RESUMEN
Genetic resistance to Quinone outside inhibitor (QoI) and benzimidazole fungicides may be responsible for a recent decline in efficacy of chemical control management strategies for Cercospora leaf spot (CLS) caused by Cercospora beticola in Michigan sugarbeet (Beta vulgaris) fields. The target genes and fungicide resistance mutations are known for these two fungicides. Based on this, two standard polymerase chain reaction restriction fragment length polymorphism (PCR-RFLP) assays were developed to detect the G143A and E198A point mutations in the fungal mitochondrial cytochrome b and the ß-tubulin genes, respectively. These mutations confer a high level of resistance to either QoI or benzimidazole fungicides. The presence of the G143A and E198A mutations was monitored within C. beticola populations recovered from Michigan sugarbeet production fields collected in 2012. Both the QoI-resistant cytochrome b allele and the benzimidazole-resistant ß-tubulin allele were detected directly from leaf tissue following a PCR-RFLP assay. Using either detection assay, the G143A and E198A mutations were detected in over 90% of the 118 field samples originating from Michigan sugarbeet production under fungicide management programs for CLS control. Monitoring of the G143A and E198A mutations in fields located in 9 counties and 58 townships indicated that the mutations were widespread in Michigan sugarbeet production areas. The PCR-based assays used and developed in this study were effective in detecting the presence of the G143A and E198A mutations in C. beticola field populations from Michigan.
RESUMEN
Diseased samples of globe mallow, Sphaeralcea grossulariaefolia and S. munroana, were submitted by an ornamental seed producer in Wyoming to our Extension Plant Pathology Laboratory in July 1997. Dark brown, amphigenous telia surrounded by chlorotic halos were present on both foliage and stems. Mean teliospore dimensions observed were 30.8 × 44.8 µm. The teliospores germinated readily on water agar at 20°C and formed basidiospores within 24 h. Aecia and uredinia were not found. Based on characteristics mentioned above, this fungus was identified as Puccinia sherardiana Körn (1). This microcyclic rust was previously described on 12 other Sphaeralcea spp. plus other plant species in the Malvaceae family (1,2). Stem and foliar symptoms were reproduced in a greenhouse on 8-week-old plants of S. grossulariaefolia and S. munroana. These plants were inoculated with teliospores removed from the original diseased plant material. Immediately after inoculation, plants were misted and placed in plastic bags and incubated for 36 h at 100% relative humidity and 20°C. Plants continued growth with natural lighting and with day and night temperatures of 20 and 15°C, respectively. Symptoms developed within 12 days with initial telia rupturing the host epidermis 13 days after inoculation. Telia were examined microscopically to complete Koch's postulates. References: (1) J. C. Arthur. Manual of Plant Rusts in United States and Canada. Hafner Pub., 1962. (2) D. F. Farr et al. Fungi on Plants and Plant Products in the United States. American Phytopatholical Society, St. Paul, MN, 1989.