Bacterial spot of pumpkin, incited by X. cucurbitae (ex Bryan) Vauterin et al. (1995) [syn. Xanthomonas campestris (Pammel) Dowson pv. cucurbitae (Bryan) Dye], has become one of the most important diseases of pumpkin [Cucurbita pepo L. and Cucurbita moschata (Duchesne) Duchesne ex Poir] in Illinois and some other pumpkin-growing areas in the United States (Babadoost et al., 2012; Babadoost and Ravanlou, 2012; Babadoost and Zitter, 2009). This disease was first reported as bacterial leaf spot on ‘Hubbard’ squash in New York in 1926 (Bryan, 1930). Subsequently, the disease was identified in other cucurbit-growing areas in Asia, Australia, Europe, and North America on cucumber, pumpkin, squash, and watermelon (Bineeta et al., 1999; Dutta et al., 2013; Lamichhane et al., 2010; Pruvost et al., 2008; Pruvost et al., 2009; Williams and Zitter, 1996).
Xanthomonas cucurbitae infects leaves and fruit of cucurbit crops (Babadoost and Ravanlou, 2012; Saddler and Bradbury, 2005; Williams and Zitter, 1996). Symptoms reported on leaves of gourd, pumpkin, and squash are small (2–4 mm), chlorotic to beige spots (Babadoost and Zitter, 2009; Williams and Zitter, 1996). As lesions coalesce, they may appear more angular and similar to the spots incited by Pseudomonas syringae pv. lachrymans (Williams and Zitter, 1996). On fruit, small, slightly sunken, circular spots, 1–3 mm in diameter, with a beige center and dark brown halo develop. Secondary fungi and bacteria may colonize the spots, resulting in enlarging spots cracking epidermis (Babadoost and Zitter, 2009; Williams and Zitter, 1996).
In the past 7 years, bacterial spot was widely observed in pumpkin and winter squash fields in Illinois, and X. cucurbitae was consistently isolated from lesions (Babadoost and Ravanlou, 2012). In wet conditions, fruit with bacterial spot usually collapses and rots. Therefore, the affected fruit is not accepted for marketing. Yield losses because of bacterial spot in some pumpkin fields in Illinois exceeded 90% (Babadoost and Ravanlou, 2012). At present, management of the bacterial spot is limited to spray applications of copper compounds, which has a limited effect on suppressing the disease. Information on the biology and epidemiology of bacterial spot is not sufficient for developing effective strategies for management of the disease.
Illinois ranks first in pumpkin production in the United States, where more than 90% of processing pumpkins are produced (Babadoost and Islam, 2003; Babadoost and Ravanlou, 2012). This study was conducted to further understand the etiology of bacterial spot in cucurbit corps. The objectives of the study were: 1) monitor development of bacterial spot on pumpkin leaves and fruit in commercial fields during the growing season; 2) define symptoms of the disease on leaves and fruit of various pumpkin cultivars; and 3) assess incidence of pumpkin fruit with bacterial spot at harvest in commercial fields.
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