Optimal Fungicide Management of Purple Spot of Asparagus and Impact on Yield.

Purple spot disease of asparagus, caused by the fungus Stemphylium vesicarium, results in lesions on spears and ferns and defoliation of ferns. In two newly established commercial asparagus fields (cvs. Jersey Giant and Jersey Knight), chlorothalonil or mancozeb was applied every 7, 10, or 14 days or according to Tom-Cast with a threshold of 15 disease severity values, and not applied to the control. Tom-Cast prompted four sprays, resulting in a 60% reduction in the number of fungicide applications when compared with the 7-day-interval treatment. When disease pressure was severe, lesions on ferns were significantly less for both cultivars when fungicides were applied according to Tom-Cast or every 7 days compared with spray intervals of 10 or 14 days. Applying fungicides according to Tom-Cast or every 7 days resulted in an increased Jersey Giant fern stand compared with applying fungicides every 10 or 14 days. Unsprayed control plots yielded 77 to 83% (depending on cultivar) of those plots treated according to Tom-Cast using chlorothalonil. Significantly higher yields of Jersey Knight were obtained for chlorothalonil versus mancozeb. When mancozeb was used, Jersey Knight yield was significantly increased with a 7-day versus Tom-Cast application regime. Using chlorothalonil in a Tom-Cast program provided a benefit per hectare (BPH) of $1,005.24 (Jersey Knight) to $2,057.69 (Jersey Giant). In comparison, using mancozeb in a Tom-Cast program provided a BPH of -$484.27 (Jersey Knight) to $1,030.55 (Jersey Giant) over a 2-year period.