I. Introduction
Sustainability is a development based on ecological and social balance, a sustainable development is the development that meets the needs of the current generation without undermining the ability of future generations to meet their own needs (Hall, 2001). As related to agriculture, sustainability describes farming systems as systems that are able to maintain their productivity to society indefinitely. These systems should be socially supportive, resource-conserving, environmentally sound and commercially competitive (Weil, 1990). Achieving sustainability requires an efficient use of the available technology by relying on cultivation techniques, equipment management and constructive materials aimed to reduce agro-chemicals, energy and water consumption as well as waste generation (Acutt et al., 1998).
The sustainability of an agroecosystem is represented by its ability to maintain a given level of productivity over time and a given quantitative-qualitative level of environmental resources (Loucks, 1977). The greenhouse is a form of agroecosystem where, unlike other agroecosystems, the environment has been adapted to the crop in order to maximize its productivity.
Greenhouse cultivation is becoming very important due to its capacity to produce vegetables all year round. The greenhouse system inputs is represented by natural resources (soil, water, air, organisms) and anthropogenic resources (chemicals, materials). The greenhouse system output is represented by the useful product per covered soil surface unit (dry matter, energy, proteins, income). The output concept should also include modifications of environmental resources (landscape, soil, water, air, organisms) associated to the production system. The greenhouse productivity depends on its specific physical/ agronomic characteristics such as environmental isolation level, direct and indirect energy inputs, tendency towards maximizing production, control level of the greenhouse climatic and agro/biological parameters associated to the system’s complexity. Greenhouse production requires a lot of energy, water and agro-chemical, and generates enormous quantities of waste. In terms of sustainability, the greenhouse agroecosystem is best when it is able to produce high yields with low resource use intensity and when it is able to keep the production factors and defense mechanisms intact (Brydges, 2001).
II. Integrated pests and disease management and sustainability of greenhouse farming technology
The occurrence, development and control of pests and diseases under greenhouse structures are influenced by the fact that the crops are enclosed. Greenhouses are designed to maintain an ideal environment for the crop, both economically and physiologically. These conditions also provide a protected and a favorable environment for pests and pathogens (optimal humidity and temperature, no rain and no wind). Pests and pathogens may therefore be more prolific and cause more damage to their hosts in greenhouses than in open field conditions. Moreover, compared with open field cultivation, natural enemies may be scarce or entirely absent, unless accidentally or purposefully introduced, on the other hand the stability of the greenhouse environment allows natural enemies of pests to be used as an effective means of control (Hanafi, 2013).
The traditional method of pest and disease management is based on the use of pesticides. The sustainability of the greenhouse farming can be maximized by applying the integrated management of pests and diseases with a significant reduction of pesticides and chemical treatments that are damaging to the soil, ground water, farmer, and consumer. Integrated crop protection combines many sustainable crop protection methods such as cultural, physical and biological methods in order to avoid diseases and pests or to suppress them. Using supported climate control management is one of the method, pest and disease infection can be reduced and influence plant development (Vox et al. 2010). Another method is the hanging sticky paper which insects fly into, stick, and consequently die. Although the sticky paper works, it only decreases and does not eliminate the usability of pesticides. By integrating predatory insects or spiders that specialize in a particular pest or pests, integrated pest management can eliminate the necessity of pesticides. These insects arrive in packaged bags (Wolosin, 2008). Biological pest control relies on the conservation of natural enemies of insect pests in agroecosystems (Letourneaua, Andob, Jedlickac, Narwanid, ; Barbiere 2015). When comparing biological pest control with chemical control, biological control tends to be long-term. Low costs of the biological approach to pest control tend to be widely accepted by growers (Yang, Zang, Wang, Guo, Xu, Zhang, ; Wan 2014). An example of the natural enemy, Amblyseius swirskii is a predatory mite that breeds extremely quickly under warm and humid environmental conditions. It predates on whiteflies, thrips, and other pests. In the absence of prey, it can also survive on the plant, feeding on pollen or mold. Biological pollination via bumblebees is a popular addition to the integrated management system. Bumblebees, Bombus terrestris, are shipped to the greenhouse when pollinators are needed and live out their four to eight week cycle pollinating the crops. The introduction of biological pesticides has introduced a non-chemical or insect combatant to the repertoire of the Integrated Pest Management system. There is a biological pesticide that contains spores of a naturally occurring strain of the fungus Paecilomyces fumosoroseus. This fungus is highly efficient against the greenhouse whitefly and can infect all stages (egg, larva, pupa and adult) of this harmful insect. (Wolosin, 2008).
