Field and glasshouse evaluations of metalaxyl, potassium phosphonate, acibenzolar and tea tree oil in managing Phytophthora root rot of papaya in far northern Queensland, Australia

Phytophthora palmivora , the causal agent of Phytophthora root rot, is widespread in the papaya-growing areas of far northern Queensland. Phytophthora root rot causes decay of the tap root and eventual death of the affected plant. In glasshouse and field experiments, metalaxyl, potassium phosphonate, acibenzolar and tea tree oil were evaluated for the control of Phytophthora root rot of papaya. In the glasshouse experiments, metalaxyl (0.625 or 1.25 g/m 2 ) controlled root rot in 8-week-old papaya seedlings while 16-week-old plants sprayed with phosphonate (6 or 12 g/L) were less affected by root rot than plants sprayed with phosphonate (3 g/L). Phosphonate had no effect on root rot incidence in 8 and 12-week-old seedlings. In the 2002 field experiment, metalaxyl applied at transplanting followed by weekly foliar sprays of phosphonate (1 g/L) plus acibenzolar (0.025 g/L) had the lowest incidence of root rot (0.6%) and the greatest fruit yield. Fortnightly sprays with phosphonate (10 g/L) plus acibenzolar were more effective at reducing root rot than fortnightly sprays with phosphonate (1 g/L) plus acibenzolar. There was no difference in disease incidence between weekly and fortnightly spray applications of phosphonate (1 g/L) plus acibenzolar. Soil drenches with tea tree oil failed to reduce the incidence of root rot compared with the untreated plants. In the 2005 field experiment, metalaxyl (0.625 g/m 2 ) applied at transplanting followed by fortnightly sprays of phosphonate (6 g/L) or fortnightly sprays of phosphonate (12 g/L) alone resulted in 97% and 93% plant survival, respectively, compared with 24% of plants in untreated plots. Phosphonate caused temporary bleaching of leaves at rates of 3 g/L or more.