if_else is true, make NA for range of columns

here is a test DF

test_df <- structure(list(plant_id = c("A", "B", "C", "D", "E",
                                       "F", "G", "H", "I", "J", 
                                       "K", "L", "M", "N", "O"), 
                          value = c(100, 105, 120, 61, 52, 
                                      30, 21, 301, 200, 92,
                                      1, 0, 81, 202, 42),
                          C1 = c(2, 5, 3, 1, 5, 
                                    1, 21, 4, 2, 3,
                                    1, 0, 8, 55, 2),
                          C2 = c(9, 7, 3, 1, 52, 
                                    30, 21, 1, 8, 2,
                                    1, 0, 1, 2, 4),
                          C3 = c(100, 3, 120, 2, 1, 
                                    3, 1, 301, 2, 3,
                                    31, 30, 2, 11, 24)), 
                     row.names = c(NA, -15L), class = "data.frame", 
                     .Names = c("plant_sp", "value", "C1", "C2", "C3"))

I want to build an if_else condition which checks if the value under column "value" is less than 100. If it is, I want it to make range of column as NA (for example column at postion 3-4 would make column C1 and C2 as NA). Pipe solution would be amazing.

Thanks a lot, Ido