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Title:Non-chemical methods for managing bacterial spot of cucurbits caused by Xanthomonas cucurbitae
Author(s):Sulley, Salisu
Director of Research:Babadoost, Mohammad; Hind, Sarah
Doctoral Committee Chair(s):Babadoost, Mohammad; Hind, Sarah
Doctoral Committee Member(s):Mideros, Santiago; Bollero, German; Kushad, Mosbah
Department / Program:Crop Sciences
Discipline:Crop Sciences
Degree Granting Institution:University of Illinois at Urbana-Champaign
Degree:Ph.D.
Genre:Dissertation
Subject(s):Bacterial spot
Xanthomonas cucurbitae
Host resistance
Biocontrol
Crop rotation
Abstract:Bacterial spot, caused by Xanthomonas cucurbitae, is an emerging and important disease of cucurbits worldwide. Studies were conducted to evaluate the effectiveness of biocontrol agents for managing bacterial spot, screen cucurbit cultivars and accessions for resistance to X. cucurbitae, and evaluate the pathogen's survival on plant debris, weeds, and different crop rotations. For evaluation of the biocontrol agents, 1,134 bacterial isolates were screened in vitro for their antagonistic activity against X. cucurbitae. Thirty-three bacterial species were identified that inhibited the multiplication of X. cucurbitae. Based on results from the in vitro assays, the strong inhibitors of X. cucurbitae were selected for greenhouse and field evaluations. Isolates of Bacillus amyloliquefaciens, Bacillus velezensis, Pantoea agglomerans, and Pseudomonas putida were able to prevent or reduce the development of bacterial spot disease on pumpkin plants in both the greenhouse and field. In addition, B. amyloliquefaciens and P. putida increased the growth of ‘Howden’ pumpkin (Cucurbita pepo). The results suggested that these biocontrol agents can be used for managing bacterial spot disease and enhance the growth of pumpkin plants. For determining the resistance of cucurbits to X. cucurbitae, 81 cultivars of gourds, pumpkins, and squashes, as well as 300 accessions of cucurbits, were screened. First, a reliable method for inoculating cucurbit plants was developed that included spraying plants with X. cucurbitae suspensions. Then, the cultivars and accessions were screened under both greenhouse and field conditions. In the greenhouse study, all commercial cultivars and the majority of the accessions developed typical symptoms of bacterial spot disease. However, symptom severity differed among the accessions. In the field studies, all 81 cultivars were susceptible to X. cucurbitae. Among 300 accessions tested, 9 and 21 accessions were designated as resistant and less resistant to X. cucurbitae, 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 is the first report of potential resistance of cucurbits to bacteria spot of cucurbits. For determining the effectiveness of crop rotation for managing the bacterial spot, field survival of X. cucurbitae in different crop rotations, plant debris, and weeds was evaluated. In the three-year rotation with nonhost crops, the severity of bacterial spot in pumpkin plots was significantly lower than the two-year rotation with nonhost crops and continuous pumpkin production. X. cucurbitae was recovered from pumpkin plant debris 15 months after harvesting pumpkins in commercial fields. The pathogen was also isolated from asymptomatic weeds found in pumpkin fields. In a greenhouse inoculation experiment, bacterial spot symptoms developed only on leaves of bur cucumber (Sicyos angulatus) and velvetleaf (Abutilon theophrasti) weeds. In soil infested with X. cucurbitae, the bacterium was not recovered after incubation for 15, 42, and 56 days at 22, 12, and 4℃, respectively. The results showed that crop rotations alone do not provide adequate protection of susceptible pumpkin plants against X. cucurbitae. The results also confirmed that X. cucurbitae is not a soilborne pathogen.
Issue Date:2021-06-22
Type:Thesis
URI:http://hdl.handle.net/2142/113253
Rights Information:Copyright 2021 Salisu Sulley
Date Available in IDEALS:2022-01-12
Date Deposited:2021-08


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