Occurrence of bacterial spot disease in Illinois tomato fields, characterization of the casual agents, and management of the disease

Bacterial spot, caused by Xanthomonas spp., is one of the most important diseases of tomato in Illinois. Field surveys were conducted in 2017, 2018, and 2019, to assess occurrence of bacterial spot disease in commercial tomato fields. Severity of foliage and fruit infections were recorded, and symptomatic samples for isolation of the pathogen were collected from three-to-five cultivars in three different farms in each of northern, central, and southern regions of Illinois. In 2017 and 2019, average disease severity was the highest in southern Illinois, while it was the highest in northern Illinois in 2018. Incidence of symptomatic fruit was generally low. During the surveys, 266 isolates were collected and identified as Xanthomonas gardneri or X. perforans using Xanthomonas-specific hrp primers. Eighty-five percent of Xanthomonas isolates identified from northern region were X. gardneri, whereas 70% of isolates from southern region were X. perforans. Isolates from the central region were 55 and 45% X. perforans and X. gardneri, respectively. Multilocus sequence analysis using six housekeeping genes (fusA, gap-1, gltA, gyrB, lepA, lacF) revealed five and four distinct genetic groups for X. gardneri and X. perforans, respectively. In addition to Xanthomonas, non-Xanthomonas bacterial genera were isolated from the diseased fields, with the majority of the isolates being classified as Microbacterium, Pantoea, and Pseudomonas. Field experiments were conducted in two different locations (Fayette and Champaign Counties), using two different tomato cultivars, ‘Red Deuce’ and ‘Mt. Fresh’. The results showed that plots treated with the combination of copper hydroxide (Kocide-3000 46.1DF) and mancozeb (Manzate PRO Stick) had the lowest disease severity. Field isolates of X. gardneri and X. perforans were grown on mannitol-glutamate-yeast agar (MGYA) amended with 0.8 mM copper sulfate. Two copper resistance genes, copA and copM, were present only in the isolates that developed colonies on the copper-amended MGYA. Additionally, in vitro laboratory studies were conducted to determine the effectiveness of selected chemical compounds and biopesticides at preventing colony formation of copper resistant and copper sensitive isolates of X. gardneri and X. perforans. The results showed that the combination of copper hydroxide (Kocide-3000 46.1DF) and mancozeb (Manzate PRO-stick) produced the largest inhibition zone for the copper-sensitive isolates of both species (X. perforans and X. gardneri). In contrast, the combination of Bacillus amyloliquefaciens strain D747 (Double Nickel LC) and copper octanoate (Cueva) produced the largest inhibition zone for copper resistant isolates of both species