Phragmites die‐back: toxic effects of propionic, butyric and caproic acids in relation to pH

Symptoms which are associated with die‐back in Phragmites : growth inhibition, root and bud death, premature shoot senescence, blocked aeration and vascular systems, especially in rhizomes and roots, and abnormal surface and internal cell‐wall lignification and suberization of roots were induced by each of three of the lower volatile organic acids, propionic, butyric and caproic. These acids were applied in nutrient media in concentrations similar to those previously associated with die‐back sites and/or in sediments containing rotting rhizomes and roots of the plant. At concentrations of 1.4 and 0.56 mM, respectively, butyric and caproic acids were each found to be highly toxic at pH 4.5, but relatively innocuous at pH 6. Propionic acid, applied at a much higher concentration of 10.4 mM, was highly toxic at both pH 4.5 and 6. The results support previous findings that the undissociated forms of the organic acids are the more toxic. Rhizomes and roots, rotting in water or waterlogged sand, released cocktails of acids and produced pH in the range 4.8–5.4. Phragmites seedlings planted in these media died within 12 h. Overall, the results support the theory that die‐back in Phragmites can be induced and/or perpetuated by organic acids released from the decaying underground parts of the plant or other sources of organic matter.