Development of laboratory tolerance to phosphorous acid, fosetyl-Al, and metalaxyl inPhytophthora capsici

A total of three phosphorous acid (H3PO3)-tolerant and five metalaxyl-tolerant mutants of Phytophthora capsici were recovered in vivo when populations of ˜ 107 zoospores were exposed to the chemical mutagen N-methyl-N-nitro-N-nitro-soguanidine. No mutants tolerant of H3PO3 or metalaxyl were recovered by mass selection procedures. Two H3PO3-tolerant isolates had EC50 values in vitro of 69 and 415 μg/mL compared with 21 and 37 μg/mL, respectively, for the parental isolates from which they were derived. These H3PO3-tolerant isolates were still pathogenic to bare-rooted green pepper seedlings treated with 900 μg/mL H3PO3, whereas sensitive isolates were controlled by only 150 mL μg/mL H3PO3. Tolerant isolates were pathogenic to peppers growing in soil drenched with either 4500 μg/mL H3PO3 or 4317 μg/mL fosetyl-Al. In contrast, sensitive isolates were controlled by a soil drench of 500 μg/mL H3PO3 or 719 μg/mL fosetyl-Al applied 24 hr prior to inoculation. Metalaxyl-tolerant isolates were also insensitive to several additional acylanilide fungicides but were sensitive to both fosetyl-Al, fosetyl-Na, and H3PO3. Likewise, H3PO3-tolerant isolates were tolerant of the fosetyl salts but were sensitive to the acylanilides tested.