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Leaf lesions and fruit warts on pumpkin are caused by Pseudomonas syringae sensu stricto

Lydia Tymon: Washington State University-Mount Vernon NWREC

<div>In 2015 and 2016, Washington field grown ‘Cinnamon Girl’ pumpkin leaves and fruit exhibited lesions surrounded by a yellow halo, and wart-like eruptions, respectively. Symptoms were reminiscent of bacterial infections, but atypical of those caused by <em>Pseudomonas syringae</em> pv. <em>lachrymans</em>, the angular leaf spot pathogen in genomospecies 2, phylogroup 1. Pathogenicity of PsAs_2015 isolated from foliage and PsFt1_2015 and PsFt2_2015 from fruit was confirmed on pumpkin seedlings, and the LOPAT profile corresponded to <em>P. syringae.</em> A multilocus sequence analysis with <em>P. syringae</em> pathotypes, demonstrated that PsAs_2015, PsFt1_2015 and PsFt2-2015 are genetically identical and members of genomospecies 1 phylogroup 2B, thus not related to <em>P. syringae</em> pv. <em>lachrymans</em>. Of the genomospecies 1 pathotypes and PsAs_2015, only PsAs_2015, and <em>P. syringae</em> pv. <em>aptata</em> were pathogenic on pumpkin leaves, although <em>P. syringae</em> pv. <em>aptata</em> lesions were distinct from those of PsAs_2015. PsAs_2015 did not cause symptoms on beet seedlings whereas <em>P. syringae</em> pv. <em>aptata</em> did. ‘Baby Boo’ pumpkins were either seed, flower, or fruit inoculated with PsAs_2015, or non-inoculated to assess how inoculation timing affects symptoms on fruit. Wart incidence on fruit was significantly correlated with fruit inoculation (<em>P≤</em>0.001) as was recovery of PsAs_2015 from symptomatic rinds (<em>P≤</em>0.001). Warts on fruit represents a newly observed symptom, likely not the result of systemic infection.</div>