Suitable substrate

The production of vegetable seedlings is one of the most critical phases in the planning of most horticultural crops, including tomato cultivation. Obtaining a successful crop depends on the quality of the seedlings used, in relation to the health or genetics of the material. For the production of high quality seedlings, the use of good techniques is essential and among the important factors is the substrate, whose quality depends on its physical structure and chemical composition and must be free of phytopathogens.

The substrate plays direct roles in the mechanical maintenance of the root system, plant stability, supply of water and nutrients, supply of oxygen and transport of carbon dioxide between the roots and the external air. Therefore, it performs more than just supporting plants.

Knowledge about the best mixtures is scarce and this information is extremely important for creating good quality substrates. The same occurs with knowledge related to the microbiota in substrates, which is an aspect little studied, as the microbial populations present in substrates perform functions similar to those naturally present in the soil, such as decomposition of organic residues with the release of nutrients and CO₂, production of substances that stimulate plant growth, establishment of mutualistic symbiosis with plants and biological control of diseases.

One of the main causes of seedling quality loss are soil pathogens, mainly fungi such as Rhizoctonia solani, Pythium sp. It is Fusarium sp., responsible for the tipping over of seedlings in several crops, and which can occur before or after plant emergence. Pre-emergent damping-off is characterized when seed infection occurs, before or during germination, causing them to rot and disintegrate, resulting in stand failures. Post-emergence damping-off in seedlings is commonly observed when seedlings are produced in trays, characterized by the attack of the pathogen at the base of the plant stem, showing symptoms of darkening and softening of the base of the plant, often resulting in tissue constriction. attacked (Lopes et al.

Chemical control is ineffective when it comes to diseases caused by soil fungi, due to the interaction of the fungicide with soil particles that can inactivate the active ingredients and the fact that pathogens can harbor at different depths in the soil. Therefore, there is a need to use alternative and effective measures to combat these diseases.

The replacement or complementation of the application of fungicides has been done through the use of biological control agents such as Trichodermasp., which reduce the pathogen's ability to reproduce, maintaining low inoculum levels. But the survival of Trichodermasp., in the soil or substrate, can be influenced by factors such as temperature, humidity, nutrients, soil type, microbiota, aeration, pH and organic matter content.

With the objective of analyzing the quality of mixtures of substrates and commercial substrates (Table 1), with the application of Trichoderma sp. and without application, several physical characterization tests, analysis of organic matter and pH of substrates were developed at the Department of Agronomy at the State University of Londrina; and quality assessments of tomato seedlings, inoculated with Rhizoctonia solani, a fungus that causes seedlings to fall over in the crop.

In the results of the physical characterization of the substrates (Table 2), percentages of 3,2% of macroporosity in the substrate were observed for most substrates. Microporosity, total porosity and maximum water retention capacity show higher percentages in the substrates SD (commercial Fertile Peat), SE (soil + sand + filter cake) and SF (commercial Bioplant) and in relation to the apparent density results , the commercial substrates SD (Fertile Peat) and SF (Bioplant) presented 0,02g/cm³ and 0,07g/cm³ and in the others, they ranged from 0,31 to 0,54g/cm³. The highest levels of organic matter were observed in the SD and SF substrates, 402,6g/kg and 228,1g/kg, respectively. And the results of pH analyzes (CaCl₂) showed a pH of 6,6 (SA); 6,4 (SB); 5,6 (SC); 5,5 (SD); 7,1 (SE) and 5 (SF).

In the disease assessment results, a lower percentage of tipping was observed in treatments where commercial substrates were used, probably due to their higher physical and chemical quality, mentioned above, and phytosanitary quality. As well as in the evaluation of seedling height, root length and aerial part weight and root weight, the best results were also observed in commercial substrates. It was also observed the efficiency of the Trichoderma sp. in tipping control, when compared to the control, without application (Figure 1).

Reports on the efficiency of biological control of plant diseases using Trichoderma sp. are increasingly common, especially diseases caused by phytopathogens, such as Rhizoctonia solani, due to the high efficiency of the action mechanisms such as parasitism through the penetration of Trichoderma sp. in the mycelia of R.solani, antibiosis through the production of metabolites that inhibit the growth and sporulation of Trichoderma sp. and competition for nutrients and oxygen.

The substrates prepared through mixtures with soil, sand, vermiculite and carbonized rice husk did not show significant results. To make better use of these materials, there must be better formulation of the substrates in order to make their use viable in the seedling production system.

The efficiency of commercial substrates is associated with their physical and chemical characteristics that provide favorable conditions for the better development of seedlings and, consequently, reducing the predisposition to pathogens. For example, due to the high levels of organic matter present in the substrates, the greater water retention capacity and the lower apparent density of the substrate. Density is also an important property for management, since the substrate and containers are transported and manipulated, which can influence the transport costs and infrastructure necessary for their use.

Knowledge regarding the performance of these substrates correlated with chemical and biological disease control is scarce. Birth et al (2003), testing only the germination of vegetables with different types of substrates, observed that the Fertile Peat substrate provided excellent germination results, as did Gomes et al (2008) working with the Bioplant substrate.

