Sources, Detection, and Inoculum Quantification of Flower Blight Pathogens in Macadamia

Abstract

Dry flower disease caused by Pestalotiopsis/Neopestalotiopsis spp., green mould caused by Cladosporium spp. and grey mould caused by Botrytis spp., collectively known as flower blight cause significant yield losses in macadamia. Potential sources of inoculum of the various pathogens in macadamia tree canopy were examined using pathogenicity tests and a multiplex quantitative PCR (qPCR) assay developed in this study. The qPCR assay detected and quantified the relative abundance of the inoculum of flower blight pathogens. The assay revealed that remnant racemes contributed a high amount of inoculum of all the three groups of flower blight pathogens, while the yellow halo leaf spot contributed only Pestalotiopsis/Neopestalotiopsis species. The amount of conidia per g of remnant racemes ranged from 7 × 103 to 2 × 104 for dry flower disease, 3 × 103 to 1 × 104 for green mould and 5–8 × 103 for grey mould pathogens. Conidia of Pestalotiopsis/Neopestalotiopsis species quantified from leaf spots varied from 1 × 102 to 1 × 103 per cm2. Pathogenicity tests performed on developing racemes under field conditions, using conidial suspensions from both sources of inoculum (remnant racemes and yellow halo leaf spot) resulted in severe flower bight symptoms. Disease severity was not significantly different when remnant racemes were incubated directly with the developing racemes compared to inoculation with conidial suspension from the materials. This suggests that racemes from preceding seasons that remain in the tree canopy carryover inoculum between seasons and should be removed as a control option of flower blights in macadamia.