M A R C H 2 0 1 8 POTATO GROWER 13 facing eggs, larvae and adults present for a longer period. The results are that insecticides are not as effective. There are no insecticides effective against eggs. The residual is only effective (or not) on first star larvae. Therefore, timing is everything. It is necessary to apply resistance management right out of the gate. Managing insecticide resistance means reducing selection pressure by rotating modes of action (MOA). MOA is on the label after “Group”. Pre-mixed products with multiple active ingredients might not be the way to go with this insect. The rates in mixed products tend to be lower than in individual products favor- ing insecticide resistance. Resistance management tactics dictate using high rates for best management practices (BMP). Many of these products are effi- cacious, but efficacy not necessarily the issue with this specific insect. Dr. Mc Rae mentioned the dip test that is available to measure tolerance for an insecticide. He suggested to wait seven days before evaluating the results because the beetles have a tendency to wake up after a few days. Dr. Neil Gudmestad, NDSU, explained Powdery Scab and Potato Mop Top, stat- ing that powdery scab is caused by a primitive “slime mold” Spongospora sub- terranea f.sp. subterranean. The disease was first reported in Germany in 1842. Now there is global distribution, which means that it can be found largely wher- ever potatoes are grown, but especially in temperate climates. Another prob- lem is that it transmits Potato mop top virus. In the lesions we can identify the cystosori, commonly known ‘spore balls’ which live a very long time in the soil. The disease has become much more important in last thirty years due to globalization of potato markets and the banning of mercury seed treatments. There are differences in susceptibility between potato varieties. Dr. Gudmestad quoted Bittara et al. ( AJPR) showing the resistance of different vari- eties such as the one that are resistant against root galls and tuber lesions such as Altura, Atlantic, Bannock Russet, Dark Red Norland, Dakota Trailblazer, Yukon Gold, Dakota Russet. There are other varieties with different levels of resistance and susceptibility. To manage powdery scab, it is recommended to use non-infected seed potatoes. Tubers and/or soil can be tested with PCR. One recommendation was not to use chloropicrin as a soil fumigant, because it has shown to increase powdery scab severity while decreasing Spongospora subteranea levels in soil. Potato Mop Top Virus (PMTV) may show seed-borne PMTV foliar symp- toms. There are varieties that are insen- sitive to both PMTV and another tuber necrosis causing virus, TRV. They are: (White:) Superior, (Russet:) Centennial Russet, Goldrush, Alturas . (Red:) Ciklamen. (Yellow:) Bintje, Gala, Oneida Gold. (Specialty:) Austrian Crescent and Russian Banana. Common Scab Biology & Management in Potatoes was the final topic on the agenda. Dr. Amanda Gevens of the University of Wisconsin indicated that Streptomyces bacteria of multiple species are the pathogens causing potato com- mon scab. They can be soil borne and seedborne. They are Gram-positive and filamentous. S. scabies was the predom- inant pathogen associated with varied common scab symptoms. S. stelliscabiei was the second most common pathogen associated with symptoms. They pro- duce a phytotoxin, thaxtomin, that induces scab and pit symptoms on pota- to tubers and other root crops (carrot, radish) They looked at two approaches: The pathology approach meant learning more about composition of individual members of soil microbial populations and the ecology approach, that is a sys- tem-wide population analyses to better understand complex soil microbiome relationships that influence crop pro- ductivity and disease. Greenhouse results were not necessarily transferable to the field. They want to look more at the effects of fumigation on soil life. The recommendations for managing potato common scab are to plant seed potatoes with little to no symptoms of common scab (5 and 50% severity on seed resulted in similar incidence and severity of common scab on daughter tubers). There was no significant differ- ence between cut and whole seed on resulting common scab severity in daughter tubers, but there was a trend of reduced common scab with cut seed which may have been caused by plant vigor and reduction in inoculum. While potato resistance to common scab, it is not well understood. There are significant cultivar differences and use of resistant varieties is the best man- agement. Also there are disease suppres- sive rotations with green manures, such as buckwheat, canola, oat, rye or millet that can inhibit Streptomyces . It is also recommended to manage the soil mois- ture, pH, and manure incorporation, if used. Unfortunately, most chemical and biological treatments have proven to be inconsistent in effectiveness in limiting common scab over time and space.