Effect of copper spiked potting mix on the emergence and growth of viola and cabbages

Abstract

Legionella longbeachae is the major cause of legionnaires’ disease (LD) in Australasia and is associated with commercially produced potting mixes and compost. Copper (Cu²⁺) has previously been shown to reduce L. longbeachae colony-forming units (CFU) in vitro. Here we report the use of 0.5 mg/mL Cu²⁺ as an additive to potting mix and the effects on viola and cabbage seed emergence and plant growth. The addition of Cu²⁺ to an all-purpose potting mix reduced the emergence of viola seeds from 68% to 39% and cabbage seeds from 91% to 66%. Days to emergence was delayed by 3 days in viola seeds potted in Cu²⁺ containing potting mix, but no difference was recorded for cabbage seeds. Dry weight of leaves, shoots, and roots were all significantly lower in viola and cabbage seedlings grown in the presence of Cu²⁺. Likewise, plant height, leaf number and total leaf area were significantly lower in plants grown in Cu²⁺ containing potting mix. After 4 weeks of growth, chlorophyll measurements were significantly decreased in both viola and cabbage plants. Inductively coupled plasma analysis showed that when grown in Cu²⁺ containing potting mix, both viola and cabbage tissues had significantly higher levels of copper compared with the controls. Although 0.5 mg/mL Cu²⁺ can reduce L. longbeachae CFU in vitro, this concentration is detrimental to seed emergence and plant growth. Further work to determine whether a lower Cu²⁺ concentration or a rinse process may be a viable option to produce a safer potting mix is required