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F E B R U A R Y 2 0 1 620 POTATO GROWER Seed Issues At The 2016 Potato Expo by Willem Schrage North Dakota State Seed Department The 2016 Potato Expo had break-out ses- sions for the processing chips seed technology and fresh market. Some break-out sessions were quite practical and applicable. The first speaker was Dr. Dennis Johnson Washington State University. He talked about Managing Tuber Blemish Diseases. He mentioned Silver Scurf Black Dot Black Scurf Common Scab Powdery Scab and Elephant Hide as important blemishes. The symptoms of elephant hide were described as thick coarse russet skin of which the cause is unknown. Dr. Johnson had selected sil- ver scurf Helminthosporium solani and black dot Colletotrichum coccodes for a more detailed description silver scurf being primarily tuber born and black dot tuber and soil born. Silver scurf is known to increase in stor- age and over seed generations. Managing silver scurf should be done by selecting clean seed. Dr. Johnson sug- gested having seed and harvested tubers tested. An effective chemical treatment such as fludioxonil Maxim Maxim MZ Cruiser Maxx or flutalonil Moncoat MZ can be effective. One should avoid close crop rotations because when planting potatoes within two years after each other the soil becomes a source of inoculum. One should also harvest within reasonable time after vine kill or senescence. It is also necessary to clean and sanitize storage facilities. Wood in storage should be eliminated. Post-har- vest treatments such as Stadium azoxy- strobin difenoconazole fludioxonil or phosphorous acid were suggested. One should keep the temperature as cool as possible and the humidity as low as possible. However a balance should be found because potatoes need high humidity and higher temperatures. Black dot was called an elusive pathogen. It infects plants during the entire season starting early. Infections may be latent. The symptoms are not evident until the plant is under stress or starts to senesce. There is an inconsis- tent influence on yield. The inoculum can be in soil tare dirt seed or air borne. The incidence of black dot is reduced the longer a field stays out of potato production. Managing of black dot starts before planting. Dr. Johnson suggested to start with as clean seed as possible with little tare dirt. One should remove grime from surfaces in contact with cut seed and avoid stress for cut seed. Select a well-drained soil and extend crop rota- tions preferably more than four years for a potato crop. One should reduce soil compaction improve soil structure and increase organic matter to reduce plant stress. During the growing season blowing sand increases stress to young plants. Plants need nutrients and water during the entire season. The fungicides Strobulirin and Vertisan were recom- mended for one to two months after planting. One should not leave the crop too long in the field. It is a good man- agement strategy against black dot to keep the storage temperature low. Dr. Amy Charkowski University of Wisconsin dealt with the biology and management of the soft rot bacterial pathogens Dickeya and Pectobacterium formerly known as Erwinia. Dr. Charkowski mentioned that Dickeya had caused losses in potatoes mainly in east- ern North America. She mentioned that Erwinia carotovora was now Pectobacterium and Erwinia chrysanthemi was now Dickeya. They are bacteria caus- ing seed piece decay black leg stem rot and tuber soft rot. Dickeya solani is the more aggressive soft rot species more aggressive than Dickeya dianthicola but D. solani has not been reported in North America. Dickeya has probably been in seed potatoes in North America over the last few years. Rain in 2013 and 2014 were favorable for the spread of the bac- teria but low temperature caused the pathogen to remain latent. When tem- peratures were higher in 2015 there were significant losses. There are measures available to help reduce the losses such as PCR tests that can identify seed lots with high inci- dence of Dickeya. Dr. Charkowski sug- gested a survey to avoid another out- break. If one wants to test 400 tubers per seed lot were considered likely to identify lots with a 1 incidence. She suggested sending multiple samples when seeing symptoms. She recom- mended to send a random sample of healthy tubers and send the least symp- tomatic tubers or stems. The challenge is that there are no cura- tive chemicals available nor is resistance available. However there seems to be a difference in susceptibility between vari- eties.