Our research on cucurbit bacterial wilt is branching into many exciting areas. When we began investigating this pathosystem in 2000, we tried several semiochemical strategies to lure cucumber beetles away from production fields. Next we evaluated using spunbond polypropylene row covers as a barrier against cucumber beetles until anthesis (the start of flowering). In 2007-2009, we suppressed bacterial wilt by delaying the removal of row covers until 10 days after anthesis (Saalau Rojas et al., 2011). Meanwhile, we began to investigate the pathogen, Erwinia tracheiphila, more intensively. PhD candidate (now Dr.) Erika Saalau Rojas found that E. tracheiphila could survive for extended periods on the surface of muskmelon leaves, even during dry periods; this result gave support to the hypothesis that epiphytic populations of the bacterium are important in the epidemiology of bacterial wilt (Saalau Rojas et al., 2012). Erika also compared more than 70 strains of E. tracheiphila from 8 states by rep-PCR; she discovered that there are at least two distinct genotypes of the bacterium, based on whether the strains were recovered from Cucurbita (squash, pumpkin) or Cucumis (muskmelon, cucumber) host crops, and that these genotypes were much more pathogenic when inoculated onto the host genus from which they had been isolated than from the other host genus (Saalau Rojas et al., 2013). Erika is currently completing a Feature Article manuscript for Plant Disease that reviews progress in understanding this pathosystem.
M.S. candidate Qian Liu has obtained the first experimental evidence that bacterial wilt symptoms progress more rapidly in younger cucurbit plants than older ones - specifically, that 2- to 4-week-old seedlings wilt more rapidly after inoculation than 6- to 8-week-old seedlings. She is preparing a manuscript for submission to Plant Disease on these findings. With guidance from phytobacteriologist Gwyn Beattie, Qian is also seeking to locate genes on the E. tracheiphila genome that control plant host specificity.
Since 2009, my lab has also been leading multidisciplinary, multi-state research and outreach projects aimed at helping cucurbit growers to integrate new tactics for bacterial wilt management into their farming practices and reduce reliance on insecticides for cucumber beetle control. Field studies with collaborators at Penn State, University of Kentucky, and The Ohio State University have reinforced the value of row covers for wilt management but also showed that the optimal timing of row cover removal differs by region. Our work with Ohio State gave encouraging preliminary results for using perimeter trap croppping (PTC) to suppress bacterial wilt and insecticide use on muskmelon; the choice of perimeter trap crop in this work was 2 rows of buttercup squash (Cucurbita maxima). With ISU ag engineer Mark Hanna and University of Kentucky horticulturist Mark Williams, we are currently evaluating ways to mechanize the handling of row covers, and also examining mesh-fabric alternatives to spunbond polypropylene row covers in cucurbit production. In addition, our USDA SCRI project is seeking ways to integrate cover crops and strip tillage into the use of row covers in order to achieve soil conservation and reduce reliance on herbicides for weed management. These projects are also seeking to understand which species of wild bees are pollinating cucurbit crops in the cooperating states, whether floral provisioning can boost wild bee populations, whether cucurbit yield can be increased by floral provisioning, and whether cover crops with strip tillage conserves populations of other beneficial arthropods.