Title : Challenges on identification and management of bacterial pathogens of plants: A case study of an emerging bacterial disease of cucurbits
Bacterial spot of cucurbits, caused by Xanthomonas cucurbitae, is an emerging disease in cucurbit growing areas throughout the world. The pathogen can infect all cucurbit crops, but its major hosts are pumpkins and winter squash. Leaves and fruits of cucurbits are infected by X. cucurbitae at all growth stages. Infected fruits by X. cucurbitae are usually colonized by opportunistic fungi and bacteria and rot. Our surveys in the North Central Region of the United States showed that 159 of 180 and 71 of 79 of pumpkin and squash fields, respectively, had fruits infected with X. cucurbitae. The mean incidence of fruits with bacterial spot in pumpkin and squash fields surveyed was 25 and 19%, respectively. We identify the pathogen based on the colony morphology on yeast dextrose agar (YDC), polymerase chain reaction (PCR) test using RST2/RST3 primers, and pathogenicity test on susceptible pumpkin ‘Howden’. X. cucurbitae survived in infected leaves and fruits in the field for more than 24 months. X. cucurbitae also survived longer than 18 months in the seeds at 4 and 22°C and remained viable. The pathogen was isolated from asymptomatic weeds in pumpkin fields. We eradicated the pathogen in the naturally-infected and artificially-infested seeds by hot-water treatment at 55°C for 15 min, and also HCl treatment at 0.5% concentration for 40 min. In our field trials, copper compounds, acibenzolar-s-methyl (ActiGard 50 WG), fungicides famoxadone + cymoxanil (Tanos 50D WG) and quinoxyfen (Quintec 250SC), and an extract from Reynoutria sachalinensis (Regalia) reduced the incidence and severity of bacterial spot on both leaves and fruits compared to the controls. However, no chemical or biocontrol agent provided satisfactory management of the disease. Crop rotation did not prevent occurrence of the disease. We screened 81 commercial cultivars of gourds, pumpkins, and squashes, as well as 300 Cucurbita spp. accessions, for their resistance to X. cucurbitae under greenhouse and field conditions. None of the commercial cultivars was resistant to the pathogen. Among the accessions tested, 9 and 21 accessions were classified as resistant and less resistant, respectively. Resistant and less resistant accessions belong to the species Cucurbita maxima, C. maxima subsp. maxima, C. maxima subsp. andreana, and C. okeechobeensis subsp. martinezii. This was the first report of potential resistance to bacterial spot of cucurbits.