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Disease Management
Since there are no resistant commercial cultivars, successful BFB management depends on exclusion of primary inoculum, cultural practices, and the use of copper-based bactericides. Currently, no single tactic is effective for managing BFB as the efficacy of chemical control depends on environmental conditions. Additionally, effective disease management must be practiced in seed and transplant production.
BFB management in seed production fields.
The most important management strategy for BFB is exclusion of A. avenae subsp. citrulli by using pathogen-free seeds and seedlings. It is impossible to guarantee pathogen-free seed, but seed health testing reduces the risk of outbreaks. Currently, the standard seed test for BFB is the seedling grow out assay (Figure 25) in which 10,000-30,000 seed/lot are planted under conditions conducive for BFB development. Seedlings are visually inspected after 18 days for BFB symptoms.
 Figure 25 |
To reduce the risk of seed infestation, cucurbit seed are produced in dry, cool climates in regions of countries with no history of BFB (Figure 26), and 3- to 5-year crop rotations with non-cucurbit hosts are routinely practiced. Seed fields are visually inspected, and only seed from BFB-free fields are used.
 Figure 26 |
Currently, no chemical or physical seed treatments are 100% effective at eradicating A. avenae subsp. citrulli. While seed treatments including thermotherapy, NaOCl, fermentation, HCl and peroxyacetic acid significantly reduce BFB seedling transmission, they can adversely affect seed physiology. Two factors that influence seed treatment efficacy are: 1) the inability of seed treatments to penetrate the seed coat, and 2) location of bacteria on/in the seed. Because the risk of BFB development in transplant houses is high for seed with low levels of infestation, seed treatment alone cannot control BFB.
BFB management in transplant production facilities
To avoid introducing A. avenae subsp. citrulli into transplant houses, only tested, pathogen-free seed are planted. To minimize secondary pathogen spread, physical contact with seedlings is kept to a minimum, and sterilized transplant flats are used for each crop to limit the risk of carry-over contamination. Adequate fertilization and pest control programs are implemented to promote healthy seedling growth.
To prevent pathogen spread, traffic and the movement of equipment between houses is minimized. Ideally, seed from different lots should be planted in separate greenhouses; however, this is usually not feasible and instead, 60 cm (24 in.)-high plastic barriers should be established between plantings (Figure 27). If possible, ebb and flow irrigation should be used instead of overhead irrigation to reduce splash dispersal of A. avenae subsp. citrulli. However, this is cost prohibitive and rarely implemented. With overhead irrigation, watering should be done at mid-day to facilitate rapid drying of plant surfaces, and the water delivery pressure should be low to limit aerosol generation.
 Figure 27 |
To facilitate early detection of BFB outbreaks, visual inspection of seedlings is routinely conducted. Since it is possible for A. avenae subsp. citrulli to survive epiphytically on asymptomatic seedlings, all seedlings produced in transplant houses with BFB should be discarded. This is a costly option since planting will be delayed and targeted market windows will be missed. Hence, in cases where it is economically unfeasible to discard exposed seedlings, symptomatic plants and those in the immediate vicinity should be discarded. The remaining seedlings should be treated with copper-based bactericides. Additionally, relative humidity in the transplant house should be reduced by increasing air-flow, and aggressive copper bactericide-based BFB management programs should be implemented in the field. After each seedling production cycle, transplant houses should be completely disinfested and left empty for at least two to three weeks before planting cucurbits.
BFB management in fruit production fields.
There are no commercial cucurbit cultivars with immunity to BFB. While several watermelon plant introductions are resistant, all commercial cultivars appear to be susceptible to the disease. To exclude A. avenae subsp. citrulli from commercial fruit production fields, tested seed and BFB-free seedlings are used to plant fields. Three-year rotations with non-cucurbit hosts are routinely employed. Debris, including watermelon culls from fields with BFB, is deep plowed at the end of each growing season. Subsequent cucurbit crops are established in new fields physically separate from the outbreak site. If possible, alternative hosts such as wild and volunteer cucurbits should be eliminated from regions around production fields and transplant houses.
For preventive management, biweekly applications of copper-based bactericides e.g., cupric hydroxide, copper hydroxosulfate, or copper oxychloride are employed at the recommended rate or weekly at half the recommended rate. Preventive sprays are initiated at or before anthesis (flowering) and continue until fruit are mature. If BFB symptoms develop, weekly applications of the full recommended rate of copper-based bactericides are employed. To prevent spread, work is not conducted in fields with BFB when the foliage is wet, and field equipment is decontaminated before moving between fields.