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.