The higher number of nematodes before the start of the experiment were detected in soil + manure substrate (SM), followed by soil (S) resp. 1498.4 and 991.8 nematodes in 100 cm
3 of soil substrate. The lowest was the number of nematodes in the compost substrate (CS)-304.8 nematodes per 100 cm
3 (Fig 1).
Within the experiment “lettuce-medicinal and vegetable crops” significant differences in the final number of nematodes after mixed vegetation were observed. Statistical analysis of the data shows differences between the variants (Table 1). The total nematode abundance of the control variants was significantly higher than that of the mixed lettuce cultivation variants. For lettuce grown individually in soil and soil + manure, the final number of nematodes was 896.8 ind.100
-3 resp. 1198.4 ind.100
-3. After the experiment, it was found that the number of nematodes increased in variants SL and SML; there was almost no change in the variants SML+S, SML+B, CSL, while a decrease in the number of plant parasitic nematodes was observed when grown together with lettuce, calendula and tagetes. The mixed cultivation of lettuce and calendula resulted in a significant reduction in nematodes, with a Pf/Pi value of 0.3 being statistically significantly lower than the other variants: arugula 0.9 and tagetes 0.6. The effect of the mixed crops was the same in the variants lettuce + savory and lettuce+basil. The value of the Pf/Pi ratios was 1.1.
The analysis of the results of the vegetative parameters, given in Table 1, confirms the reporting changes in nematode populations. At an average height of 12.81 cm for the experimental plants, the lettuces with calendula were 66.5% higher and those developed together with savory and basil resp. 15.3% and 14.5%. There are statistically proven differences in plant height between the lettuces in the control variant and all other variants except for the arugula variant SML+A. The plants developed jointly with arugula (11.24 g/plant) and calendula (11.22 g/plant) have the largest leaf mass exceedingly significantly that of the other variants. With an average above-ground test mass of 8.41 g/plant, the excess in variants SML+A and SML+C was 33.7 and 33.4%, respectively. The leaf mass of the other mixed-growing lettuces ranges between 5.78 and 8.71 g/plant. The differences between the arugula variant and those with tagetes and savory, as well as between the lettuce containers grown with calendula and those with tagetes and savory, are highly proven. The lettuces of the variant grown together with calendula also have the most developed root system, 2.96 g/plant, in line with the well-developed above ground part of the plant. The measured root mass of these plants was more than twice the average (1.42 g/plant) for the test. Lettuce plants from basil containers have the least developed roots 0.63 g/plant. There are highly significant differences in the root mass of lettuces between the calendula containers and the variants with compost substrate, tagetes, savory and basil. As a result of the decrease in the number of nematodes,there was an increase in lettuce yields when mixed grown compared to when they were grown alone. The total average weight of the plants in the study was 9.82 g/plant. The variant with calendula, with the highest plants, also had the highest total plant mass of 14.18 g or 44.4% heavier plants compared to the average total mass of lettuce plants in the experiment. For lettuces grown with arugula, no large masses of 13.06 g (33% more than the average total mass) were accounted for, despite their low height. However, there was a slight decrease in lettuce yield when grown together with basil 6.41 g total mass (34.7% less than the average total mass), tagetes (7.80 g; 20, 5% less than the average total mass) and savory (9.63 g; 1.9% less than the average total mass). This is anticipited because the different plants share nutrients and space in the same nutrient volume. Statistically, the differences between the calendula variant and the compost, tagetes, savory and basil variants are well represented. Although the results of the influence of the soil substrate on the vegetative parameters of lettuce were not discussed in detail here, then during the experiment, differences in the root masses of lettuces between the experimental and control variants with compost and manure were also proved statistically. Similar results have been observed by other authors, who assess the growth and physiological characteristics of lettuce in the delivery of different N-organics and prove that the fertilizers derived from animal manure give good effects on leaves, LAI, the rate of photosynthesis and biomass lettuce
(Slamet et al., 2017).
Due to the complexity of the process involved in determining the species affiliation of nematodes and the multivariate of the experiment, individuals from nematode communities were analysed only quantitatively. In addition, some nematode species have a very short life cycle and reproduce rapidly and changes in the soil environment can cause significant changes in the species structure (
Villenave, 2001).
The results obtained here are close to those reported by
Tringrovska et al., (2015). In mixed crops the possibilities for growing tomatoes with tagetes, ustard, basil and lettuce under greenhouse conditions were evaluated. The mustard and the tagetes suppress the attack by nematodes, the genus
Meloidogyne, but the mustard causes a decrease in the yield of tomatoes. No negative impact on the yield and quality of the tomato fruits was found in the tagetes. Tagetis is known to contribute to decrease in the density of nematodes, but the decrease is not as pronounced as in the calendula variant. There are numerous studies on the use of tagetes (
Tagetes spp.).
Shah et al., (2018) by applying leaf extracts to some medicinal plants prove that margosa or marigold (
Tagetes erecta) extract treatment of brinjal seedlings prior to transplantation can avoid severe damage by suppressing the multiplication of root-knot nematodes and inducing the tolerance in treated inoculated plants. Tagetis is well known for its ability to produce compounds such as a-terthienyl, which are allelopathic to many species of plant-parasitic nematodes. However, there is contradictory data in the literature about how this compound is released
(Hooks et al., 2010).
Our results are in line with the information obtained by other authors, who prove that the role of root exudates in interactions between plant roots and other plants, microbes, and nematodes present in the rhizosphere
(Bais et al., 2006). The results are also confirmed by the data reported by
Elzawahry et al., (2014) examining the effect of five plant species (
Tagetes erecta L.,
Datura stramonium L.,
Nerium oleander L.,
Ipomea palmata L. and
Santolina chamaecyparissus L.) on the plant parasitic nematode
Tylenchulus semipenetrans under greenhouse and laboratory conditions. The authors prove that the tested plant species reduce the population of
T. semipenetrans juvenile stages and their effect increases over time, with
T. erecta giving the lowest percentage of reduction. Interpretation of mechanisms that suppress the development of nematode populations is complex because several mechanisms usually occur simultaneously.
Future research should focus on identifying the exact causes of suppressing the development of nematodes in the mixed intercropping and on developing IPM programs that take advantage of this potential. To confirm the influence of the “hostile” plants used, it is necessary to monitor the populations of nematodes during the subsequent growing seasons and let the lettuces be grown individually in the same soil substrate. Before this system can be recommended for use by farmers, its profitability needs to be significantly better.
