Published online: Jul 19, 2011 Fungicide
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ACEQUIA, Idaho-Miller Research has issued an early blight warning for the Magic Valley in Idaho. Early blight lesions have been found on the lower leaves of both Russet Burbank and Western Russet. Fungicide applications for early blight/brown leaf spot control should be made at this time if they have not already been initiated.

Several products are effective for control of early blight such as Endura, QoIs (Gem, Tanos, Headline, Quadris, Reason), and products with difenoconazole (Revus Top, Quadris Top and Omega Top).  These products should be tank-mixed with a standard protectant such as a chlorothalonil or EBDC product.  Reduced sensitivity has been found with Endura, Quadris and Headline in the early blight pathogen population. In a trial sponsored by the Idaho Potato Commission and industry, in 2010 Gem and Tanos showed greater disease control than Headline and Quadris. These products are still useful and provide value, but should be used with a protectant.

The standard protectant (chlorothalonil or EBDC) will provide good protection against late blight. In this semi-arid environment, the standard protectants have resulted in very good late blight control when applied before an outbreak has occurred. Miller Research research has shown that repeated use of the same protectant will build fungicide residue in the potato canopy.

Standard protectants have a broad spectrum of activity and the risk of resistance developing due to repeated use is very low. Endura is not effective against late blight, and most QoI products need to be applied at higher rates (relative to early blight) to provide late blight protection. Revus Top and Tanos both have active ingredients effective against late blight. Gavel is also very effective against late blight and has an EBDC component to provide protection against early blight.
Applications should be made on a 10-14 day schedule during this time of rapid vine growth. Recent thundershowers have also made the environment favorable for both early and late blight development. Miller Research has published a discussion of products, application methods and application intervals at: