Integrated management of late blight in tomatoes and potatoes

The disease popularly known as mela or late blight is caused by the oomycete Phytophthora infestans and its main hosts are potato and tomato. It is an extremely aggressive disease, with the capacity to cause severe damage to crops, and can also decimate entire crops in a short space of time. Even though it has been widely studied, its control is still difficult, which makes tomato cultivation unsustainable at certain times of the year, in regions with environmental conditions favorable to the development of the pathogen.

Under favorable conditions, that is, mild temperatures (15ºC to 22ºC) and high humidity (> 90%), this microorganism can completely destroy a crop in a few days, due to its high aggressiveness and rapid multiplication. Thus, in areas with a high level of infection and inefficient control, losses can reach 100%.

In recent years, new breeds of the pathogen have appeared, which are even more aggressive and, consequently, require a greater amount of agrochemical application. It is estimated that the losses caused by this disease worldwide exceed 8 billion dollars. Some estimates indicate that in Brazil, more than 20% of the production cost is directed towards controlling tomato late blight.

disease symptoms

Phytophthora infestans is a pathogen in which sporangia (spores) adhere to the surface of the plant within a few minutes, germinating and penetrating plant tissues in less than three hours. After the plant is infected, the microorganism can attack all parts of the tomato plant. Symptoms appear three to four days after infection and are most evident on the leaves, with irregularly shaped and dark colored lesions. Initially, it presents a waterlogged appearance, which culminates in necrosis or burns (Figure 1). When the infection occurs in the shoots, it can cause the death of the apical buds, which leads to limited development of the tomato plant. As the pathogen progresses, stems, stems and petioles can break, resulting in the death of these organs. In tomato fruits, the symptoms are characterized by irregular, moist, deformed and brown spots, increasing the degree of depreciation and, consequently, compromising the commercialization of the product.

Favorable conditions

Tomato late blight is facilitated by mild temperatures between 15ºC and 22ºC and high relative humidity, above 90%. Prolonged leaf wetness due to rain or sprinkler irrigation leaves the plant more susceptible to the proliferation of P. infestans. This oomycete can migrate between tomato leaflets, spreading between plants, and under continuous wetness, all aerial organs of the plant can become infected and necrotic. The spread occurs through drops of water and the wind that transports it to new locations.

Preventive measures

Some control methods such as cultural practices, crop rotation, adequate soil management, elimination of host plants and the choice of healthy seeds with high vigor help in the process. Another intervention consists of promoting greater spacing between plants and reducing surface leaf wetness, thus reducing the area of ​​water film that favors dissemination.

Main ways to control the disease

In Brazil, there are no varieties with a satisfactory level of resistance to P. infestans. Therefore, its control occurs almost exclusively through the application of chemical products using protective anti-oomycetes (which form a barrier that prevents the fungus from penetrating) and systemic (absorbed and moved to other parts of the plant) in preventive sprays, making which inhibits the germination of spores. To ensure significant effectiveness of control, daily monitoring of crops must be carried out, always seeking to identify the first symptoms of the disease.

Alternative forms of control

Integrated disease management (IDM) is an important factor in controlling P. infestans, as it integrates several methods, which together have the ability to inhibit the development of the disease in tomato cultivation, in addition to reducing negative impacts on the environment and avoiding cultivars resistant to conventional chemicals. In this context, several studies prove the efficiency of fungi used as antagonists of P. infestans, which can reduce the impact of the disease on the crop, also increasing plant growth. The fungi of the genus Trichoderma spp have demonstrated effectiveness in control due to different mechanisms of action, these being competition, antibiosis or hyperparasitism.

Other measures adopted in the MID capable of preventing negative impacts reside in choosing the appropriate time for planting, management and cultural practices. Drip irrigation systems are used for control, avoiding the production of spores due to the reduction of leaf wetness and, additionally, savings in water consumption. As P. infestans is a pathogen capable of surviving in crop residues, it is recommended that they be eliminated from the planting area, along with any potential hosts.

The use of essential oils and plant extracts is a management strategy that has gained a lot of space among researchers and producers in recent years. For P. infestans extracts of pepper, black pepper, cloves, turmeric and garlic have already been evaluated, from which the incidence of the disease was reduced due to the inhibition of zoospore formation. Furthermore, neem oil, combined with chemical fungicides, provided less progress of the pathogen, indicating that controls with these compounds are effective and beneficial to the crop. It is also possible to mention research with phosphite-based products, which have fungal and anti-oomycete action, with the potential to control diseases in field conditions.

