Role of Maize rayado fino virus in the Etiology of "Red Stunt" Disease in Brazil.

Corn production in several areas of Brazil recently has been seriously afflicted by a disease commonly referred to as "red stunt," characterized by stunting and leaf reddening. Early observations that a phytoplasma was associated with the disease were confirmed through molecular analysis, which revealed the presence of maize bushy stunt phytoplasma (MBS) (1). Another disease of corn, corn stunt, is considered to be caused by one or more of a complex of MBS, corn stunt spiroplasma (CSS), and Maize rayado fino virus (MRFV), which can all be transmitted simultaneously by their leafhopper vector Dalbulus maidis (Delong & Wolcott). The contributions of CSS and MRFV to the recently described "red stunt" disease in Brazil are unknown. A virus serologically related to MRFV, Brazilian corn streak virus, was first identified in Sao Paulo State, Brazil, in the early 1970s; serological studies indicated that isolates of Brazilian corn streak virus were related to, but distinguishable from, MRFV isolates from other Latin American countries (4). Therefore, there was a high probability that MRFV would be found in maize tissues collected from plants exhibiting symptoms of "red stunt" disease. Maize leaf samples exhibiting symptoms of "red stunt" disease were collected and preserved by drying from four Brazilian States during 1995 and 1996 (1). Total nucleic acid extracts were prepared from dried leaf tissue and aliquots of the extracts were spotted onto a nylon membrane, which subsequently was hybridized to an MRFV-specific cRNA probe (3). Of the 37 samples tested for the presence of MRFV by nucleic acid hybridization, 16 were positive for MRFV. It was present in some, but not all, samples that were positive for MBS (1). MBS was detected in six, and CSS was detected by polymerase chain reaction (PCR) (2) in 12 samples. In 8 of the 37 samples, both CSS and MRFV were present, 4 of 37 were positive for MBS and MRFV, and in 3 of 37 samples, all three pathogens were detected. Therefore, there is not a clear correlation between the presence of MRFV and the symptoms of "red stunt. " The coat protein gene and adjacent 3' nontranslated region of MRFV were amplified from infected tissues by reverse transcription-polymerase chain reaction (RT-PCR) using MRFV-specific primers (3). Three cloned PCR products were sequenced (deposited at GenBank under accession nos. AF186177 to AF186179), which revealed that the nucleotide sequences of the Brazilian isolates were 98% sequence identical and shared 90 to 97% identity with other MRFV isolates (3). Phylogenetic analysis of the sequences revealed close relationships to MRFV isolates from Peru and Bolivia, which neighbor Brazil (3). The contribution of MRFV to the stunting and leaf reddening symptoms exhibited by maize plants with "red stunt" disease is unknown. Of the 37 samples examined in this study, MRFV was detected in 16. A more complete epidemiological study of the association of MBS, CSS, MRFV, and their insect vector with "red stunt" disease will provide information on the significance of these pathogens in the current disease outbreak. References: (1) I. P. Bedendo et al. Plant Dis. 81:957, 1997. (2) R. E. Davis and E. L. Dally (Abstr.), Phytopathology 88:S20, 1998. (3) R. W. Hammond et al. J. Gen. Virol. 78:3153, 1997. (4) E. W. Kitajima et al. Proc. Am. Phytopathol. Soc. 2:76, 1975.