The peach replant problem—some causal agents

Abstract

The incorporation of peach ( Pranus persica (L.) Batsch) roots, dried at 80°C, in soil, subsequently planted with peaches, retarded growth. Fresh roots, root and soil extracts and amygdalin did not affect growth. Peach roots contain about 13 mg g fresh wt −1 of the cyanogenic glycoside prunasin. On hydrolysis prunasin yields toxic hydrocyanic acid (HCN) and benzaldehyde. Addition of peach roots to aqueous soil suspensions resulted in production of HCN, the amount produced being largest if oven-dried roots were added to the soil surrounding the roots of 4 month old peach plants, less so with soil from an aged peach orchard, and still less with soil not planted previously with peaches (“virgin soil”). HCN evolution from peach soil which had been replanted with peaches, and to which amygdalin had been added, was larger if the replanted trees were in an almost healthy condition, than trees declining severely. Hence, the rhizosphere of actively-growing young peach trees appears to be particularly rich in a microflora able to hydrolyze cyanogenic glycosides. As a result, the prunasin content of replanted old peach soil, 30 months after replanting, was lower than that of soil from the same plot which was not replanted. Heat-resistant bacilli, able to hydrolyze cyanogenic glycosides, were isolated from the surface of peach roots, dried at 80°C. Planting pomegranates ( Punica granatum L.) in peach soil, to which amygdalin was added, had no effect on HCN evolution, but planting applies ( Malus domestica Brokh.) or peaches increased HCN evolution. The application of a KCN solution to soil, planted with young trees, belonging to various species, resulted in reduced growth and, subsequently, death of the plants. The damage was less severe with apples than with peaches, and particularly less than with almonds ( Prunus amygdalus Batsch.). Thus differences in the response of various species to replanting after peaches may in certain cases originate from a lack in the augmenting effect of their roots on the microflora responsible for the hydrolysis of prunasin, in other cases they may be caused by differences in the response of their roots to cyanide. Detrimental effects caused to peaches and almonds by adding benzaldehyde (the second constituent of the aglucon of prunasin) to the soil were only slight.