Susceptibility of Fourteen New Cucurbit Species to Gummy Stem Blight Caused by Stagonosporopsis citrulli Under Field Conditions

At least 24 species of cucurbits from 13 genera are known to be susceptible to gummy stem blight, caused by three species of Stagonosporopsis. Cankers that are formed on crowns and stems play an important role in the disease cycle and the survival of the pathogen. Fourteen cucurbit species of unknown susceptibility representing 12 genera, four taxonomic tribes, and four geographic origins were inoculated with Stagonosporopsis citrulli in Charleston, SC, in spring 2015, spring 2016, and fall 2016 to evaluate their level of susceptibility to gummy stem blight and the ability of the pathogen to reproduce on crown cankers. An additional species, Cucumis melo, was included as a reference due to its known high susceptibility. Data sets of area under the disease progress curve (AUDPC) for foliar severity and crown cankers, final percentage of diseased leaf area, final percentage of plants with cankers, final percentage of plants with fruiting bodies, and rates of increase in canker incidence were analyzed to evaluate susceptibility. Results were similar for datasets of AUDPC and final ratings but there were more differences for AUDPC. In all experiments, Apodanthera sagittifolia, Ecballium elaterium, and Kedrostis leloja were at least as susceptible to foliar blight as the reference C. melo. K. leloja was as susceptible to crown cankers as C. melo in all experiments and A. sagittifolia and E. elaterium were among the species most susceptible to crown cankers in two experiments. Coccinia grandis was highly resistant to gummy stem blight and had a few cankers only in fall 2016. Sicana odorifera and Zehneria pallidinervia also consistently grouped with the most resistant species. Incidence of crown cankers on Cucumis melo and K. leloja increased at the fastest rate of all species in all experiments and had, along with E. elaterium, the highest incidence of crowns with fruiting bodies. In general, the most susceptible species also were most suitable for reproduction of the pathogen and had the fastest disease progression. The tribes Benincaseae and Cucurbiteae had consistently lower levels of foliar blight than Bryonieae and Coniandreae. The tribe Benincaseae had a consistently lower AUDPC for canker incidence than Bryonieae and Coniandreae. The species originating from Europe (E. elaterium) was consistently most susceptible to both symptoms, while African species grouped with the least susceptible species in all experiments. To the best of our knowledge, this is the first report of susceptibility to gummy stem blight of 14 species and the first report of susceptibility of the cucurbit tribes Coniandreae and Gomphogyneae. This expands the host range of Stagonosporopsis citrulli to 37 species representing 21 genera and seven tribes in the family Cucurbitaceae. This study demonstrates the importance of crown cankers as reproductive sites for S. citrulli.