Effect of fungicides applied as foliar sprays and trunk injections on nut rot of chestnuts caused by Phomopsis castanea in Victoria

Abstract

Summary. Fungicides applied in vitro, as foliar sprays and trunk injections were evaluated for the control of phomopsis nut rot, an important storage disease of chestnuts in Australia caused by Phomopsis castanea. Benomyl, imazalil, prochloraz and propiconazole were most effective in vitro against mycelial growth, with EC₅₀ values <0.1 µg/mL. Iprodione was less effective with an EC₅₀ value between 1 and 10 µg/mL, while chlorothalonil and phosphorous acid were ineffective with EC₅₀ values >100 µg/mL. Up to 4 foliar sprays of benomyl, or phosphorous acid, applied to trees during the growing season reduced (P<0.05) the incidence of rotted nuts during cold storage in 2 out of 5 trials, or 1 out of 3 trials respectively. In one trial, nut rot was reduced from 42.6% in unsprayed trees to 23 or 23.6% in nuts from trees sprayed with benomyl or phosphorous acid respectively. Prochloraz sprays were not effective. At harvest kernel nut tissue from trees sprayed with benomyl, prochloraz or phosphorous acid showed residues of <0.03, 0.14 or from 16 to 145 µg/g respectively.

Trunk injection with imazalil or phosphorous acid at flowering in December did not control the disease, although in 1 trial, phosphorous acid reduced rot levels in stored nuts from 51.1% in untreated trees to 41.8% (P = 0.088). Imazalil treatments caused leaf scorching and in some cases limb death above the point of injection. Occasional slight leaf scorching was associated with some phosphorous acid injections. Residue studies showed that phosphorous acid moved into nut tissue, but showed little lateral redistribution around the tree.