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Disease Cycle and Epidemiology

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Seedborne inoculum is the most important primary inoculum source for BFB epidemic development. Seed of most, if not all cucurbits, can be infested and transmit BFB. Unfortunately, little is known about the biology of cucurbit seed infestation by A. avenae subsp. citrulli. Additionally, it is not known whether the pathogen survives on or under the seed coat. Seed within infected fruit become infested by contact with contaminated tissues; however, in commercial seed production, visual field inspection is conducted, and only symptomless fruit are used. Hence, the significance of infested seed produced in symptomatic fruit is questionable. Recently it was found that female watermelon blossom colonization could lead to seed infestation by A. avenae subsp. citrulli although BFB symptoms did not develop on the fruit. This mechanism of seed infestation provides a possible explanation for the production of contaminated seedlots from visually inspected fields. Despite these findings the epidemiological significance of blossom colonization in seed infestation remains to be determined. While it is clear that infested seed are important for BFB outbreaks, other possible sources of inoculum exist, including volunteer seedlings, cucurbit weeds and decaying infested debris.

Direct planting of infested seed results in BFB seedling transmission under ideal environmental conditions. BFB seedling transmission is heavily dependent on high relative humidity; hence the risk of disease outbreak is reduced with direct seeding. Increasingly, commercial cucurbit production involves transplanting greenhouse-grown seedlings to the field to ensure rapid plant stand establishment. Transplant house conditions including high temperatures, high relative humidity, and high plant populations are conducive to BFB seedling transmission and rapid disease spread (Figure 23) . The exact mechanism of BFB seedling transmission is unknown, but bacteria move from seed to seedling tissues where they multiply in intercellular spaces and initiate water-soaked lesions. Bacteria in lesions are dispersed by overhead irrigation to seedlings throughout transplant house facilities. Splash-dispersed bacteria initiate secondary infections that are responsible for rapid epidemic development and high populations of infected seedlings. Even in the absence of symptoms, epiphytic A. avenae subsp. citrulli populations may initiate BFB outbreaks under ideal field conditions. Because of these factors, the risk of BFB development is high, even for seedlots with low levels of contamination.

Figure 23

Under field conditions, high relative humidity and high temperature favor BFB development. A. avenae subsp. citrulli is disseminated by wind-driven rain and irrigation water, and causes foliar lesions and blight symptoms. Infected vines are usually not killed by infection, and in most cases, symptoms on mature foliage are difficult to recognize. Foliar lesions and epiphytic populations serve as reservoirs for A. avenae subsp. citrulli inoculum and contribute to secondary infections. The bacteria penetrate leaves via stomata and wounds and remain in the apoplast (intercellular spaces) of infected tissue (Figure 24). There is no evidence of systemic bacterial migration through the plant. For fruit infection, bacteria deposited on the rind of fruit two to three weeks after anthesis (flowering) swim through open stomata to initiate infections. After this period, stomata become blocked by the deposition of waxes on the fruit surface that prevent further invasion. Hence, mature fruit are generally not susceptible to natural infection by A. avenae subsp. citrulli unless wounding occurs.

Figure 24

During the early stages of fruit development, BFB symptoms are inconspicuous or absent, but appear suddenly at, or just prior to, harvest maturity. Surface lesions expand rapidly, and fruit may eventually rot in the field. A. avenae subsp. citrulli can overwinter in decaying infested rind tissues or in infested seeds that can produce volunteer plants in subsequent crops.

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