with stem canker. M. phaseolina, like DPC, also infect soybean plants early, with symptoms remaining unobserved until plants are under stress. For charcoal rot, disease development occurs when plants are water stressed during hot and dry periods. As disease develops, charcoal rot can be distinguished from stem canker by the appearance of tiny, black, “grains” in stem and root tissues called microsclerotia. Infection by M. phaseolina was noted in soybeans grown throughout Wisconsin but disease development was most abundant in sandy and well-drained soils.

 

Figure 4. Charcoal rot of soybean. A, Microsclerotia inside stem tissue. B, Tattering of stem tissue by Macrophomina phaseolina with microsclerotia underneath. C, Microsclerotia just below the outer stem tissue.

 

 

 

Both DPC and M. phaseolina are seedborne and can survive in soybean residue. The microsclerotia of M. phaseolina can also survive in the soil outside of host tissue between seasons. Management of stem canker and charcoal rot is best when using a multi-tactical approach. Planting seeds from plants grown in areas free of these diseases, reducing plant stress by maintaining soil fertility and water, managing crop residue, and rotating with corn, wheat, or oats, can minimize disease incidence. For soybean varieties adapted to Wisconsin, resistance to stem canker and charcoal rot are unknown. For seed companies who provide ratings for stem canker, it is important to note that these rating are for southern stem canker (D. phaseolorum var. meridionalis) and not for northern stem canker. Although the two diseases are similar, resistance to one does not automatically signify resistance to the other.

From both field and sample data, the Rps 1k gene is still effective against PRR. Of the soybean varieties sampled during 2008, those with Rps 1k and a high field tolerance rating or varieties with multiple Rps genes (1k, 6) expressed minimal symptoms of stem canker. Soybeans collected from fields with a high sand content were susceptible to charcoal rot regardless of the presence of an Rps genes. In selecting varieties to combat these diseases, use variety performance information from previous years. For information related to soybean variety selection please see the 2008 Wisconsin Soybean Variety Test results (http://soybean.uwex.edu/soytrials/printable/documents/2008WISoybeanReport.pdf).  

Within the fields sampled in 2008, plants were noted in all stages of disease development from early symptom development to plant death, suggesting infection was occurring throughout the season. Also, plants were observed that had succumbed to disease prior to flowering. Both DPC and M. phaseolina take advantage of soybeans in stressed environments. Recognizing and sampling plants during the first signs of stress will provide invaluable information as to how these fungi exploit stressed conditions and the process of disease development. During the 2009 season, we suggest scouting your fields early (V3-V4) and reporting initial signs of stress to your local county agent.

 

For more information see:

University of Wisconsin Soybean Plant Health Website at www.plantpath.wisc.edu/soyhealth/

Esker, P., Grau, C., Hudelson, B., Conley, S., and Ballweg, M. Soybean stem canker in 2007. Wisconsin Crop Manager, Vol. 14, Number 31, Pages 168-169.

Esker, P., Conley, S., Gaska, J., and Hughes. T. 2008. Charcoal rot – A disease of drought

stressed environments. Wisconsin Soy Sentinel, Vol 5, Page 16.