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

Abstract

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. 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 2020seasons 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 1.1kPa VPD for P. macadamiae and N. macadamiae and <0.6kPa VPD for B. cinerea and C. cladosporioides. 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. This information provides a firm basis for the development of prediction tools for flower blights in macadamia.