A Holistic Approach to Managing Important Potato Diseases in Idaho.
Since 1999, growers in eastern Idaho have experienced significant losses due to storage rots, such as pink rot and Pythium. In 2006, the number of reports of problems in the Magic and Treasure Valley's increased and in 2007 and 2008, many growers experienced an increase in the amount of pink rot and Pythium coming out of the field. In 2008 and 2009, many growers also reported increased problems with lack of control of early blight using traditionally effective fungicides. The majority of these increases in disease problems may be due to the emergence of new fungicide resistant fungal pathogens. In recent years it has been reported that most of the pink rot problems in eastern Idaho have been caused by pathogen isolates that are resistant to mefenoxam (Ridomil Gold) and increased applications of Ridomil Gold have proven ineffective in controlling
the disease. Mefenoxam resistant strains of Pythium leak have also been found in Idaho in the past several growing seasons. Like pink rot, Ridomil Gold has been the main fungicide used for the control of Pythium leak in Idaho. In 2006 and 2008, Pythium was responsible for significant storage losses. With the development of mefenoxam resistance in Idaho, Pythium leak could become a much more severe problem in the field and storage. In 2008 and 2009, many Idaho growers reported the failure of the traditionally effective strobilurin fungicides (e.g. Headline and Quadris) to control early blight. This may also be due to the development of fungicide resistant isolates of Alternaria solani (the early blight pathogen). Fungicide resistant isolates of A. solani have been reported in Idaho and are common in other potato growing regions (e.g. Wisconsin). However, these problems may also be due to
misdiagnosis of early blight. Brown leaf spot, caused by A. alternata is also a common disease of potato in Idaho and is often confused with early blight as its symptoms are very similar. The pathogens causing these two diseases are also closely related. However, unlike the early blight fungus (A. solani) the brown leaf spot pathogen (A. alternata) is inherently more resistant to strobilurins and has never been well controlled by this class of fungicides. Due to the similarity of these two pathogens and their disease symptoms it may be possible that growers are confusing brown leaf spot for early blight and thus reporting poor disease control of early blight. The objective of this study are to take a holistic approach to management of pre- and postharvest diseases in Idaho. Firstly, fungicide resistance surveys will be carried out to assess the extent of fungicide resistance in the
pathogen populations of Pythium and early blight. Secondly, field and storage trials will be carried out to determine the best fungicides and management practices for the control of these pathogens. Thirdly, the information generated by this research will be used to produce new extension bulletins to help growers make proper and timely diagnoses of these diseases. With the emergence of new fungicide resistant fungal pathogens, proper and timely diagnosis of diseases is paramount to effective crop health management.