Epidemiology of damping-off disease in greenhouse cucumber crops in the sultanate of Oman

Greenhouse cucumber production is the fastest growing sector of the landbased agricultural industry in Oman. The number of greenhouses has increased by over 400% in the last 4 years. Greenhouses are used almost exclusively to produce cucumbers. Expansion of monocultured cucumber production has brought a subsequent increase of disease problems, most notably damping-off caused by Pythium aphanidermatum. Seedling losses as high as 25% are not uncommon despite a high number of fungicide applications. Crops receiving over 20 fungicide applications per season are not uncommon and fungicide resistance is consequently emerging as a serious risk on many farms. The epidemics caused by damping-off have been modelled and reveal a two-stage process with a primary focus establishment followed by a secondary spread within crop rows. Secondary spread results in disease foci and gaps of greater or lesser size depending on the duration and speed of the epidemic. Probabilities of focus location vary within the greenhouse depending not only on pathogen density in the soil, but also on soil characteristics, especially water content and possibly salinity levels, both of which vary considerably. Epidemic characteristics are largely unaffected by cultivar, but planting date has a significant effect with higher disease levels occurring in the warmer seasons, corresponding to the temperature preferences of P. aphanidermatum. Disease management practices vary considerably between farms and the effect of these management practices on disease epidemiology is discussed in the context of reducing fungicide dependence.