Management of Fire Blight Using Pre-bloom Application of Prohexadione-Calcium

Abstract

Fire blight, a bacterial disease of rosaceous plants caused by Erwinia amylovora, is one of the most important diseases affecting commercial apple production worldwide. Antibiotics, applied at bloom to protect against blossom infection, are the most effective means of management but raise concern due to the potential for antibiotic resistance in both the pathogen population and nontarget organisms. In addition, most fire blight outbreaks in New York State often emerge in late June to July as shoot blight, calling into question the role of blossom infections and the antibiotic applications made to manage them. Prohexadione-calcium (PhCa) is a gibberellic acid inhibitor used post-bloom to control shoot vigor and to manage shoot blight. However, the magnitude of shoot blight management is directly related to the suppression of shoot growth, which is undesirable, especially in young orchards during establishment years. PhCa is believed to control shoot blight by thickening cell walls in cortical parenchyma, preventing invasion of host tissues by E. amylovora. We hypothesize that PhCa applied pre-bloom could similarly prevent invasion of blossom pedicels following infection, leading to reduced disease incidence. We evaluated novel pre-bloom PhCa programs for their effects on disease management (blossom and shoot blight) as well as their impact on shoot growth for three years in a mature ‘Gala’ orchard in New York. In all three years of the study, all PhCa programs resulted in less than 27% incidence (71% control) of blossom blight and less than 13% incidence (77% control) of shoot blight with minimal effect on tree growth. Inclusion of a biopesticide during bloom further reduced the incidence of blossom blight in one year of three. Using light microscopy, we found that cell walls in the cortical parenchyma of fruitlet pedicels on trees receiving pre-bloom PhCa applications were significantly thicker than those of untreated trees 40 days after full bloom and inoculation. Overall, we found that pre-bloom applications of PhCa had utility in reducing blossom blight and shoot blight with minimal impacts on tree growth. These pre-bloom programs would fit with standard production practices and may contribute toward the development of fire blight management programs without the use of antibiotics.

Proof of Concept for Shoot Blight and Fire Blight Canker Management with Postinfection Spray Applications of Prohexadione-Calcium and Acibenzolar-S-Methyl in Apple

“…programs 3 (one PCA high) and 4 (two PCA high) on shoot blight severity in the present study suggest that the high PCA rates require less than 10 to 14 days to take effect after application, which is contrary to this timeline currently viewed as required for PCA's mode of action to express (Wallis and Cox 2020). This implies that the immediately accessible (i.e., absorbed) dose of PCA that reaches inside the apple shoots plays a key role in achieving the desired host changes (McGrath et al 2009), triggering quicker interruption of E. amylovora invasion through shoots.…”

“…Prohexadione-calcium (PCA) is a valuable PGR that activates structural resistance in vegetative apple tissues to E. amylovora (Sundin 2014) (Apogee, BASF Corporation; Kudos 27.5 WDG; Fine Americas). PCA inhibits gibberellin biosynthesis, which reduces longitudinal apple shoot growth and increases the thickness of cell walls in cortical parenchyma, thus preventing invasion of shoot tissues by E. amylovora and reducing shoot susceptibility to fire blight (Evans et al 1999;McGrath et al 2009;Ramirez et al 2005;Wallis and Cox 2020). Preventive spray applications of PCA are recommended in commercial apple orchards starting at the 2.5-to 5-cm shoot growth stage and continuing at 1-to 4-week intervals after the first application because they reduce shoot blight incidence as much as 83% and shoot blight severity as much as 86% (Momol et al 1998;Yoder et al 1999).…”

“…In addition to its use for fire blight, PCA is often used to control fruit tree vigor and size, especially on vigorous rootstocks, and can mitigate a currently established fire blight outbreak and prevent the shoot infections from renewing in the growing season following an outbreak. Recently, PCA applied at pink bud stage was shown to thicken the cell walls in the cortical parenchyma of apple flower and fruitlet pedicels and prevent their invasion by E. amylovora, leading to 71 and 77% control of blossom and shoot blight, respectively (Wallis and Cox 2020). Because of its indirect mode of action impacting the plant host, resistance development in E. amylovora to PCA is unlikely (Spinelli et al 2005).…”

Proof of concept for management of shoot blight and fire blight cankers on pear with preventive spray applications of giant knotweed extract

“…In contrast to fungal plant diseases, to control a bacterial plant disease a combination of multiple control approaches and methods is necessary (Agrios, 2005). For example, control of fire blight usually involves planting resistant cultivars, spray application of copper bactericides, antibiotics, biological materials, activators of Systemic Acquire Resistance (SAR), shoot growth retardants, and mechanical removal of infected tree parts by pruning or whole tree removal (Johnson and Temple, 2013;Johnson and Temple, 2015;Wallis and Cox, 2019;Acímovićet al, 2021;Philion and Joubert, 2021).…”

Prohexadione-calcium on downy mildew and botrytis bunch rot epidemiology, and yield and technological and phenolic parameters of the Merlot vine cultivar in the highland region of southern Brazil

“…Additionally, many reports have indicated that ProCa can reduce the intensity of many diseases, such as fire blight in pears, scab in apples, and gray molds in grapes (BAZZI et al, 2003;McGRATH et al, 2009;WALLIS & COX, 2020). McGrath et al (2009), suggested that ProCa triggers the production of flavanoids, not normally reported in rosaceous species, that provide antimicrobial activity against E. amylovora, and this activity is the primary mechanism of fire blight control by ProCa.…”