Influence of climatic factors on dry flower, grey and green mould diseases of macadamia flowers in Australia

Aims Flower blights (grey mould, green mould and dry flower) are important diseases of macadamia. Lack of information on pathogen biology and disease epidemiology in macadamia has hampered control options. Effects of climatic variables including temperature, relative humidity (RH) and vapour pressure deficit (VPD) on the abundance, germination and growth of conidia of four fungal pathogens that cause various flower blights in macadamia were studied. Methods and Results Mycelial growth, sporulation, conidial germination and germ tube growth for five isolates each of Botrytis cinerea, Cladosporium cladosporioides, Pestalotiopsis macadamiae and Neopestalotiopsis macadamiae, at eight temperatures, seven RH and the corresponding VPD regimes were determined in vitro. The optimal climatic range of each of the four pathogens was validated during macadamia flowering periods in the 2019 and 2020 seasons by conidia detected and quantified using quantitative PCR. Several growth models were fitted to the data with high significance; predicted optima from these models ranged from 0.9 to 1.1 kPa VPD for P. macadamiae and N. macadamiae and B. cinerea and C. cladosporioides. Conclusions This study showed that VPD, as a determinant of the fecundity and growth of the four fungal pathogens, was predictive of flower blight incidence in macadamia. The importance of temperature, RH and, thus, VPD for defining the conditions for infection and flower blight epidemics was established. Significance and Impact of the Study This information provides a firm basis for the development of prediction tools for flower blights in macadamia.

Automated summary

This study investigated the effects of temperature, relative humidity, and vapor pressure deficit on the abundance, germination, and growth of four fungal pathogens that cause flower blights in macadamia. The results showed that VPD was predictive of the fecundity and growth of the pathogens, while temperature, RH, and VPD were important factors defining conditions for infection and epidemics. This information provides a solid foundation for the development of prediction tools for flower blights in macadamia.