Effect of sick rhizosphere soil under tomato continuous cropping on soil nematodes, microbes and tomato growth

Continuous cropping of tomato is a widespread practice that severely restricts sustainable tomato production. The interaction between soil nematodes, soil microbes and plants may be related with root-knot nematode disease due to continuous cropping. However, a little attention has been paid to the interrelatedness of these factors or the effect of continuous cropping on the relationship among soil and other 3 factors. Thus, this study explored the effects of sick soil (root-knot nematode infecting soil) on tomato root-knot disease, mechanism of micro-ecological obstacles and plant growth of continuously cropped tomato with a pot experiment. The abundances and communities of microbes and nematodes in root-zone soils and the activities of defensive enzymes in seedling leaves of tomato were analyzed to determine what pathogenic mechanism existed in such cropping systems. Compared with healthy soil, sick soil caused the following changes: (1) at seedling stage, root-knot nematodes began to infect roots and therefore root-knots appeared on tomato roots. Compared with the healthy soil, silk soil increased the abundance of soil nematode by 390.4%. At maturity, the infection rate of root-knot nematode was 62.7% and the related disease index was 80.0%. (2) The growth of tomato was restrained and the activities of defense enzymes reduced. The dates of blossoming and fruiting delayed too. Fresh biomass of shoot and root at maturity significantly (P < 0.05) decreased by up to 50.2% and 33.1%, respectively. Also the quantity and fresh mass of fruit significantly (P < 0.05) decreased by up to 59.7% and 68.2%, respectively. While compared with healthy soil, PPO activity of seedling leaves significantly (P < 0.05) decreased (by 15.8%), and POD activity significantly (P < 0.05) increased (by 24.0%) for silk soil. (3) Tomato roots became easily infected by harmful bacteria (Pseudomonas brassicacearum). The number of pathogenic bacteria P. brassicacearum in tomato roots in sick soils was 463 times greater than that in healthy soils. The total number of bacteria, fungi and actinomycetes in rhizosphere soils increased by 46.3%, 94.5% and 134.0%, respectively. (4) The abundance of soil nematodes increased nearly 3 times, among which the abundance of fungi-feeding nematodes, bacteria-feeding nematodes and plant-parasitic nematodes increased by 1.6, 3.3 and 7.3 times, respectively. The abundance and diversity of plant-parasitic nematodes greatly increased, of which root-knot nematode accounted for 95.6%. In conclusion, the inoculation of sick soils from the root-zone under continuous tomato cropping had complex effects on root-zone soil ecology. It inhibited tomato growth and increased the incidence of root-knot nematode disease by influencing the abundance and diversity of microbes and nematodes in root-zone soils and also by influencing biochemical metabolism of tomato. Mass propagation of plant parasitic nematodes and plant pathogenic bacteria decreased defensive enzyme activity and stress resistant ability of tomato, which in turn led to more severe root-knot nematode infection with significant inhibitory effect on tomato production. The negative effects of continuous cropping on tomato growth were caused by the interactions among root-zone soil microbes, root endophytes and soil nematodes.