Potato late blight in tubers—The role of foliar phosphonate applications in suppressing pre-harvest tuber infections

Several phosphonate biopesticides were evaluated in three field trials over 2 years for efficacy in suppressing tuber infections caused by Phytophthora infestans. Tuber disease was assessed at harvest and (for two trials) after 2 months in storage; foliar disease was assessed during the growing season. The phosphonates were dipotassium phosphonate–dipotassium phosphate; mono- and dipotassium phosphorous acid, potassium phosphite; mono- and dibasic sodium, potassium and ammonium phosphites; and aluminum tris O-ethyl phosphonate. Each phosphonate was applied at weekly intervals to the foliage. Additionally some phosphonates were applied in combination with, or in alternation with a conventional fungicide. Although tuber blight was highly variable in these small plots, tubers from plots treated with phosphonates generally had less tuber blight relative to tubers from plots treated with chlorothalonil. The effect on tuber blight was particularly visible after 2 months in storage. All phosphonates applied alone or in combination with chlorothalonil suppressed foliar disease.