Hygiene in greenhouse management as a basis for crop protection against infections

The importance of hygiene in greenhouse farming

For the most part, people have a very clear idea of the role of hygiene and everything associated with it, especially if it concerns themselves or their home. No one doubts the importance of maintaining hygiene; there have been too many examples where a lack of hygiene led to disease and even death. The concept of the role of hygiene in greenhouse farming is much less understood, although the principles here are practically the same as in a household.

A grower's task — to minimize any risk of disease outbreaks in the greenhouse — is only feasible if they have done everything possible to eliminate sources of infection. Here, the economic factor also comes into play: what is the importance of maintaining hygiene from an economic perspective? It is very difficult to quantify all the benefits provided by the time spent on maintaining hygiene; however, there is no doubt that in a facility where basic requirements are met, epiphytotics occur much less frequently than where hygiene is discussed a lot but little or nothing is done.

Overall, attention to hygiene requirements is interconnected with tidiness and, naturally, with an increase in labor productivity, as opposed to conditions where chaos, dirt, and clutter prevail. In relation to greenhouse crops, compliance with hygiene requirements can be characterized as a way to weaken or suppress diseases and/or pests using physical and chemical means. In practice, hygiene standards come down to the eradication of infection sources, and the most important method — tillage — has already been discussed above.

Infected or contaminated residues of the previous crop represent the most common source and reservoir of phytopathogens. If the greenhouse is treated with a disinfectant solution or fumigant before the start of sowing operations, the contamination of all surfaces will decrease noticeably. The bulk of the roots, as well as trellis wires, must be removed from the greenhouse.

The main sources of pathogens that require strict adherence to hygiene:

• greenhouse structures;
• benches, trellises, supports;
• water sources;
• plant residue disposal sites;
• pots, crates;
• staff (can be carriers of infection).

Disinfection of greenhouse structures

Spores or other forms of pathogens that are resistant to significant fluctuations in humidity and air temperature survive even in the absence of crops (during rotation) on any surface of the greenhouse. Sometimes spores penetrate deep into cracks and crevices, which are numerous in every structure, and remain there, posing only a potential threat. They can leave this refuge and, upon finding a suitable host, subsequently cause its disease.

The possibility of such secondary infection emergence generally exists only in cases where spores are released during the reconstruction or renovation of a greenhouse. For example, epiphytotics in refurbished mushroom houses are often caused by exactly this type of spore release. Usually, however, disinfection of all surfaces is carried out in the greenhouse, since any surviving pathogen may sooner or later infect the next crop.

To clean greenhouse structures, two main methods are used: treatment with a disinfectant solution and fumigation. Disinfectant treatment is quite effective, but it is not a particularly pleasant procedure for the worker. It is best to perform it with a strong jet of water, which also washes plant residues off the benches. Satisfactory results are obtained by using, for example, 2% formalin or a 2% phenol solution with the addition of a wetting agent, especially for very dry surfaces. In addition, fumigation can be used for disinfection. Previously, fumigation was carried out mainly with sulfur vapors, burning it in the greenhouse at a rate of 450 g/28 m³. The resulting sulfur dioxide is a very powerful fungicide; however, when dissolved in water, it forms sulfuric acid, which is very harmful to humans and plants, and also causes corrosion of metal structures. Sulfur fumigation can only be carried out in a hermetically sealed room, impermeable to its vapors, to avoid damage to neighboring crops. Furthermore, sulfur cannot be used if there are metal surfaces in the greenhouse.

A poisonous chemical and a very strong biocide in vapor form. Formaldehyde fumigation is very simple: it is carried out using a strong oxidant, for example, crystals of potassium permanganate, when mixed with which formaldehyde begins to evaporate under the influence of the heat generated. The doses of the mixture components vary significantly, but on average, 100 g of fine potassium permanganate crystals and 0.25 L of commercial formalin (38% formaldehyde) are sufficient to treat a room with a volume of 28 m³, as the minimum consumption of permanganate yields the optimal amount of formaldehyde vapor. The treatment method is as follows. Metal containers with formalin (9-liter containers containing 1 L of formalin each) are placed in the greenhouse at a certain distance from each other. Do not use containers that are too small, from which the reaction products will leak and less formaldehyde vapor will be released, nor containers with a large amount of mixture components (in any case, no more than 10 L of formalin per 90 L container). Several small containers with small doses of both components are most convenient; they are easier to place in the greenhouse and, furthermore, working with them is safer for the operator. To obtain the amount of formalin vapor sufficient for a full treatment, the volume of the greenhouse in cubic meters is determined. Then, the corresponding dose of potassium permanganate is poured into each container. It is essential to ensure that no paper gets into the container, as the heat generated during the reaction could lead to a fire. The operator must move quickly through the greenhouse, as the reactions proceed at a high speed.

Formalin treatment is most effective at a greenhouse air temperature of at least 10 °C and relative humidity of 50—80 %. If there are puddles in the room or surfaces are heavily moistened, formalin vapors dissolve in water and their concentration decreases rapidly. The greenhouse is left closed for 24 hours and then fully ventilated. Planting can begin 24 hours after ventilation.

Similar results are provided by formaldehyde aerosol; a thermal fogger is used to produce it. Formaldehyde aerosols are commercially available, the recommended application rate is 1 L per 250 m3.

If fumigation is carried out while the crop is in the growing season, it is necessary to remove all dead plants within a few days, otherwise a portion of the pathogens will begin to sporulate from deep-seated necroses.