At crop change time, an 'accident' is often lurking in odd corners. For example, if there are diseased or infected plants in the greenhouse, there is a risk that sources of infection could still be present here and there. "Pathogens could, for instance, be hidden in the greenhouse structure or in the gutters. Proper cleaning and disinfection are therefore essential to ensure a good start with a new crop," explains Jan-Willem Spaargaren, Manager R&D at Cultilene.
First clean, then disinfect
In most of the Dutch greenhouses, all plant material and substrate are removed from the greenhouse at the end of the growing cycle. Polythene and ground coverings are sometimes left in place, ready for the next crop. "Before you start disinfecting, ensure that all the weeds have been removed from the greenhouse," says Spaargaren. "It is best to remove weeds without using herbicides. If you decide that they are necessary, choose a contact herbicide with no after-effects."
Ines van Marrewijk, product manager at Groen Agro Control, is a specialist in hygiene during crop rotation and says that glyphosate-based products often have a vapour effect. "These vapours can be released when the temperature in the greenhouse is raised. Not all growers are aware of this."
Spaargaren believes that once everything has been removed from the greenhouse, it is best to clean it thoroughly before applying a disinfectant. "Many growers clean and disinfect in one go, but this does not always leave the surfaces clean enough. Disinfectants do not work well on dirty surfaces, which may harbour pathogens which are released later on."
Van Marrewijk also believes that the minimum temperature in the greenhouse during disinfection should be 15°C. "Disinfectants work better at higher temperatures."
Inside of the drippper pipes
Spaargaren highlights some more specific issues. "Various disinfection methods are used for drippers, which are normally left in place. If the drippers hang along the gutters with the pegs facing downwards, you can fill them with disinfectant. However, some growers do this slightly differently: they remove the dripper pipes, roll them up and place them in a large tank filled with an acid solution. This method disinfects all the hoses at the same time, while also dissolving any lime scale that has built up on the stakes. However, this method does not remove all the organic contamination. If there is still visible soiling, the hoses can then be immersed in a peroxide solution."
According to Spaargaren, the inside of the dripper pipes and supply pipes requires extra attention. "They must be flushed with a disinfectant several times until the water runs clear. At the start of a new crop cycle I often notice that growers don’t devote enough time or attention to this and the water used for the first irrigation cycle is dirty. It is logical that this increases the risk of problems."
Decaying plant material as a disease source
According to Spaargaren, particular attention should also be paid to the area immediately surrounding a greenhouse. "Weeds often grow outside the greenhouse and it’s not unusual to have a heap of old plant material lying around. Traces of decaying plant material can easily blow into the greenhouse, with all the consequences that brings."
Drain reservoirs and other types of reservoir must also be borne in mind, according to the Cultilene specialist. "Over time, a layer of sludge forms at the bottom of the reservoir. This contains all kinds of contaminants, including organisms that could be harmful to plants. There is a risk that they could be drawn in with the irrigation water one day. It is therefore advisable to clean the bottom thoroughly every year. Crop change is the perfect time to do this as you don’t have to water the crop at that time."
According to Van Marrewijk, another advantage is that the oxygen content of the water in the reservoirs can be maintained at a better level if they are emptied regularly.
Once the greenhouse is completely clean, Spaargaren believes it is important to observe strict hygiene rules. "Equipment and materials taken into the greenhouse must be clean and disinfected. In fact, access to the greenhouse should be restricted until the new plants are in place. Other cultivation materials – e.g. harvesting carts and other equipment – must be cleaned and disinfected before they are taken back into a greenhouse that has been cleaned and decontaminated."
Hygiene during crop rotation: top tips at a glance
- Remove weeds in the greenhouse.
- First clean with water and a brush and/or a high-pressure washer, then disinfect.
- Block the gutter drainage points during disinfection. Make sure that no disinfectant ends up in the reservoir.
- Ensure that the ambient temperature is at least 15°C during disinfection.
- Flush the drip pipes and supply lines thoroughly, until the water runs completely clean.
- Ensure that there are no pathogens in the immediate vicinity of the greenhouse (e.g. in decaying plant material).
- Remove the sludge from the bottom of the (drain) reservoirs every year.
- Ensure that only clean and disinfected materials are taken into the greenhouse after decontamination.
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