Comparisons of virulence of pathogens associated with crown and root diseases of strawberry in Western Australia with special reference to the effect of temperature

Abstract

Strawberry production in Western Australia (W.A.) is severely compromised by crown and root diseases. A range of fungi and oomycetes, including Fusarium oxysporum, binucleate Rhizoctonia, Cylindrocarpon destructans, Phoma exigua, Gnomonia fructicola, Phytophthora cactorum, Pythium ultimum and Macrophomina phaseolina, are associated with such disease outbreaks. Studies were conducted for the first time to determine and compare the virulence of the fungal and oomycete isolates, and to determine the effect of temperature on virulence of these isolates in terms of disease severity on strawberry plants. We firstly determined the virulence of these isolates at 22 °C, F. oxysporum severely affected crown, crown vascular and root tissues, resulting in the rapid wilting and death of strawberry plants; binucleate Rhizoctonia, particularly AG-A, also caused severe disease on roots and crowns, resulting in the eventual death of plants; C. destructans, P. exigua, G. fructicola, P. cactorum and P. ultimum caused variable levels of symptoms on crowns and roots, and while generally less virulent compared with F. oxysporum and Rhizoctonia, still severely retarded the growth and development of inoculated plants. Subsequent studies showed that F. oxysporum, Rhizoctonia and M. phaseolina were much more virulent under warmer conditions (>22 °C), evident as more severe disease. F. oxysporum and Rhizoctonia were most virulent and caused most severe disease at 27 °C, but M. phaseolina was most virulent and caused most severe disease at 32 °C. This relates to the dominances of these pathogens in the warmer months of the year and, in particular, the dominance of M. phaseolina in the late spring/early summer when temperatures often exceed 30 °C. This is the first study to demonstrate that F. oxysporum, binucleate Rhizoctonia and M. phaseolina are the most virulent pathogens causing crown and root diseases of strawberry in W.A., and to show that the virulence of these pathogens is influenced by the prevailing seasonal temperature regime.