Early blight management in tomato cultivation

The tomato (Solanum lycopersicum L.) is the second most planted and consumed vegetable in Brazil and the world. Among the productivity limits of this vegetable, diseases caused mainly by fungi, bacteria, viruses and nematodes stand out.

The black spot (Alternaria Solani), which is also known by producers as alternaria spot, is one of the most economically important fungal diseases in tomato cultivation, as it occurs in all producing regions and its control costs are high, mainly because it is a disease quite aggressive, also observed in other nightshades, such as potatoes.

Temperatures in the range of 25ºC to 32ºC, high humidity and high leaf density are favorable for the occurrence of early blight on tomato leaves. But the fruiting phase is the phase of greatest susceptibility to attack by the fungus. Producers in regions with a semi-arid climate are not free from early blight attacks, because frequent dew provides the necessary moisture for the disease to progress.

Tomato plants attacked by early blight present a reduction in leaf area, a drop in vigor, depreciation of fruits and tubers and a consequent reduction in productive potential. The use of cultivars with genetic resistance to this disease is a viable alternative, especially when associated with chemical control.

Symptoms

It is possible to observe the symptoms of black spot at any age and at any time.remove the aerial part of tomato plants. However, the lesions are more abundant on older leaves, where the first symptoms appear. Necrotic, dark brown lesions can be observed, with or without very pronounced concentric zones, where the fungus' fruiting structures (spores) are formed, defined edges, circular or elliptical at the beginning and irregular later, with a diameter of 3mm to 20mm. Under favorable conditions, the lesions quickly increase in size and can unite and destroy a large part of the leaf blade. When the lesions become older, there is a chlorotic halo that can expand over large areas of the leaflets. When it reaches the vein of the leaf, it causes its destruction, interrupting the circulation of the sap and causing the affected part to turn yellow and die. On younger leaves and especially when the plants show vigor and active vegetative development, the lesions are smaller in size.

On the stem, petiole and rachis, the lesions are similar to those on the leaf, however, they are more depressed, tending to circumscribe the affected organs and lead to their death. On the fruits, the lesions begin with a brown or black color from the sepals, where they cause dry rot with a zoned appearance. In conditions of high humidity, the entire lesion becomes covered with a black velvety growth due to the fruiting of the pathogen.

Characteristics of the fungus

Conidia are spread mainly by wind, insects, seeds, people entering the area and contaminated agricultural implements. The fungus is able to survive in crop residues, seeds and volunteer plants, remaining viable for a long period of time. Germination occurs in the range of 6ºC to 34°C, with excellent germination power between 28ºC and 30°C e 35 to 45 minutes of leaf wetness. The fungus penetrates directly through the cuticle or cell wall after appressorium formation. Lesions become visible under favorable conditions within two or three days after penetration.

How to control

To correctly manage the disease, several measures can be adopted, such as: seed treatment with fungicides; crop rotation with grasses in order to eliminate or reduce the source of inoculum. Choosing a cultivar resistant to the disease, a suitable location for producing seedlings and installing the crop, avoiding lowland areas or places subject to fog and areas close to tomato crops at the end of the cycle are also recommended. Balanced fertilization and use of organic matter and preventive spraying with fungicides. In general, the application interval varies from three to seven days for fungicides with protective action.

Severity in different tomato accessions

In order to assess the severity of Alternaria Solani in different tomato accessions, work was carried out at the State University of the Center-West, on the Cedeteg campus in Guarapuava, Paraná, using 64 tomato accessions (Table 1) from the University's germplasm bank and a commercial cultivar as a control .

Sowing was carried out in polystyrene trays (200 cells). After 20 days, the plants were transferred to the field, when the seedlings had four permanent leaves. The tomato plants were trained in two stems, in the cross fence staking system. Water was supplied to the culture using dripper tubes (20-20cm). The design consisted of randomized blocks, with three replications and four plants per plot, totaling 195 plots. The spacing used was 0,5m between plants and 1m between rows. Phytosanitary control and fertilization were carried out in accordance with the recommendations for the crop. The harvests were carried out from January to March 2012, corresponding to at least one harvest per week. All fruits that were at the ripening stage were collected and immediately counted and weighed.

Five assessments were carried out regarding the severity of A.solani, all during the harvest period, with an interval of seven days, following the diagrammatic scale for the disease, showing 2%, 4%, 8%, 16% and 32% of leaf area damaged by early blight, on tomato leaflets measuring 18cm² of total leaf area, assigning grades 1, 2, 3, 4 and 5, respectively, and grade 6 for plants that had more than 32% of their aerial part taken over by the disease (Figure 1).

Among the accessions evaluated, it was found that IAC404 presented a higher level of severity of the disease evaluated, the accessions Viçosa-MG, 0224-53; Guaraciaba-SC, RVTC-58 and Santa Cruz do Rio Pardo-SP, IAC392-59, also presented high levels of severity. Lower results were observed in the commercial cultivar Sweet Grape.

Click here to read the article in issue 81 of Revista Cultivar Hortaliças e Frutas.