Cucumber (Cucumis sativus L.) is one of the most grown crops under protected cultivation systems (greenhouses, etc.) throughout the world. Due to monoculture or narrow crop rotation in greenhouses, the cucurbit crops are frequently cultivated under unfavorable soil conditions caused by soil-borne diseases and environmental stresses . Root-knot nematodes, Meloidogyne spp., are obligate endoparasites and are among the most common biotic stressors that can cause serious problems in soil-grown cucurbits .
The traditional method of nematode control is based mainly on chemical nematicides. However, the potential negative impact on environment and ineffectiveness after prolonged use have led to a total ban or restricted use of most chemical nematicides and need for safe and more effective alternatives . Biological control, including use of biocontrol agents and organic amendments promises to be one of these alternatives [4, 5, 6]. Application of microorganisms antagonistic to root-knot nematodes or compounds produced by these microbes could provide an additional option for managing the damage caused by root-knot nematodes. Fungi and bacteria are among the most dominant soil-borne groups in natural soil ecosystem and some of them have shown great potential as biological control agents for root-knot nematodes .
The rhizospheric microorganisms target nematodes mainly depend on parasitizing, such as Pasteuria penetrans  and Paecilomyces lilacinus ; and producing nematicidal substances, such as Bacillus thuringiensis , Pseudomonas spp. . Bioproducts contain a microorganism (bacterium, fungus, virus, protozoan) as an active ingredient often referred to microbial pesticides, they are host specific and they are potential candidates with regard to integrated pest management . Many of the soil amendments used as nutrient sources for crop production have been found to control plant parasitic nematodes. Such materials include green manure, cow dung, poultry droppings, dried crop residues, botanicals, and composted agro-industrial wastes . A remarkable reduction in nematode populations both in greenhouses and field conditions with an attendant increase in crop yield and growth has been achieved [14, 15, 16].
The aim of the study was to establish the biological effects of fungal and bacterial bioagents on the root-knot nematode Meloidogyne incognita in soil used to grow cucumber seedlings in greenhouses. Trials were done with and without compost.
Materials and methods. The experiments were conducted under greenhouse conditions in the Maritsa Vegetable Crops Research Institute in 2013–2015 with the cucmber cv. Kiara F1. Cucumber plants were inoculated with 2000 second-stage juveniles in each 5 L pot. Suspension from Bacillus thuringiensis strain Bt1 + Bacillus amyloliquefaciens strain 2/7А (titer 104 spores/1 cm3 substrate) and BioAct WG (Paecilomyces lilacinus strain 251, titer 1х1010 spores per g product) was added three times (while pricking out seedlings, transplanting and one month after transplanting) – 0,2 g/plant.
Trials with compost: 1. Control (untreated); 2. B. thuringiensis strain Bt1 + B. amyloliquefaciens strain 2/7А (Bacterial strain); 3. BioAct WG (Pricking out seedlings in mixture: Peat:perlite:compost — 1:1:0.7). Trials without compost: 1. Control (untreated); 2. B. thuringiensis strain Bt1 + B. amyloliquefaciens strain 2/7А (Bacterial strain); 3. BioAct WG (Pricking out seedlings in mixture: Peat:perlite:soil — 1:1:1).
Root systems were rated for root galling on a scale of 0–5: 0 = no galling, 1 = trace infections with a few small galls, 2 = <25% roots with galls, 3 = 25–50% roots with galls, 4 = 50–75% roots with galls and 5 = >75% roots with galls . The plants were removed 60 days after transplanting and the following indices were recorded: shoot length (cm), fresh shoot weight (g), root length (cm) and fresh root weight (g).
The microorganisms, B. thuringiensis strain Bt1 and B. amyloliquefaciens strain 2/7А, had been tested previously for biological compatibility. It was established that being in contact together does not limit their growth. The compost composition was 78% rye-grass and 22% farmyard manure with the following characteristics: pH = 7,88, EC = 3,61, N = 600 ppm, P = 12,0 ppm, K = 1547,6 ppm, Ca = 2100,0 ppm and Mg =115,2 ppm. Data were processed using Duncan’s multiple range test .
Results and discussion. The root galling in all the treatments was remarkably reduced compared to the untreated control. The lowest rate of root galling was observed in treated plants with bionematicide BioAct WG in both variants with and without compost, 1,35 and 1,45 respectively, followed by Bacillus thuringiensis strain Bt1+Bacillus amyloliquefaciens strain 2/7A treatment (Fig. 1). These results were similar to those previously reported by  which reported that the Paecilomyces lilacinus product was the best treatment in suppressing the root-knot populations in the soil, followed by those with Bacillus subtilis and B. thuringiensis. Also, P. lilacinus increased the shoot length and fresh weight of the root system .
Figure 1. Rate of root galling by Meloidogyne incognita in cucumber treated with bacterial strain (B. thuringiensis strain Bt1 + B. amyloliquefaciens strain 2/7А) and BioAct WG
The biometrical parameters were relatively higher in plants grown with compost, suggesting beneficial effects on plant growth. The highest values of the shoot length (213,00 cm), fresh shoot weight (277,40 g) and the fresh root weight (52,92 g) in this variant were recorded with the application of BioAct WG, followed by the treatment of bacterial strains (B. thuringiensis strain Bt1 + B. amyloliquefaciens strain 2/7А) in which are recorded at higher values of parameters length of the shoot (193,03 cm) and the fresh root weight (51.62 g). Similar findings were reported in an integrated application of P. lilacinus with acacia compost recorded where maximum growth parameters and yield with least root-knot index were recorded .
Effect of bacterial strain (Bacillus thuringiensis strain Bt1 + Bacillus amyloliquefaciens strain 2/7А) and BioAct WG on plant growth of cucmber cv. Kiara F1
|Control||169,91 b||200,81 b||50,74 n,s,||35,45 b|
|Bacterial strain||168,10 b||212,43 ab||53,61 n,s,||37,71 b|
|BioAct WG||170,23 b||204,65 ab||54,95 n,s,||40,36 ab|
|Control||192,05 ab||221,05 ab||51,09 n,s,||51,16 a|
|Bacterial strain||193,03 ab||263,25 ab||54,81 n,s,||51,62a|
|BioAct WG||213,00 a||277,40 a||54,24 n,s,||52,92 a|
a, b, c … – Duncan’s multiple range test (p < 0.05)
Conclusions. The lowest rate of root galling was observed in treated plants with bionematicide BioAct WG in both variants with and without compost. Plant growth parameters were relatively higher in plants grown with compost and treated with bioproducts.
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