Control of Early Blight in the San Luis Valley, Colorado

Early blight of potatoes caused by Alternaria solani is a disease that growers in Colorado often do not consider a major problem. However, past and this current research suggest that judicious use of a well-managed fungicide program can significantly reduce disease and increase yields. Additionally, wide spread use of fungicides in various rotations (azoxystrobin, pyraclostrobin, chlorothalonil, dithiocarbamate, mancozeb and boscalid) was shown to substantially decrease the disease levels of plants in treated versus untreated plots. Field trials evaluating fungicide rotations and application scheduling were conducted using the cultivar Russet Norkotah Selection 8. Treatments were first applied to either coincide with the dates as indicated by the early blight degree day model currently used in the San Luis Valley (similar to 60 days after planting - DAP), or starting later in the season (similar to 80 DAP). While significant differences (P < 0.05) in disease reduction were noted each year using fungicides, there was no corresponding yield advantage of fungicide treated versus the control (un-treated) plots. However, when data were analyzed over three years, a significant higher (P < 0.05) foliar disease control and subsequent yield increase among treatments was observed when a strobilurin product was used first in the fungicide rotation at 78 DAP. Results indicate that for a cost of $100-125/ha, growers can use fungicides with an appropriate rotation of active ingredients to control early blight and potentially increase yields.