Survey of Potential Root Pathogens in Softwood Cuttings Collected from Georgia Blueberry Nurseries

Abstract

Survey sampling of 18 blueberry nurseries propagating softwood cuttings was conducted in 2007 and 2008 to determine which soilborne plant pathogens were most prevalent in commercial blueberry propagation systems in southern Georgia. Samples were collected four times: June (early), September (mid), and October (late) in the 2007 growing season, with additional sampling of overwintered cuttings in April 2008. This survey revealed that Calonectria ilicicola (Cylindrocladium parasiticum), causing Cylindrocladium root rot, is a primary pathogen of blueberry cuttings in southern Georgia. The organism was isolated consistently during all four survey dates with a cutting-level incidence of 3.6, 10.2, 36.4, and 14.3% in the first through fourth samplings, respectively. Rhizoctonia spp. were recovered less frequently and were only present during the first (10.9% cutting-level incidence), third (4.6%), and fourth survey dates (3.6%), while the Oomycetes Pythium and Phytophthora were detected only on the second survey date. Fusarium spp. were isolated commonly (29.1% incidence in the first, 12.2% in the second, 18.2% in the third, and 7.1% in the fourth sampling), but pathogenicity remains uncertain. Across all survey dates, Calonectria, Rhizoctonia, Oomycetes, and Fusarium were recovered at least once from 41.2, 17.6, 0.1, and 82.4% of nurseries surveyed, respectively. When nursery-level pathogen presence-absence data collected from this survey was analyzed in relation to a survey of production practices used by these same propagators, the practice of reusing growth media was found significantly associated with Calonectria presence.

Changes in Production Practices Used for Disease Management in Blueberry Nurseries in Georgia, USA, Over a 15-year Period

“…Softwood cuttings were taken in September during the second flush of vegetative growth, and these were treated with the fungicide captan only when disease was observed. Calonectria was recovered from this nursery (Haralson et al 2022).…”

“…In Georgia blueberry nurseries, the predominant pathogens are C. ilicicola and Rhizoctonia sp., with water molds (Phytophthora and Pythium species of the Oomycete class) so far contributing little to plant losses from disease (Haralson et al 2021). C. ilicicola is spread primarily through contaminated plant material or propagation substrate, and reuse of propagation media has been strongly linked to Cylindrocladium root rot in Georgia and North Carolina, USA, rooting beds (Cline 2004;Haralson et al 2022). Likewise, a similar trend was observed in Georgia for Rhizoctonia root rot (Haralson et al 2022).…”

“…C. ilicicola is spread primarily through contaminated plant material or propagation substrate, and reuse of propagation media has been strongly linked to Cylindrocladium root rot in Georgia and North Carolina, USA, rooting beds (Cline 2004;Haralson et al 2022). Likewise, a similar trend was observed in Georgia for Rhizoctonia root rot (Haralson et al 2022).…”

“…Although softwood cuttings are the primary choice for propagation material in Georgia, many different propagation systems and methods are used by growers (Haralson et al 2022). Propagation systems can be separated into two main types: open and closed.…”

“…Diseases have been a particularly difficult challenge for blueberry propagation (Haralson et al 2013(Haralson et al , 2021(Haralson et al , 2022. In other regions, the four main diseases affecting blueberry cuttings are Cylindrocladium root rot [Calonectria ilicicola (Cylindrocladium parasiticum)], Phytophthora root rot (Phytophthora cinnamomi), Pythium root rot (Pythium sp.…”