Iprodione resistance of Alternaria alternata pv. citri from Minneola Tangelo in Israel and Florida

In an orchard of Minneola tangelo at Suffa, Israel, where iprodione had been used in the preceding 3 years, iprodione treatments in the current season failed to control Alternaria brown spot, caused by Alternaria alternata pv. citri. All 200 A. a. pv. citri isolates from this orchard were iprodione resistant, as evidenced by growth on potato-dextrose agar (PDA) amended with 25 mg of iprodione per liter. The average ED 50 of 10 resistant isolates was 280 mg of iprodione per liter. In addition, in Florida and Israel, many isolates of A. a. pv. citri were recovered from brown spot lesions of Minneola tangelo in randomly selected orchards differing in iprodione history. The average ED 50 of these iprodione-sensitive isolates ranged from 0.20 to 0.62 mg of iprodione per liter. All Florida isolates of A. a. pv. citri were iprodione sensitive, with ED 50 values of 0.20 to 0.62 mg of iprodione per liter. When plates with mycelial plugs were incubated for 7 to 14 days on PDA amended with 6 to 200 mg/liter, most isolates from Florida and Israel developed resistant colonies as sectors. The subcultures from these sectors, designated as laboratory-selected iprodione-resistant isolates, grew well on PDA supplemented with 100 mg of iprodione per liter. Detached leaves of Minneola tangelo that had been sprayed with iprodione at 250 or 500 mg/liter were well-protected from infection by iprodione-sensitive isolates of A. a. pv. citri. However, when leaves were inoculated with an iprodione-resistant isolate from Suffa, disease was not controlled. The severity of infection of leaves inoculated with laboratory-selected resistant isolates from Florida or Israel was not affected by spraying iprodione at 250 mg/liter, but was reduced slightly at 500 mg/liter.