Corn lethal necrosis
The disease appeared in the Republican River valley and appeared elsewhere only rarely. Presumably the biology of river valley soil and/or the southerly location of the valley influenced the geographic restriction.
When CLN first appeared in Nebraska (~1980) it was closely tied to the soil. In fact one field was solidly yellow with disease symptoms except for a strip where a drainage ditch had been the previous year. It's now clear than MCMV is the soil associated component. Interestingly MCMV symptoms appear only in July about the time corn rootworm beetles emerge. Rootworm beetles are the primary aerial vector of MCMV. It seems probable that beetle larvae influence soil transmisson of MCMV, but their role is unclear. The MCMV component of the disease can be controlled by crop rotation. There's no obvious natural genetic resistance to MCMV in maize, but it's likely that transgenic beetle resistance would control MCMV. MCMV is native to South America and presumably arrived in the valley (perhaps decades earlier) by seed transmission.
Potyviruses are the other disease component. Mixed infection stimulates MCMV replication and greatly enhances symptoms. Wheat streak mosaic is occasionally the potyvirus component, but most Nebraska maize is WSMV resistant so the association is rare. Two strains of sugarcane moasic virus, maize dwarf moaic virus strain A and maize dwarf mosaic virus strain B are more commonly associated with CLN. MDMVA overwinters in johnsongrass which itself was common in part of the CLN range. Johnsongrass overwinters only in the southern part of Nebraska, but has largely disappeared due to herbicide treatments. The major potyvirus component of CLN in the Republican River valley was MDMVB. MDMVB appeared only sporadically in the valley. I suspect seed tranmission was the culprit. Commercial resistance to CLN appears to be predominantly resistance to MDMVB derived largely from Pa405.
The disease has been rare over the last 25 years, likely because MDMVB is rare either due to resistance or due to less seed tranmission. Perhaps MDMVB is seed transmitted only in a few maize varieties which are no longer planted. MCMV by itself has mild symptoms and may be easily overlooked. In the absence of potyviruses the low level of MCMV in plants likely minimizes soil overwintering.
When the disease appears in new regions it's worthwhile to identify specific viruses, vectors, alternate hosts and other factors that influence local ecology.