北卡罗来纳州和美国东南部诸州栽植的烟草属的烤烟常常受一些土壤内的病害严重侵袭。造成这些病害的病原有高度毁灭性的真菌、细菌和线虫。这些病原大部份完全能使植株遭受伤害而不借助于其它生物。然而,已知某些病源一起存在时,毁灭性常较一种病源单独存在时大得多,特别是根瘤线虫。例如,黑胫病(Phytophfhora Parasitica var.nicotianae)、青枯病(Pse- Flue-cured tobacco grown in North Carolina and the states of the southeastern United States is often severely affected by some soil-borne diseases. The pathogens causing these diseases are highly devastating fungi, bacteria and nematodes. Most of these pathogens can completely harm the plants without resorting to other organisms. However, it is known that when certain disease sources are present together, the destructiveness is often much greater than that of a single disease source alone, especially root-knot nematodes. For example, Phytophthora Parasitica var. Nicotianae, bacterial wilt (Pse-