Severity of crown and root diseases of strawberry and associated fungal and oomycete pathogens in Western Australia

Abstract

The decline/death of strawberry plants is considered to be a serious challenge to strawberry production in Western Australia (WA). Although crown and root diseases have been found to be important contributing factors, little is known on the extent of crown and root diseases or the associated fungal and oomycete pathogens. Surveys were undertaken in 2008 to determine the severity of crown and root diseases in commercial strawberry fields in WA, to determine the influence of cropping history, fumigation, soil nutrients and pH on severity of such diseases, and to identify the associated pathogens. Among the sites surveyed, there was a significant variation with the decline/death index (%DI) ranging from 2.9 to 39.7. The level of strawberry plant decline/death progressed significantly from August to October, with the %DI rising from 13.0 to 39.2; There was a significant difference in strawberry plant decline/death between fumigated and non-fumigated beds, with a %DI of 7.1 for fumigated and 45.2 for non-fumigated beds. However, it was evident that in situations where there were periods of 3 or more years involving crop rotations between successive strawberry crops that the level of plant decline/death was lower, even in the absence of fumigation. Both the severity of crown disease and root disease were positively correlated with that of strawberry plant decline/death. There was a significant negative correlation between soil pH and the decline/death of strawberry plants. The main pathogens associated with crown and root diseases of strawberry were Fusarium oxysporum, Rhizoctonia spp. (AG-A, AG-C, AG-I, AG-K and others), Cylindrocarpon destructans, Phoma exigua, Gnomonia fructicola, Phytophthora cactorum, Pythium ultimum and Macrophomina phaseolina. F. oxysporum was most frequently isolated from crowns, at a frequency of 41.2%. Rhizoctonia spp. and C. destructans were most frequently isolated from roots, at a frequency of 11.8% and 12.0%, respectively. This study demonstrates, for the first time, that strawberry production in WA is severely compromised by crown and root diseases, especially late in the production season, and impacts are clearly greater in non-fumigated field beds as well as those with relatively lower soil pH (4.5 ~ 6 in CaCl2). This study also demonstrates that F. oxysporum is the major pathogen associated with crown disease, and Rhizoctonia spp. are important pathogens associated with root disease of strawberry in WA.