The MAR system of genetic improvement identifies and pyramids MAR genes causing a broad biological resistance effective against most diseases, most insects and causes at the same time increases in early and total yield. The question as to what causal factors may be controlled by MAR genes causing such a broad genetic change has been addressed. The hypothesis is that MAR genes influence quality and quantity of exudates, intercellular fluids, etc. that as energy media selectively favor plant colonization by symbionts which are beneficial in causing resistance to adversities. Research results supporting the hypothesis are briefly reviewed. It has been shown that plant tissues of MAR varieties have more bacteria that suppress disease and insect damage and more bacteria in the rhizosphere which are antagonistic to soil-borne pathogens than non-MAR varieties.

A reason for accepting symbiotic organisms as causal factors in MAR resistance is they are associated with resistance and when isolated and applied as treatments they cause resistance in susceptible varieties.

The given results show that an isolate (Fu-10) of Fusarium solani (Mart.) Appel & Wr., from the rhizosphere of a MAR variety, applied to the rhizosphere, causes plants to resist Phymatotrichum root rot and seedling pathogens, and enhances early and total yield. The fungus is more effective when combined with complementing bacteria.