Biological control against tomato seedling tipping

Diseases in seedling production are responsible for serious losses in tomato cultivation. This is the case of tipping over, caused by Pythium ou Rhizoctonia solani. Biological control, with the use of Trichoderma, associated with other management measures, is one of the alternatives to combat the problem.

The tomato plant (Solanum lycopersicum) is one of the most important vegetables in the world. It is a crop with a relatively short cycle and high yields. In Brazil is considered one of the main vegetables with production of 3,6 million tons, with emphasis on the states of Goiás, São Paulo and Minas Gerais.

The production of seedlings is one of the key stages in tomato cultivation, as the performance of the plants in the production areas depends on it. Among the important factors for obtaining good quality seedlings is the selection of the substrate. The quality of a substrate depends on its physical structure and chemical composition, as in addition to its support function, it provides nutrients and stores water for plants. Furthermore, native microbial populations present in the soil perform several functions such as decomposition of organic residues with the release of nutrients and CO₂, production and release of substances that stimulate plant growth, establishment of mutualistic symbiosis with plants or biological control of pests and diseases that affect plants. There is a positive relationship between plant disease suppression and the diversity or abundance of the microbial community in the soil.

In tomato cultivation, more than 200 diseases have been reported that result in significant reductions in productivity. Although rarely more than five occur simultaneously, the occurrence of some of them can result in significant damage and losses. The lack of effective measures to control these diseases or the high production cost due to the intensive application of agrochemicals limits tomato production to certain growing seasons and regions where the incidence of diseases in the crop is lower. Seedling tipping over is one of the most common diseases in the production of tomato seedlings, being caused mainly by oomycetes belonging to the genus Pythium and the fungus Rhizoctonia solani.

Pythium sp. It survives saprophytically as a soil inhabitant, being disseminated by water. Symptoms in seedlings are thinning of the stem, necrosis in the neck region, wilting, tipping over and consequent death. Seedling tipping occurs pre- or post-emergence. The first case is characterized when the pathogen attack results in the death of the seed or the germinating seed, still in pre-emergence, often confused with the low germination capacity. Post-emergence damping-off is characterized by the plant falling onto the substrate. Pre-emergence damping-off often occurs in soils with high inoculum density. In field conditions, in rainy seasons, it can attack plants over 10cm in height, forming lesions with a watery, dark appearance that can extend up to 4cm above the plant's neck. The incidence of the disease is favored in areas with successive planting of the crop. The disease can occur in the form of lumps in flowerbeds.

the fungus R.solani can cause seedlings to fall pre- and post-emergence. It is a fungus that attacks a large number of host plants and can remain in the soil, surviving as a saprophyte in crop residues for long periods. Its occurrence is generally associated with excessive rain or irrigation and with plants with excessive vegetative vigor as a result of high humidity in the substrate. Density of plants and excessive irrigation in the nursery can favor the spread and multiplication of the pathogen. Parts of the plant containing mycelium and sclerotia (survival structures) of the pathogen are sources of inoculum for other plants in the nursery or crop.

In addition to the management measures already mentioned (avoid crowding of seedlings and excessive irrigation), other important actions are: avoiding areas subject to flooding, using high-vigor seeds and planting at appropriate depths, in addition to the correct use of nitrogen. Excess of this nutrient causes the tissues to become very succulent, making the plant susceptible for a longer period of time. On the one hand, the aim is to promote the rapid development of the seedling, enabling the maturation of tissues that become more resistant, and, on the other, to prevent the occurrence of environmental conditions that favor pathogens. The use of healthy seeds, chemical, biological or thermal treatment of seeds, soil and/or substrate treatment and crop rotation are measures that aim to reduce the pathogen inoculum.

Biological control has been used as an alternative control measure to reduce damage caused by tipping over in seedlings. It is characterized by the introduction of non-pathogenic microorganisms so that the host, when in the presence of the pathogen, does not suffer significant damage. The use of this natural disease control method can replace or complement the use of chemical products.

the fungus Trichoderma sp. It is one of the most used biological control agents in agricultural production. It stands out for being efficient in controlling phytopathogens that have structures considered difficult to attack by other microorganisms such as spores, sclerotia, chlamydospores and microsclerotia. Recently in Brazil, different formulations based on Trichoderma sp. have been commercialized for application in substrates intended for the production of seedlings, especially vegetables and ornamentals, where significant reductions in damage caused by Pythium sp.e R. solani.

At the Agronomy Department of the State University of Londrina, experiments were carried out in a greenhouse to evaluate the efficiency of the fungus Trichoderma sp. in controlling tipping caused by R.solani e Pythium sp. in the production of tomato seedlings. Seven wild isolates of Trichoderma sp. collected from fields growing tomatoes, corn, kale and cabbage, and four commercial products based on Trichoderma sp. Two different experiments were carried out, one to test the control of tipping caused by Pythium and another for that caused by R.solani.  To achieve this, phytopathogenic fungi and Trichoderma were inoculated into the substrate where the tomato was sown.

In general, Pythium sp. was responsible for the pre-emergence tipping and R.solani post-emergence. In the first case, the control exercised by Trichoderma was inefficient, ranging from 0% to 12%. In the case of tipping caused by R.solani, control ranged from 25,6% to 58,9% (Figure 1). Although there are many reports about the efficiency of Trichoderma sp. control or reduce damage caused by Pythium, In the present work, control of this pathogen was not observed by the majority of isolates. Only the Trichoderma isolated from a tomato field (TH tomato), controlled damping-off caused by this pathogen by 12%. The greatest difficulty in controlling tipping caused by Pythium is that the propagules of this pathogen respond to the exudates of the germinating seed by quickly colonizing the host tissue. In this way, the colonization of Pythium occurs before the fungus Trichoderma effectively establish around the seed, resulting in a high incidence of pre-emergence damping-off. In case of R.solani, the infection process would be slower, facilitating the establishment of Trichoderma. In general, the use of wild isolates and commercial products showed similar efficiency in reducing damping-off caused by R.solani, with the exception of Trichoderma isolated from a corn field (TH corn2), which showed damping-off control of almost 60%, significantly differentiating itself from all other isolates.

In recent years, many studies have been developed reporting the success of biological control of plant diseases, specifically with the use ofTrichoderma sp. in the seedling production process. However, there are still many difficulties in relation to the standardization of the formulations used, a process that goes from the stages of selecting the isolate, the formulation of commercial products, to the establishment of the fungus. Trichoderma sp. under field conditions. Selecting biocontrol microorganisms in the same region where they will be used later could facilitate their establishment in the crop. Furthermore, the use of new technologies in the development of formulations based on Trichoderma, such as spore encapsulation to maintain viability, would also contribute to improving establishment under field conditions. In this way, the use of biological control would become a more effective control tactic, especially when carried out in association with other recommended management measures.

Ciro Hideki Sumida, Maria Isabel BalbiPeña, UEL

Article published in issue 80 of Cultivar Hortaliças e Frutas