Seedling Disease Triangle

Interactions of host, pathogen and environment certainly affects corn seedlings.  This will probably be emphasized this year in those western US corn fields that were planted and then covered with several inches of snow a few days later.  Also, only 1 day after I blogged about how Fusarium species are often more or less innocent endophytes in corn seedlings, the most recent publication of Plant Disease includes a study showing a Fusarium species could cause corn seedling rot and one from China claiming another Fusarium species causing stalk rot. Both of these studies indirectly point out the difficulty of separating the environmental affect on the host from the pathogen aggressiveness. Simulating the field environment that a crop in the field really undergoes with laboratory conditions is problematic for all pathologists. 
 
If a seed germinates at 60°F in moist loam soil, emerges in 7 days to sunny warm weather but is surrounded in the soil by fungi, some of which may penetrate the primary root tissue and may even progress between cells, causing the plant to produce the anti-fungal chemicals to restrict the fungus, is the fungus an aggressive pathogen of concern?  How about if the seed was old with damaged mitochondria delaying emergence and having a weakened response to the invading fungus?  Is the fungus now defined as an aggressive pathogen?  Or, if the cold environment delays emergence and response time from the host to the invaders, is the fungus the cause of low plant stand?  If the seed is planted in environment with high concentration of the fungus, causing the plant defense system to be overwhelmed, is the fungus now an aggressive pathogen?
 
It is not easy to separate the biology of the plant, complexity of environment and the biology of the potential pathogen when determining the cause of a corn emergence problem in the field. Usually it is related to each component of the disease triangle.