Cotton (Gossypium hirsutum L.) normally has good tolerance to pyrithiobac {2–chloro–6–[(4,6–dimethoxy–2–pyrimidinyl)thio] benzoic acid, sodium salt] applied postemergence. Occasionally, though, moderate-to-severe injury has been observed in the field. Empirical evidence suggests this injury is related to cool temperatures during or near the time of application. A growth chamber experiment studied the effect of temperature on cotton cv. Deltapine 51's response to pyrithiobac applied postemergence. Cotton was grown at 31/24 °C (day/night), except for exposure to cool temperatures (21/8 °C) for 5 d before, 5 d after, or 5 d before and after pyrithiobac application. Pyrithiobac caused visible chlorosis and reduced leaf chlorophyll content 4 to 5 d after treatment but had no effect on other parameters. Cool temperatures reduced cotton height and shoot dry weight 14 d after pyrithiobac application, with reductions dependent upon length but not time of exposure. Results were similar when exposure occurred either 5 d before or 5 d after application, but exposure for 10 d caused greater reductions. Exposure to cool temperatures reduced square production, with exposure for 5 d before pyrithiobac application having a greater impact than exposure for 5 d after application. Exposure to cool temperatures for 5 d before or 5 d after pyrithiobac application increased nodal position of the first sympodium. The number of main stem nodes was reduced only on plants exposed to cool temperatures for 10 d. Lack of a temperature-by-pyrithiobac interaction suggests that the damage occasionally observed in the field under cool conditions may be due to a combination of stress factors.
