Postinfection activity of selected late blight fungicides

Two sets of experiments were done in the greenhouse with 'Russet Burbank' potatoes infected with Phytophthora infestans. The first determined the effectiveness of selected late blight fungicides in restricting lesion length and spore production when applied after stem lesions had occurred on potted plants. The second quantified the curative activity of selected fungicides applied 12 to 48 h after inoculation of excised leaves with P. infestans. Such information may be beneficial in selecting an effective fungicide for late blight management when symptoms of the disease are present in a field. Expansion of established stem lesions was consistently restricted by one application of either dimethomorph plus mancozeb or metiram plus triphenyltin hydroxide. Sporulation on established stem lesions was consistently inhibited by either one application of dimethomorph plus mancozeb or cymoxanil plus mancozeb. Two applications of propamocarb hydrochloride plus chlorothalonil were required to restrict lesion expansion and inhibit sporulation. Size and number of lesions and sporulation on leaflets were reduced when propamocarb hydrochloride plus chlorothalonil was applied up to 48 h after inoculation.