Composted Cotton Stalks and Cotton Gin Trash Substrate Amendments and Irrigation/Ground Cover Management II. Effect on Growth and Disease Suppression for Azalea and Juniper

Abstract

‘Sunglow’ azalea and ‘Blue Pacific’ juniper were grown in pine bark (PB) and pine tree (PT) substrates that were amended with cotton stalks composted with a N source (CSN), cotton stalks composted without an N source (CS), and cotton gin trash (CGT) to evaluate the substrate's effect on plant growth and disease suppression. The plants were grown under two different, commonly used, irrigation/ground surface management regimes — overhead, sprinkler irrigation with black geotextile weed fabric covering the ground (OH) or low-volume, spray stake irrigation with gravel covering the ground (LV). In 2010, with OH, all PB-amended substrates produced significantly larger azalea shoots than PT-amended substrates. In 2011, with OH, all azalea shoots were similar in size when grown in all substrates except for PT:CS, where plants were significantly smaller. With LV, in 2010 and 2011, azalea shoot growth was largest when grown in a PB substrate amended with CSN or CGT and lowest in PT:CS. Junipers with OH produced generally larger shoot growth with the PB-based substrates in both 2010 and 2011 compared to the PT-based substrates. With LV, PT:CGT produced the numerically smallest juniper shoot growth for both years. Overall, PT-based substrates appeared to produce greater consistency in growth, because responses were more similar in 2010 and 2011, however irrigation method and management can impact growth regardless of substrate composition. CGT added to PB- or PT-based substrates enhanced Ca and Mg uptake by both species but not P uptake. OH generally kept ground surface and substrate temperatures lower than LV regardless of substrate composition. The substrates tested neither enhanced nor deterred P. cinnamomi infection in azalea or juniper.