Use of silicon in late blight control

Regarding the physiological aspects of plants, adequate nutrition provides better tolerance to attack by pathogenic microorganisms. Silicon (Si) is not considered essential for plant growth, but it is still important, as several studies prove beneficial effects on crops such as tomatoes. Silicon, over the years, has had a beneficial effect recognized by several researchers in the agricultural field, as it acts as a barrier to various pathogens, reducing insect attacks and the incidence of diseases, in addition to increasing the photosynthetic capacity of plants, promote the development of more upright plants, reduce the rate of transpiration, increase the mechanical resistance of cells, reduce the toxic effect of Mn, Fe and other heavy metals, and increase the absorption and metabolism of elements, such as phosphorus.

In addition to being a physical barrier, due to its accumulation in the leaf epidermis, silicon activates some genes involved in the production of secondary compounds in the plant's metabolism, such as polyphenols and enzymes related to plant defense mechanisms. Thus, the increase in Si in plant tissues causes the plant's tolerance to attack by Phytophthora infestans increases, due to the additional production of toxins that can act as pathogen-inhibiting substances.

Self Management

The moment of application occurs when the plants are in the vegetative stage, with green shoots, flowers and fruits. The idea is to apply it to young leaves so that the cells form layers of protection against the entry of the fungus. Silicon is not very mobile in the plant, so even though it is absorbed by the roots, it finds it difficult to reach the aerial parts, requiring frequent applications. Therefore, the supply of silicon via foliar spraying is a viable alternative for supplying plants, stimulating its beneficial effects, in addition to significantly reducing the germination rate of fungal spores.

Silicate application methods are mainly in solid (powder or granulated) or liquid (soil or foliar) form. The use of powdered silicates occurs through incorporation into the total area, while the granules are applied to the planting lines, normally accompanied by other fertilizers, such as NPK.

The doses of silicon to be applied to the soil depend on the source used and the Si content in the soil. There are still many disagreements about the ideal dose for applying silicon to crops, however studies indicate that doses for the fine material (powder) to be broadcast ranging from 1,5t/ha to 2t/ha of calcium silicate are efficient. For soils that are already corrected, the dose should not exceed 800kg/ha. Other studies with granulated material have demonstrated that silicate doses ranging from 0,5t/ha to 0,8t/ha are sufficient in the planting furrow, as the combination of silicate with fertilizer material (NPK) is a great alternative. For application via mechanical spraying, research used doses of silicate between 1L/ha and 8L/ha, in installments throughout the crop cycle.

Some more recent studies with foliar application of Si showed that a dose of 2L/ha of a commercial product with 0,8% soluble Si, such as a stabilized concentrate of salicylic acid, divided into four applications during the potato cycle, was sufficient to reduce the severity of late blight by more than 40% compared to plants that did not receive any dose of Si.

Technology based on the use of silicon in agriculture is clean, viable and sustainable, with enormous potential to reduce the use of agrochemicals and increase productivity through more balanced and physiologically more efficient nutrition, with more productive and vigorous plants, with low disease severity. The gain in productivity is already enough to pay the cost of applying Si, not to mention the help in managing late blight and other pathogens and insect pests.

Measures with better results for the management of P. infestans

• Planting more tolerant cultivars

• Balanced fertilization, without excess nitrogen

• Silicon fertilizer

• Avoid planting in regions subject to the occurrence and persistence of fog for long periods

• Avoid planting in low, shaded land or close to water reservoirs

• Rotate crops with plant species that do not host the disease

• Do not cultivate in lowland areas and close to old potato and tomato crops

• Carry out less dense plantings, for better aeration

• Destroy crop residues immediately after harvesting

• Monitor climate conditions favorable to P. infenstans

Rodrigo Vieira da Silva, UFV and IF Goiano – Campus Morrinhos; Ana Paula Gonçalves Ferreira, Brenda Ventura de Lima e Silva and Gabriela Araújo Martins, IF Goiano – Campus Morrinhos 

Growing Vegetables and Fruits September 2019 

With each new edition, Cultivar Hortaliças e Frutas publishes a series of technical content produced by renowned researchers from all over Brazil, which address the main difficulties and challenges encountered in the field by rural producers. Through research focused on controlling the main pests and diseases in vegetable and fruit cultivation, the Magazine helps farmers in the search for management solutions that increase their profitability. In the September 2019 edition you can also see: 

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