Infection Processes Differ Among <i>Botrytis cinerea</i> , <i>Cladosporium cladosporioides</i> and <i>Neopestalotiopsis macadamiae</i> in Macadamia Flowers

ABSTRACT Botrytis cinerea , Cladosporium cladosporioides and Neopestalotiopsis macadamiae are the foremost causative agents of the flower blight disease complex in macadamia, threatening the industry worldwide. Detailed descriptions of the infection process of these pathogens on macadamia flowers are scant. Therefore, this study tested the hypothesis that these three pathogens differ in conidial germination, penetration, colonisation and sporulation on macadamia floral tissues. These processes were assessed from 1½ to 48 h post‐inoculation (hpi) using scanning electron microscopy and light microscopy. Our analyses revealed that regardless of flower development stages, N. macadamiae conidia germinated within 1½ hpi, while conidia of the other two pathogens germinated at 3 hpi, all producing a single terminal germ tube. Mechanisms for flower penetration differed in that, at 9 hpi, B. cinerea penetrated the petal epidermal cells directly without forming appressoria. In contrast, C. cladosporioides used both direct penetration and appressorial structures, whereas N. macadamiae exclusively penetrated the host tissue through appressoria, completing this process within 6 hpi. Initial disease symptoms appeared following tissue colonisation by the pathogens at 24 hpi. Unlike the other two pathogens, calcium oxalate crystal deposits were observed at 24 hpi for B. cinerea ‐inoculated tissues. All flower blight fungal pathogens produced copious amounts of conidia within 48 hpi, suggesting the polycyclic nature of the pathogens in the macadamia flower blight pathosystem. This knowledge will aid in further investigation of pathogenicity, disease monitoring and management decisions in the macadamia industry.