During April 2002 to September 2003 in unheated plastic greenhouses located in Fondi and Sperlonga (Latium Region of central Italy), in which more than 100 ha of cucumber (Cucumis sativus L.) were cultivated, an unusual disease causing decay of roots and plant wilting was observed. Many of the most common cultivars showed susceptibility, and in some farms, severe economic losses occurred. Disease symptoms observed on young plants included stunting, wilting, black root rot, and marked reduction of root development where pseudosclerotial structures were produced. The degree of root symptoms was proportional to the wilting. During periods of high evapotranspiration, wilting was severe in plants at the early stages of disease development, and even lightly infected plants wilted rapidly. Symptoms resembled those caused by vascular wilt fungi and were generally more severe in greenhouses with poorly drained soils. Samples from each of four greenhouses were collected during different periods of the growing season. Each sampling unit consisted of five to eight root pieces that were surface disinfected in 0.1% HgCl2 for 30 s, rinsed in sterile water, placed on petri dishes containing potato dextrose agar (pH 5.5), and incubated for 7 days at 25°C. Phomopsis sclerotioides van Kesteren (1,2) (identification confirmed by R. A. Samson, Centraalbureau voor Schimmelcultures of Utrecht, the Netherlands) was consistently recovered from affected tissues. Subcultures of three isolates were prepared and evaluated for pathogenicity. The experiments were conducted in a greenhouse with a 12-h photoperiod at 25 to 32°C. Seven-week-old seedlings (20 representatives per isolate) of a susceptible hybrid were dipped for 2 min in an agar slurry suspension of the pathogen and then returned to pots. Within 4 to 5 weeks after inoculation, all plants inoculated with each P. sclerotioides isolate showed the same symptoms observed in the field and caused wilting and death of approximately 80% of the inoculated plants. P. sclerotioides was consistently reisolated from the symptomatic test plant, whereas the fungus was never isolated from control plants. Another experiment using naturally infested soil in comparison with sterilized soil confirmed the soilborne nature of the fungus and its pathogenicity. To our knowledge, this is the first report of P. sclerotioides on cucumber in Italy. According to the experience of farmers and agricultural consultants, the disease was first observed in the last 3 to 4 years in unheated plastic greenhouses. However, we cannot exclude the possibility that the disease may have been present in central Italy prior to our observations, since it can be misdiagnosed and the symptoms can be masked by symptoms of other diseases. For these reasons, an accurate monitoring of the pathogen is necessary to determine the magnitude of the problem and its impact on the industry. Management practices that include long-term crop rotation with nonsusceptible hosts, removal and destruction of infected crop debris, and steam soil sterilization are suggested to reduce the economic losses. References: (1) E. Punithalingam et al. No. 461 in: Descriptions of Pathogenic Fungi and Bacteria. CMI, Kew, Surrey, U.K., 1975. (2) H. A. Van Kesteren. Neth. J. Plant Pathol. 73:112, 1967.
Effect of Soil Inoculum Density and Temperature on the Incidence of Cucumber Black Root Rot