STRAINS OF SORGHUM MOSAIC-VIRUS CAUSING SUGARCANE MOSAIC IN LOUISIANA

The viruses found causing sugarcane mosaic in Louisiana during 1978-19 88, 1990, and 1992 were strains H, I, and M of sorghum mosaic virus. T he percentages of plant samples that were infected with the three stra ins within the three production areas of the sugarcane belt were deter mined (because approximately 4% of all plants sampled were infected by more than one strain, incidences totaled more than 100%). In the Bayo u Lafourche area, strain H was found in 99% of infected plants, strain I in 1%, and strain M in 2%; in the Mississippi River area, strain H was found in 98% of infected plants, strain I in 1%, and strain M in 2 %; and in the Bayou Teche area, strain H was found in 90% of infected plants, strain I in 12%, and strain M in 5%. Strains were identified a nnually by inoculating differential host plants (sugarcane cultivars C P 31-294 and CP 31-588, sweet sorghum cultivar Rio, and johnsongrass) with leaf juice from diseased sugarcane plants. The highest incidence of strain I (12-31% of samples assayed) occurred in the Bayou Teche ar ea during 1978-1982. The subsequent decline in incidence of strain I i n this area corresponded with the decline of cultivar NCo 310. Strain M appeared intermittently at low levels in all areas, but in 1987 and 1988 in the Bayou Teche area, strain M appeared in 17 and 13%, respect ively, of the samples-most often those of the cultivar CP 79-318.