Citrus replant problem in Iraq III. Interactive effect of soil fungi and allelopathy

The interactive effects of citrus root residues and soil fungi on citrus replant problems were investigated. The results indicated that incorporation of citrus root residues in combination with the pathogenic fungiPhytophthora citrophthora, Pythium aphanidermatum andFusarium solani in soil caused more reduction to sour orange growth than did the root residues alone. Subsequent experiments showed that extracts of different parts of sour orange and leachates of some soil fungi increased the disease index of citrus roots grownin vitro. The citrus extracts did not affect growth of the test fungi.Thus, it appears that allelopathic compounds of plant and microbial origins build up in old citrus soil and may act as predisposal agents for the infection of citrus roots by soil pathogens.