This study aimed to determine the inhibitive or stimulatory effects of leaf extracts from two Brassica rapa subspecies on the hyphal growth of two well-known entomopathogenic fungi, Cordyceps fumosorosea and Beauveria bassiana. Extract concentrations of 50, 25, and 10% w/v based on leaf fresh weight were prepared from turnip (B. rapa subspecies rapa) and bok choy (B. rapa subspecies chinensis) leaves. Each concentration was individually incorporated into potato dextrose agar plates for in vitro bioassays. The center of each plate was inoculated with 20 µL of a fungal suspension that was allowed 24 h to soak into the agar before sealing the plates and incubating them at 25 °C under a 14-h photophase. The fungal colony perimeter was marked 5 days after inoculation on two perpendicular lines drawn on the bottom of each plate. Radial colony growth was measured from 4 marks per plate 5, 10, and 15 days later. Radial growth rates for both fungi were 1.3–2.0 and 0.9–1.4 times faster with bok choy and turnip extracts, respectively, at the 25% and 50% concentrations compared to the no-extract control treatment. Therefore, bok choy and turnip leaf extracts can stimulate entomopathogenic fungus growth within 15 days. Biochemical compounds in the extracts include sesquiterpenes, α-copaene, β-selinene, γ-gurjunene, calamenene, cubenene, and α-calacorene.
Phytophthora root and stem rot (PRR) is a limiting factor for soybean production. Seed treatments are used for early-season management, but efficacy can depend on seed selection and the local environment. Ethaboxam is a new fungicide commercially available as a seed treatment to control oomycetes. Field experiments were established in Illinois and Iowa in 2017 and 2018 to evaluate the effect of ethaboxam + metalaxyl on PRR. Experiments included soybean lines with no resistance gene, Rps1c or Rps1k, and different levels of partial resistance. Seed treatments increased soybean stands in all locations and years. Significant yield effects were observed only in two locations that were inoculated with Phytophthora spp. Groups of soybean lines with the same Rps gene responded differently in each location, showing how Rps gene usefulness depends on the field. A comparison of the effect of seed treatment on lines with different levels of partial resistance showed that partial resistance alone cannot always protect against stand losses. Soybean lines with high levels of partial resistance had consistently higher yields than those with low levels of partial resistance across Illinois locations. These results show that ethaboxam seed treatment can protect early-season stands and that selection of cultivars with high levels of partial resistance is important for PRR management.
Phytophthora root and stem rot (PRR), caused by Phytophthora sojae, is one of the most devastating oomycete diseases of soybean in Illinois. Single resistant genes (Rps) are used to manage this pathogen, but P. sojae has adapted to Rps causing failure of resistance in many regions. In addition to P. sojae, recent reports indicate that Phytophthora sansomeana could also cause root rot in soybean. Soil samples and symptomatic plants were collected across 40 Illinois counties between 2016 and 2018. P. sojae (77%) was more abundant than P. sansomeana (23%) across Illinois fields. Both species were characterized by virulence, aggressiveness, and fungicide sensitivity. Virulence of all isolates was evaluated using the hypocotyl inoculation technique in 13 soybean differentials. Aggressiveness was evaluated in the greenhouse by inoculating a susceptible cultivar and measuring root and shoot dry weight. On average, P. sojae isolates were able to cause disease on six soybean differentials. P. sojae was more aggressive than P. sansomeana. All isolates were sensitive to azoxystrobin, ethaboxam, mefenoxam, and metalaxyl. The characterization of the population of species associated with PRR will inform management decisions for this disease in Illinois.
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