Many unusual situations may occur in the next few years as various Rp genes are used in sweet corn and virulence differs among populations of the rust fungus. For example, it is likely that one Rp-resistant hybrid will be completely resistant while another is susceptible because the two hybrids have different Rp genes and the popula tion of P. sorghi is virulent against only one of the Rp genes. Another possibil-ity is that an Rp-resistant hybrid will be completely resistant at an early planting but susceptible at a later planting because virulent isolates of P. sorghi are introduced into an area in the middle of the growing season.

A similar situation is that an Rp-resistant hybrid will be completely effective in one region of North America but the same hybrid will be susceptible in another region because the rust population includes virulent isolates in one area but not in the other. Still, another possibility is that one Rp-resistant hybrid will be severely infected while another Rp-resistant hybrid is only mildly infected. This might be due to different background reactions of the two hybrids or due to differences in the frequency of rust isolates that are virulent against different Rp genes in each hybrid.

It is not likely that Rp-resistance will control rust as effectively in the next 20 years as in the past nor will it add more than $20-$40 million to the on-farm value of sweet corn during the next 20 years as it has during the past 20 years.

However, with close scouting and increased preparedness to apply fungicides when necessary, Rp-resistance can be one of the methods used to manage rust. Rp-hybrids may be completely or partially effective or they may need to be protected with fungicides depending on whether virulence occurs in rust fungus.