Phragmites die-back: toxic effects of propionic, butyric and caproic acidsin relation to pH

Symptoms which are associated with die-back in Phragmites: growth inhibitio n, root and bud death, premature shoot senescence, blocked aeration and vas cular systems, especially in rhizomes and roots, and abnormal surface and i nternal cell-wall lignification and suberization of roots were induced by e ach of three of the lower volatile organic acids, propionic, butyric and ca proic. These acids were applied in nutrient media in concentrations similar to those previously associated with die-back sites and/or in sediments con taining rotting rhizomes and roots of the plant. At concentrations of 1.4 a nd 0.56 mM, respectively, butyric and caproic acids were each found to be h ighly toxic at pH 4.5, but relatively innocuous at pH 6. Propionic acid, ap plied at a much higher concentration of 10.4 mM, was highly toxic at both p H 4.5 and 6. The results support previous findings that the undissociated f orms of the organic acids are the more toxic. Rhizomes and roots, rotting i n 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 decayi ng underground parts of the plant or other sources of organic matter.