On the antagonistic efficiency of wild isolates of Trichoderma spp. in disease control there are numerous reports, such as by Elad et al (1980), Hadar et al (1987), Larkin et al (1998), Naseby et al (2000), Howell (2003), Harman et al (2004) Asran-Amal et al (2005) and Woo et al (2006). Hadar et al (1987), in studies with Rhizoctonia solani, reported the ability of wild isolates to Trichoderma harzianum to colonize the mycelium of R.solani, effectively reducing the damage caused by this pathogen. Asran-Amal et al (2005), also tested isolates from Trichoderma sp. against Rhizoctonia solani in tests in vivo presenting satisfactory data for disease control. In work carried out by Woo (1986), a decrease in the population of Trichoderma sp. in soils with low moisture content and, in relation to the incorporation of organic matter, a significant increase in population was observed. Humidity is considered one of the factors that most influence the maintenance of the natural distribution of various species of Trichoderma sp. and in their ability to colonize the rhizosphere to compete with pathogens, as well as the phyllosphere. Melo (1998) reported that the fungus Trichoderma sp. It has a wide distribution around the world, in practically all types of soils and natural habitats, especially those that contain or are formed from organic matter. Howell (2003) reported that biological control action mechanisms are influenced by substrate or soil, temperature and pH. Confirming work by Papavizas (1985) and Harman and Taylor (1988), who reported that pH can influence the parasitism of biocontrollers such as Trichoderma sp., which are generally favored by acidic soils.

In conclusion, in the seedling production system, it is strongly recommended to use commercial substrates of proven quality, being safer due to their ideal physical, chemical and phytosanitary characteristics for seedling formation, influencing the control of pathogens such as Rhizoctonia solani. Mainly, when biological control is used with application of Trichoderma sp., which helps combat various diseases that occur in seedlings, maintaining a greater balance of the substrate's microbiota, favoring the development of seedlings and, consequently, increasing crop productivity in the plant's adult phase.

Table 1 - Composition of the evaluated substrate mixtures
substrates			Composition
SA		Soil + sand + carbonized rice husk + filter cake + vermiculite (3:3:2:2:2)*
SB		Soil + sand + vermiculite (1:1:2)*
SC		Soil + sand + carbonized rice husk (1:1:2)*
SD		Fertile Peat (peat + perlite + limestone + mineral fertilizer)
SE		Soil + sand + filter cake (1:1:2)*
SF		Bioplant (material of plant origin and expanded vermiculite)

Table 1 - Composition of the evaluated substrate mixtures

substrates

Composition

SA

Soil + sand + carbonized rice husk + filter cake + vermiculite (3:3:2:2:2)*

SB

Soil + sand + vermiculite (1:1:2)*

SC

Soil + sand + carbonized rice husk (1:1:2)*

SD

Fertile Peat (peat + perlite + limestone + mineral fertilizer)

SE

Soil + sand + filter cake (1:1:2)*

SF

Bioplant (material of plant origin and expanded vermiculite)

*proportion of substrate mixtures by volume.

Table 2 - Physical attributes of the substrates: macroporosity (MA), microporosity (MI), total porosity (PT), maximum water retention capacity (CMRA), substrate apparent density (DAS), organic matter (OM) and pH
substrates*		MA (%)		MI (%)		PT (%)		CMRA (50ml/cm3)		DAS (g/cm3)	MOg/Kg	pH (CaCl2)
SA		4,8		34,6		39,4		17,3		0,31	32,2	6,6
SB		3,2		38,0		41,2		19,0		0,51	2,7	6,4
SC		3,2		37,2		40,4		18,6		0,54	2,7	5,6
SD		3,2		43,6		46,8		21,8		0,02	402,6	5,5
SE		3,2		43,0		46,2		21,5		0,48	40,3	7,1
SF		3,2		42,4		45,6		21,2		0,07	228,1	5,0

Table 2 - Physical attributes of the substrates: macroporosity (MA), microporosity (MI), total porosity (PT), maximum water retention capacity (CMRA), substrate apparent density (DAS), organic matter (OM) and pH

substrates^*

MA (%)

MI (%)

PT (%)

CMRA (50ml/cm³)

DAS (g/cm³)

MOg/Kg

pH (CaCl₂)

SA

4,8

34,6

39,4

17,3

0,31

32,2

6,6

SB

3,2

38,0

41,2

19,0

0,51

2,7

6,4

SC

3,2

37,2

40,4

18,6

0,54

2,7

5,6

SD

3,2

43,6

46,8

21,8

0,02

402,6

5,5

SE

3,2

43,0

46,2

21,5

0,48

40,3

7,1

SF

3,2

42,4

45,6

21,2

0,07

228,1

5,0

^*Substrates: SA: soil + sand + vermiculite + carbonized rice straw + filter cake; SB: soil + sand + vermiculite; SC: soil + sand + carbonized rice straw; SD: commercial Turfafertil; SE: soil + sand + filter cake and SF: commercial Bioplant.

Figure 1 - Evaluation of seedling height, root length, weight of the aerial part, root part, germination percentage and tipping percentage, grown under different substrates for the production of tomato seedlings, with Trichoderma sp. treatment. (Figure 1A, 1B and 1C) and without treatment (Figure 1D, 1E and 1F). SA: soil + sand + vermiculite + carbonized rice straw + filter cake; SB: soil + sand + vermiculite; SC: soil + sand + carbonized rice straw; SD: commercial Turfafertil; SE: soil + sand + filter cake; SF: Bioplant commercial