Inoculum sources of Fusarium oxysporum f.sp. asparagi in asparagus production

Fusarium oxysporum f.sp. asparagi (Foa) incites crown and root rot of asparagus which causes early decline of asparagus plantings. The aim of the present study was to identify the main inoculum sources of the pathogen in the Netherlands. As has been reported for foreign seed lots, Dutch seed lots can be infested with Foa at low levels. We found that seed infestation occurs mainly during the seed harvesting process through infested soil adhering to fallen berries. Soil samples from 59 fields without a history of asparagus growing and differing in their distance from asparagus plantings were tested for infestation with Foa, using a bioassay with asparagus as a bait plant. A high correlation was found between the incidence of infestation and proximity to asparagus fields; Foa was found in 69% of the samples from fresh fields in an asparagus production centre, and in only 6% of the samples from fields at a distance of 1 km and more from asparagus fields and outside a production centre. To evaluate planting material as an inoculum source of Foa, 49 lots of one-year-old crowns from 23 nurseries were collected and rated for disease symptoms. Infestation was found to be common with only two lots free of symptomatic plants. Most of the lots had more than 75% of symptomatic plants. Although most of the plants were infested, they showed only slight root rot symptoms. The procedure for production of Foa-free planting material is discussed. Persistence and infestation of asparagus root residues in former asparagus fields was assessed by retrieving the residues from eight former asparagus fields with an asparagus-free period of one to 25 years, and three fields with a standing asparagus crop. Even after an asparagus-free period of 25 yr asparagus root residues were retrieved from soil, although at low levels. Mean population densities of Fusarium spp. declined from 2 times 10⁶ to 1 times 10⁵ colony forming units g^_1 air-dry root tissue during the first 10 years and were still > 10⁴ c.f.u. g“¹ air-dry root tissue 20 to 25 yr after asparagus produced was stopped. The population was dominated by F. oxysporum. Eighty-three of the 112 isolates (74%) of F. oxysporum belonged to the forma specialis asparagi. The proportion of Foa in the population did not decrease in time. It was concluded that persistence of Foa in asparagus root residues is a major reason for its long-term survival.