When farmers grow the same crop in a greenhouse over and over again without rotation, diseases and pests become a big problem, crop rotation prevents pests and diseases associated with any crop family to accumulate in the soil. Black and white plastic mulching is an important way of controlling weeds and some insect pests (thrips, leafminers etc.). Other sanitary measures during the growing period include removal by hand before the weeds set seed. Preventive measures may be taken at the end of the previous crop cycle, using solarization to reduce pest and disease inoculums in the soil. After several years of complete prevention, infestation becomes very low, and pest and pathogen inoculums in the soil decrease to manageable levels (Hanafi, 2013). Another integrated pest management tool is the use of insect proof net. These nets keep the insects out, without using any chemicals. Most vegetable greenhouses are sealed with insect-proof nets.
Integrating those method achieve an optimal environment to protect crop from pests and diseases and to harm the environment as little as possible. Chemical agents are only used to a very limited extent. Consumers have become more and more aware of the risk of pesticide residues in fresh plants and crops, a huge demand for non-chemical control methods is emerging in many countries. (Albajes et al. 2002) In the particular case of Morocco, IPM adoption in greenhouse vegetable farms has increased from 5 ha in 1999 to 4 230 ha in 2011. In terms of the percentage of greenhouse area adopting IPM in Morocco, tomato is by far the leading crop (61.4%), followed by pepper (22.4%), strawberries (10.6%) and green beans (2.6%). The biological control of insects and mites has resulted in a significant reduction (over 60%) in pesticide use. IPM has become the general crop protection policy in greenhouse crops, and is the wise answer to the overuse of chemical pesticides (Hanafi, 2013).
III. Energy source and sustainability of greenhouse farming technology
Majority of greenhouse farming systems depend on non renewable energy sources which has a high impact on the environment. For a sustainable greenhouse farming, greenhouse equipment such as fans, heaters, fertigation and lightning need to feed on electricity which can be generated from renewable energy sources in order to maximize crop production in terms of quantity and quality and thus to increase overall efficiency. One source of sustainable energy is the use of solar power which can be converted to electricity by means of photovoltaic systems (Knier, 2002). The high manufacturing cost and the rate of converting sunlight into electricity are the drawbacks of the solar panel application to the greenhouse usage (Bagher, 2014). The sun is a cleaner energy source than fossil fuel. The energy that the solar panels produce can power everything from water pumps to lighting systems, from electric fencing to greenhouse heating. The excess energy can be stored in a battery pack from which energy can be tapped during the night.
The solar panels consist of blackened tubes containing circulating water. During daytime, the sun heats up the water, which is then stored with a temperature of 500C in a buffer tank. The following morning, this warm water is pumped into the greenhouse pipes by the system. The heated air dries plants quickly, and mold has no chance. There is still a boiler installed for any cloudy periods, used to provide supplementary heat to the system.
The solar collectors are installed to heat the greenhouses. The heating prevents diseases and increases production. It is possible to run a greenhouse independent of local energy suppliers. Energy costs can be reduced by 40 percent. Production quantity and quality can be increased. The greenhouse climate can be more balanced. The use of pesticides can be reduced. Lower system maintenance is possible. The risks and costs resulting from power cuts can be lowered (Hoogerwert, 2013).
IV. Covering material and sustainability of greenhouse farming technology
Covering materials protect crops from weather conditions, influence greenhouse microclimate altering the growing conditions of the crops comparing to the external climatic condition (Vox et al. 2010). Glass, semi plastics and plastic films are the most common covering materials and are widely used. Nevertheless, those materials are rather unsustainable especially plastics since it is petroleum based and energy losses through those covering material are high. Even so replacing conventional plastics with biodegradable materials in agricultural applications does not reduce the amount of waste, but it does provide the opportunity to choose an alternative waste treatment strategy, i.e. organic recovery (Kapanen et al. 2008). To increase the sustainability in greenhouse, innovative covering materials should be applied along with effective heating and cooling systems. Biodegradable materials formed with raw materials from renewable origin have been developed in recent years to be used as environment ally friendly alternatives to petroleum based material.

