1. Introduction
Almeria is the Mediterranean region with the highest concentration of greenhouses in Europe and in the world, with a greenhouse land surface of 32,827 ha in 2021 [
1]. Mediterranean greenhouses are mainly composed of tensile steel cable structures supported by columns and covered with transparent plastic roofs [
2]. These greenhouses protect crops from external environmental conditions using mainly passive climate systems without energy inputs [
3], making them more sustainable with a lower carbon footprint [
4].
Plastic covers can increase the level of diffuse radiation inside greenhouses, reaching diffuse radiation in the shaded areas and providing higher rates of photosynthetic activity in crops [
5]. Under cover materials that transform direct light into diffuse light, the light profiles are more homogeneous and can increase yield [
6,
7,
8] of tomato (9%) [
9] and cucumber (
Cucumis sativus L.) (11–15%) [
10,
11]. The increase of transmissivity to photosynthetically active radiation (PAR) of the greenhouse cover improves photosynthetic activity and consequently can enhance tomato yields [
12]. These changes in the light that reaches the crops, and the effect on them, could influence the microclimate inside the greenhouse and affect the development of certain fungal diseases. The incidence of light in the canopy affects some diseases such as powdery mildew. The quality of light can affect the conidiophore development [
13], which can modify the severity of powdery mildew on the crop depending on the PAR, ultraviolet or infrared radiation that reaches the plants [
14]. In addition, host plants of some diseases show increased transpiration and decreased photosynthetic activity [
15].
Normally, farmers apply phytosanitary products periodically throughout the crop, without regard to the severity of the disease, which leads to excessive applications of phytosanitary products [
16]. It is an unsustainable agricultural practice, which causes environmental and health risks; it also contributes to the emergence of disease resistance to phytosanitary products due to their excessive use [
17,
18]. The European Economic Community has reduced in the last years the active materials allowed for use in the fight against diseases and agricultural pests. In the current regulatory framework, any knowledge or technique allowing reductions of damage to crops generated by agricultural pests acquires significant interest. The reduction of pests by the use of new plastic films does not increase the work of the producers and generates an environmental benefit by reducing the use of pesticides, making the farms more sustainable.
The main fungal disease in tomato and pepper crops developed in the spring–summer cycles in Almería greenhouses is powdery mildew. This endemic disease in the southwest of Spain is one of the biggest concerns for greenhouse growers. It is one of the diseases with the highest expenditure on phytosanitary products at the European level [
19] and one of the most widespread worldwide [
20]. The climate of the area, characterized by warm temperatures with high humidity (due to the proximity of the sea) and dew at the beginning of the day, favors the development of powdery mildew [
21,
22,
23]. Additionally, the characteristic winds of the Almería region that reduce the ambient humidity can facilitate the dispersion of the spores [
21]. In addition, Mediterranean greenhouses (mostly naturally ventilated without forced ventilation to reduce excess humidity [
24]) also benefit the presence of powdery mildew.
Powdery mildew is one of the fungal diseases that most affects the production of tomato and pepper in greenhouses [
19] and could cause losses in the final production of the crop between 2–4 kg m
−2 [
25]. This disease is the main cause of the increase in the use of pesticides in Europe due to the important damages that are produced in agricultural production [
19,
26].
Leveillula Taurica is the pathogen that causes powdery mildew in tomato and pepper, which in turn is one of the most important fungal diseases in these crops. Powdery mildew in most cases is an epiphytic disease, generating main damages to the outer parts of the leaf.
L. taurica is an endophytic fungus that penetrates the internal tissues of the leaf (the mesophile), causing internal damage to the leaf and defoliation, which generates production losses in crops [
19].
Alternaria solani is also one of the most common pathogens in solanaceous crops, causing early blight on all tomato varieties worldwide that can reduce production [
27]. The symptoms of early blight are the appearance of necrotic spots in concentric rings with a yellow chlorotic halo on the leaves, causing the reduction of the photosynthetic area of the plant [
28,
29,
30,
31]. After colonization, the damage is visible 2–3 days after infection and spores occur between 3 and 5 days later. The pathogen can remain in the greenhouse between different crop cycles, with mycelia on hosts or conidia in the soil, in crop residues and in seeds [
32]. This pathogen attacks at all stages of growth and in all plant structures of the tomato, including fruits, leading to crop losses that can reach up to 79% of the yield [
33,
34].
Tropical climates with elevated temperatures and rainfall favor the development of the pathogen; this requires the use of specific fungicides to control crop damage [
35]. Frequent rains, high humidity and high temperatures (24–29 °C) help the development of early blight, but also semi-arid climates with wet nights causing frequent and prolonged night dew favor the proliferation on crops [
29].
