Interviews with Dr. Scandiani of the Laboratory Rio Parana, Argentina (AR), and Eng. Sillon of the Littoral National University, AR.
October 11, 2012
One important disease that impacts Root Health is Rhizoctonia. Research on the prevalence and importance of Rhizoctonia in Argentina has been conducted by Dr. Mercedes Scandiani from Laboratorio Rio Parana, as well as by Eng. Margarita Sillon from Littoral National University. During the Latin America South (LAS) Root Health Forum held in Argentina in August, we asked Dr. Scandiani and Eng. Sillon – who each gave a presentation – about their experiences researching Rhizoctonia in Argentina.
With Dr. Mercedes Scandiani –
How does Rhizoctonia impact yield crops in Argentina?
There is not a lot of information available on diseases caused by Rhizoctonia in Argentina. This pathogen has been called Rhizoctonia sp. and R. solani in soybean, wheat, corn, beans, and chickpeas, but not much is known about the correct identification. In 2008, corn fields in the west Buenos Aires province were analyzed and showed small patches of affected plants. The causal agent in those fields was identified as Rhizoctonia solani. The symptoms were stunted plants, foliar discoloration and dead plants. Yield losses of 33 to 60% were reported. Rhizoctonia is frequently mentioned in soybean as the causal agent of pre-emergence and post-emergence damping-off and root rot in adult plants.
What approach did you use to gather knowledge and data on Rhizoctonia?
In 2010/11, we conducted a survey in collaboration with Syngenta. For this survey, soil samples were analyzed from commercial fields in the west centre of Buenos Aires, Santa Fe and Entre Rios provinces, where soybean and wheat were previously grown. Using the phytopathological classic methodology, isolations from soybean seedlings with postemergence infected stems used as trap crop were carried out. Based upon morphological description and DNA sequence analyses of the twenty isolates obtained, and considering this small Rhizoctonia collection, a broad spectrum of Rhizoctonia spp. was obtained. Rhizoctonia multinucleate, as well as binucleate Rhizoctonia, from different AGs were detected. The most abundant R. solani (Thanatephorus cucumeris) anastomosis groups (AG) were AG-4 HGI (4 isolates) and AG-8 (4 isolates), followed by AG-3-TB (3 isolates), AG-6 (2 isolates), AGGV2 (2 isolates), AG9 (1 isolate) and 4 unidentified isolates. The population of binucleate Rhizoctonia (Ceratobasidium species) obtained included AG-Fb and AG-A.
Did you see a relationship between the AG and the geographic origin of the sample?
We found that AG4 was isolated from samples of historical soybean fields under no tillage conditions, while AG8 was obtained from samples with wheat/soybean history and AG3 from tillaged fields in rotation with potatoes.
Why do you think correct identification of the pathogen is relevant?
In Argentina, it was always assumed that AG4 was the pathogen of soybean; however, we found that Rhizoctonia (Ceratobasidium species) AG-Fb was highly pathogenic to soybean in this study. Knowledge about the pathogen identity makes it possible to carry out research about its distribution, hosts range, chemical and biological control through seeds treatments, genetic behavior of the crop and establish suitable management strategies.
With Eng. Margarita Sillon –
How do you see the relevance of Rhizoctonia solani for central Argentina?
Rhizoctonia is a pathogen that is increasing its prevalence over the last 10 years in this region. In general, we see that several soil pathogens have a stronger and more damaging impact on crop development. The minimum tillage and reduced crop rotation in Argentina allows survival of soil fungi on crop debris and exposes the plant roots to a complex of diseases impacting their development if they are not protected. We have identified Rhizoctonia as causing problems in various crops: wheat, soybeans and corn from 2002 onwards and more recently in chickpeas, peas and canola.
Do you have experiences with the use of sedaxane on Rhizoctonia ?
We carried out some greenhouse studies on corn with very good results. The emergence of plants showed an increase of 40% when treated with sedaxane, when compared to the inoculated checks. We also studied the roots of inoculated check plants without symptoms and found that 23% of these asymptomatic plants had roots colonized with Rhizoctonia. When treated with sedaxane, only 1% of the roots showed asymptomatic Rhizoctonia. The percentage of roots colonized with Rhizoctonia in asymptomatic plants represents a potential problem in the field because the farmer cannot see the symptoms and will not know that his field is affected.
Do you also have results from field studies with sedaxane?
In the Santa Fe province, we have registered a 30% difference in the vigor of the plants and up to 87% reduction in affected plants (dead or smaller) for seeds treated with sedaxane. That will have an impact on yield together in combination with other important pathogens in our region. For corn, Fusarium graminearum and Colletotrichum graminicola play an important role as well. Corn is affected by numerous pathogens during its development; therefore, it is critical to ensure and protect the initial health of the roots in order to reduce sensitivity to other diseases at later growth stages.
What are your observations about Rhizoctonia in soybean?
An increasing problem, especially in the wheat-soybean sequence, is the often low water availability in the soil at the time of planting. This makes it difficult for plants to develop a robust root system, especially in adverse weather conditions. Due to such conditions, we recorded losses of up to 50% on initial stand that forced replanting. In 2009 and 2010, Rhizoctonia attacks in reproductive stages in the northern area of the province resulted in 10 to 20% losses in yield. All efforts need to be made to improve the initial health of the roots in order to fight the increasing impact of soil pathogens. The results we obtained with sedaxane show that it could help provide a strong root system. Its impact on final yield should further be investigated.
Dr. Mercedes Scandiani from Laboratorio Rio Parana and Eng. Margarita Sillon from Litoral National University were interviewed by Fernando Biffis, Seed Care Technical Manager for Syngenta, about their presentations during the Latin America South (LAS) Root Health Forum hosted by Syngenta in Argentina in August. 2012.