V. Modern irrigation and sustainability of greenhouse farming technology
For greenhouse cultivation good water quality is vital. The plant material is always exclusive, and they cannot take any risks. Because water can only be re-used if it is clean, a reverse osmosis installation should be built, where water is fed into a special installation using membranes. This reduces water EC level and also removes any bacteria, viruses and mold. Reverse osmosis leaves re-used water just as clean as rainwater, and therefore fertilizers can be applied more precisely. By using a combination of different technologies, it is possible to reduce water consumption by sixty to eighty per cent. Hydroponic is a technology for growing plant in media other than soil (substrate culture) or in a nutrient solution (water culture) (Vox et al. 2010). Hydroponic system enriches the water with nutrient necessary for plants in a closed system, which protects the water from evaporating and prevents discharge to the outside environment (Roberto 2003). Hydroponic is necessary for optimization of water and nutrient delivery to the plants in order to reduce water and nutrient consumption and drainage with ground water and soil preservation. There are 6 basic types of hydroponic system which are wick, water culture, ebb and flow, drip, nutrient film technique and aeroponic. Hydroponics has many advantages, it eliminates soil borne pest and disease, increases yield, reduces crop maturation cycle and crop and plant are free of chemical traces (Roberto 2003; Vox et al. 2010). There are also disadvantages but the most important one is the high capital cost. However, hydroponics is worth investing it aims to increase the sustainability of the greenhouse.
Irrigation has been researched intensively since the early 1950s. It became clear that water use is much more efficient when pressurized irrigation is used as opposed to surface irrigation. Pressure irrigation systems enable better control and monitoring of irrigation that can be translated into higher water use efficiency. In recent years, approximately 25% of greenhouses with soilless substrates have switched to recycled irrigation systems. Recycling the water and nutrients by reusing water drainage either back to the same or nearby field appears to be the most efficient, environmental and economical solution: approximately 30% to 40% of water and fertilizer inputs are saved. Potential polluting of the aquifer by open irrigation systems is reduced. The switch to recycled irrigation systems increases yields. The water must be tested so the farmer may clear it of any organisms that can negatively affect the crop. The system has to be installed properly, otherwise leaks and water loss can occur and lead to crop failure; the crops will suffer from this and may die. The pipes connecting the tank to the plants can get clogged with residues from unclean water due to the filtration process, so maintenance is essential. At their early stage, crops require at least 0.75 to 1 liter per plant, per day and this increase at the blooming, fruiting, and harvesting stages (Smith, 2008). The availability of water is an essential requirement for greenhouse growing of high added value crops (Castilla ; Baeza, 2013).
VI. Technical training and sustainability of greenhouse farming technology
Working with modern technologies requires new skills. Agricultural extension service deals with three main fields: Instructing to provide updated agricultural knowledge for farmers, training to provide farmers with concentrated professional knowledge as a basis for adopting new and advanced technologies, and production of applied knowledge where agricultural extension service professional personnel conduct lots of experiments, observations and studies each year, with the aim of finding solutions to problems that emerge in the field and applying them in the field. This way, the agricultural extension service makes available to farmers the forefront of current information for use in the agricultural production process (Giacomelli, 2002). The fields of knowledge that the unit assists farmers in adopting include: efficient use of water , use of recovered and salty water, adoption of technologies and automations for personnel saving, improvement of agricultural produce quality to meet international standards, variety diversification, reduction in the use of pesticides, advancement of agricultural subjects related to environmental protection, improving the image of agriculture and training a young generation to continue to work in their parents farms (Hanks, 1983).
VII. Conclusion
The traditional greenhouse in the food production industry is very efficient when it comes to providing food for the majority of the population. However, it is extremely unsustainable and it is heading toward an inevitable end. When a traditional greenhouse is integrated with a sustainable system, the result will be a sustainable greenhouse. Integrated pest and disease management assures that pest and diseases are enemies of sustainability and that pesticides cannot be the only mode for dealing with pests and diseases. The natural enemy, sticky paper, black and white plastic mulching and insect proof nets are being used as integrated pest management methods. The sustainability of greenhouse farming would also be assured if farmers can consider renewable sources of energy. The use of modern irrigation, the reduction of water EC level, types of irrigation and recycling of water are considered as determinants of sustainability. Technical training can also assure greenhouse sustainability through regular farm visits by extensional officers to enlighten farmers and also how trainings and workshops can be used to enhance knowledge among greenhouse farmers. Sustainability of greenhouse farming technology has been a worldwide challenge. The goal of sustainable greenhouse is to reduce the usage of energy, water, and agro-chemical; and minimize the amount of waste from the production.