It is known that there are differences in the development of fungal diseases in different crops, depending on the quality and quantity of light received by the plants [
13,
14,
36,
37,
38,
39]. The light in the canopy could be related to the development of diseases that a priori would be more influenced by environmental humidity, as would be the case of early blight. In a previous cucumber crop developed in the same experimental greenhouse, we observed that powdery mildew infection decreased by 30% under the influence of a plastic cover with high transmissivity [
11].
The aim of this study was to evaluate the effect of an experimental cover film with high transmittance and high light diffusivity on the development of fungal diseases and yield and fruit quality in tomato and pepper crops developed in the spring–summer cycles in a Mediterranean greenhouse.
4. Discussion
For tomato and pepper crops developed in the spring–summer cycles, a greater development of some fungal diseases was generally observed in the greenhouse sector with the commercial film; it could be related to the higher light transmissivity of the experimental film (90%) compared with the commercial film (85%). Powdery mildew survives and grows well in shady conditions; the conidia are sensitive to direct sunlight and ultraviolet radiation [
51,
52,
53,
54]. However, the whitewashing effect results in lower temperatures and higher relative humidity due to the lower incidence of radiation, which also influences the development of pathogenic fungi.
The quantity of light reaching the plant leaves can influence the development of powdery mildew [
55], as we can see in
Figure 3 and
Figure 7. The areas covered with the plastic that allowed a lower total light transmission showed a higher development of the disease, which could be related to a lower incidence of ultraviolet radiation (UV-B, 280–300 nm) that inhibits the development of powdery mildew in the tomato [
54]. The differences in the disease development under different plastics may be because of different light qualities under different plastic films or an indirect effect on host plant susceptibility [
14,
56]. Light intensity and light quality can affect the pathogen or host plant in powdery mildew infections, as has been shown with
L. taurica on pepper [
57] and powdery mildew on cucumber [
58].
The cover whitewashing also influenced the light transmitted through the films tested in this work. In the tomato cycle, the whitewashing did not have a significant effect on the results because the rains removed it from the greenhouse canopy (
Figure 4). In this case, we observed the different incidences of diseases from the beginning of the crop. In the pepper cycle, there was practically no rain, so the whitewashing remained on the roof of the greenhouse until it deteriorated. Therefore, even with infection rates of 20%, the two areas covered with different films did not differ in disease level. In July, with the whitewashing severely degraded, we observed statistically significant differences. The different levels of disease in both sectors clearly showed the importance of the light that reaches the crop for the development of powdery mildew.
In the tomato cycle, early blight development was also observed. In this case, there was also a lower level of disease in the greenhouse sector covered with the experimental plastic (
Figure 4). Early blight development is known to be strongly affected by the need for high humidity and elevated temperatures [
35]. Rainfall during the trial could benefit the development of early blight in the crop.
Different wavelengths of light affecting crops can influence the development of diseases as well as the host plant, which can make it more susceptible to these diseases [
59]. Early blight needs elevated temperatures and high humidity values for its correct development [
35], so the effect of light reaching the crop acts indirectly on the development of the disease [
60]. Under the experimental film, there is a greater transmission of total light, which increases photosynthetic activity [
12] and may retard the development of the disease. In addition, other studies show that low light in tomato plants reduces the production of antioxidants and hydrogen peroxide and increases the presence of malondialdehyde [
61], reducing the plants’ natural defenses against pathogens.
The analysis of the marketable yield of the tomato showed an increase of 0.25 kg m
−2 in the western sector of the greenhouse with the experimental film (
Figure 5). This 4.2% increase in marketable yield could be due to the higher radiation transmissivity of the experimental film. For cucumber crops, the yield increased by 14% [
11], 10% [
9] and 4.8% [
7] with diffuse film covers. Similarly, Dueck et al. [
10] found an increase in tomato yield of 4.8%. Fruit quality showed no statistically significant differences (
Table 4 and
Table 5) in any of the parameters analyzed. In the tomato crops, sugar contents associated with fruit flavor and pigments associated with fruit maturity can be influenced by the cover materials [
62,
63]. Plastic covers increase the level of diffuse radiation inside greenhouses, providing higher rates of photosynthetic activity in crops since this diffuse radiation reaches areas that are shaded in outdoor crops [
5].
The increase of 0.06 kg/m
2 in the yield of marketable fruits obtained in the sector with the experimental film with respect to the commercial film for the pepper crop (
Figure 8) could be also attributed to the higher light transmissivity of the cover. Hee et al. [
64] obtained a higher productivity of peppers in a greenhouse with a plastic cover with a high light diffusion rate with respect to a polyethylene plastic cover. The production increased by 3.1%, values similar to those studied by Hemming et al. [
65], which were found to increase by 5–6% during the summer months due to the use of a diffuse cover film. There were no statistically significant differences in any of the cases studied (
Table 5 and
Table 6) in the quality parameters of the pepper fruit studied, although they were higher in the sector with the experimental film. Other authors did detect differences in dry matter content, with higher values in the sectors with the higher diffuse radiation film [
64].