The influence of host phenology on infection and severity of rachis tip dieback caused by flower blight pathogens in macadamia

Abstract Rachis tip dieback (RTD) is an emerging disease affecting macadamia racemes and is associated with the flower blight complex. However, the timing and site of RTD infections and the effects of these infections on flower development remain poorly understood. Trials were conducted to investigate the relationship between disease severity, raceme developmental stages at the time of infection, and infection courts. Five fungal species associated with macadamia flower blight and RTD were tested, namely Botrytis cinerea , Cladosporium angustisporum , Epicoccum italicum , Neopestalotiopsis macadamiae and N. maddoxii .. Isolates of each species were inoculated individually and as a mixed inoculum onto three macadamia cultivars at three raceme developmental stages. The results showed that RTD was most severe when racemes were inoculated at the elongation stage (< 100 mm in length), whereas disease severity was mild or negligible when inoculation occurred at later stages of raceme development. All fungal species tested caused RTD symptoms, and disease severity following inoculation with individual species was comparable to that observed with the mixed inoculum, indicating no additive effect. RTD infection entry site occurred exclusively at the distal end of the raceme, with no symptoms observed when inoculations were conducted at the proximal end, suggesting that the disease progresses basipetally along the rachis. These findings demonstrate that RTD development is strongly influenced by host phenology and the infection court. The improved understanding of temporal and spatial dynamics of RTD provides valuable information for disease monitoring and the development of effective disease management strategies in macadamia production systems.