Diversity and Pathogenicity of Rhizoctonia Species from the Brazilian Cerrado

Abstract

Eighty-one Rhizoctonia-like isolates were identified based on morphology and nuclei-staining methods from natural and agricultural soils of the Cerrado (Brazilian savanna). The nucleotide similarity analysis of ITS1-5.8S-ITS2 regions identified 14 different taxa, with 39.5% of isolates assigned to Waitea circinata (zeae, oryzae, and circinata varieties), while 37.0% belonged to Thanatephorus cucumeris anastomosis groups (AGs) AG1-IB, AG1-ID, AG1-IE, AG4-HGI, and AG4-HGIII. Ceratobasidium spp. AG-A, AG-F, AG-Fa, AG-P, and AG-R comprised 23.5%. Rhizoctonia zeae (19.8%), R. solani AG1-IE (18.6%), and binucleate Rhizoctonia AG-A (8.6%) were the most frequent anamorphic states found. Root rot severity caused by the different taxa varied from low to high on common beans, and tended to be low to average in maize. Twenty-two isolates were pathogenic to both hosts, suggesting difficulties in managing Rhizoctonia root rots with crop rotation. These results suggest that cropping history affects the geographical arrangement of AGs, with a prevalence of AG1 in the tropical zone from central to north Brazil while the AG4 group was most prevalent from central to subtropical south. W. circinata var. zeae was predominant in soils under maize production. To our knowledge, this is the first report on the occurrence of W. circinata var. circinata in Brazil.

A broad diversity survey of Rhizoctonia species from the Brazilian Amazon reveals the prevalence of R. solani AG‐1 IA on signal grass and the new record of AG‐1 IF on cowpea and soybeans

“…zeae , with R. zeae as the asexual stage) was detected on maize and rice sampled from Pará ( N = 9). The association of R. oryzae and R. zeae with diseases on another grass species other than rice or maize has been reported in the United States (Chen et al , ) and more recently in the Brazilian Cerrados (Blanco et al , ). In fact, W. circinata species complex ( zeae , oryzae , and circinata varieties) was indicated as an important group of Rhizoctonia ‐like pathogens, totalling 39.5% of the isolates sampled from the Cerrado agricultural fields (Blanco et al , ).…”

“…We have not detected R. solani AG‐1 IE in our survey. However, its occurrence has been reported from Mato Grosso agricultural areas cropped with common bean and in Roraima natural soils under forest vegetation (Blanco et al , ). The Brazilian AG‐1 IE isolates included in our phylogenetic analyses were formerly described and identified as AG‐1 IA and were associated with the soybean foliar blight disease sampled from Mato Grosso, Tocantins, and Maranhão fields in 2002 (Ciampi et al , ).…”

“…The AG-1 IE and AG-1 IF pathogens are considered the most common and aggressive within the AG-1 complex, and also tend to infect common bean cultivars with partial resistance (Godoy-Lutz et al, 2008). The occurrence of AG-1 IE on common bean was recently reported in a diversity survey conducted in Mato Grosso, Brazil (Blanco et al, 2018). In field conditions, isolates from AG-1 IE and AG-1 IF produce abundant sclerotia, which are a relevant source of inoculum besides basidiospores, infected plant organs and infested soil (Cárdenas-Alonso, 1989).…”

“…Outside T. cucumeris ( R. solani species complex), there are two other main Rhizoctonia‐ like species described as important plant pathogens: the binucleate species Ceratobasidium spp. (classified as Ceratorhiza in the asexual phase), and the multinucleate species Waitea circinata (asexual phase Rhizoctonia , with five known varieties: zeae , oryzae , circinata , prodigus , and agrostis ; Blanco et al , ).…”

“…On common bean in particular, the web blight disease can be caused by four genetically distinct groups within the AG‐1 complex (IA, IB, IE, and IF), by AG‐2‐2 WB (Godoy‐Lutz et al , ; Blanco et al , ), and to a lesser extent by AG‐2‐2 IV and AG‐4 (Carling et al , ; Godoy‐Lutz et al , ). The AG‐1 IE and AG‐1 IF pathogens are considered the most common and aggressive within the AG‐1 complex, and also tend to infect common bean cultivars with partial resistance (Godoy‐Lutz et al , ).…”