Faculty of Agricultural Technology, King Mongkut's Institute of Technology Ladkrabang, Ladkrabang District, Bangkok 10520, Thailand

Background and objectives
Avoidance of root diseases was one of the motivating forces underlying the development of hydroponics. Although cultivation in hydroponics has resulted in a decrease in the diversity of root infecting microorganisms compared with conventional culture in soil, root diseases still occur [1, 2]. Therefore this research was conducted in order to study the disease occurrence on hydroponically-grown English cucumbers, including the spread of fungi in the recirculated culture, which could give us knowledge on the avenues of pathogen introduction. The potential of growing this crop in hydroponics was also evaluated.

Materials and methods
Disease occurrence was studied and monitored from 3 crops covering all three seasons.

Results and conclusions
From the results, the diseases found can be grouped as follows: airborne disease -- powdery mildew (Oidium sp.); water-borne disease -- collar and root rot (Pythium aphanidermatum); and the deformation of leaf and apical shoot caused by broad mite, aphid and viral disease. Powdery mildew was found on mature-plants of 6-12 weeks in the first and third croppings, that is, during winter and rainy seasons, respectively. Besides, disease incidence in winter cropping was higher than that in rainy season cropping. However, disease severity overall was not so high. Collar root rot (Pythium aphanidermatum) was found in all three crops. Damage was serious during the fruiting stage of cucumbers resulting in the sudden death of plants. Disease incidence and disease severity were varied depending on the growing season. Deformation of leaf and apical shoots were found to be the most serious problem in all three growing seasons. Damage was mainly caused by broad mite and aphids which could also transmit virus. Consequently, the plant would be infected by virus resulting in serious damage which affected plant flowering and fruiting. From the study of fungal spread in the recirculated culture, 12 genera of fungi -- Aspergillus spp., Chaetomium spp., Conidiobolus sp., Emericeifa sp., Fusarium sp., Mortierella sp., Mucor sp.,Penicillium sp., Rhizopus sp., Sartoya sp., Syncephalastrum sp., Trichoderma sp., including two genera of zoosporic fungi -- Saprolegnia sp., and Pythium sp., were found. For Pythium spp., they were identified into four species, namely P. aphanidermatum (Edson) Fitzp., P. carolinianum Matthews, P. 'group G' and P. 'group HS'. A large amount of inocula of P. carolinianum was frequently detected in the system. For P. aphanidermatum, its amount was found highest in substrate slab in accordance with the occurrence and severity of collar and root rot disease. In terms of the potential of growing English cucumber in hydroponics, it was shown to be satisfactory, particularly the relative merits of local substrates (coir dust and rice husk-charcoal). Growth and fruit weight of plants grown in winter were highest compared to those grown in rainy and summer seasons. Among the three nutrient solution compositions -- Coic-Lesaint,1983; Benoit, 1992 and modified from Belgium [1] employed in this experiment, they gave no statistical differences in growth. Nonetheless, the modified one gave the best result in terms of fruit weight.

All in all the results obtained such as diseases occurrence, an accurate identification of the specific pathogen involved, are essential to the selection of development of an appropriate strategy for control or for maintaining a pathogen-free environment for hydroponics.

1. Jaenaksorn T, Ratanopas S, 1996. Proceedings of the 10th Asian Agricultural Symposium: Present Status and Future Prospects of Sustainable Agriculture, pp. 137-146.
2. Stanghellini ME, Rasmussen SL, 1994. Plant Disease 78, 1129-1138.