A series of diseases occur in tomato that can significantly reduce productivity and fruit quality, whether through direct or indirect damage caused by pathogens. Among the diseases of greatest concern among producers is septoria or septoria spot, which in recent years has increased in potted or staked table tomato crops and in industrial tomato crops across the country.
The disease is caused by the fungus Septoria lycopersici Spegazzini and occurs practically in all tomato producing regions of the world, being more common in hot and rainy seasons. The pathogen causes severe defoliation of plants, significantly reducing fruit productivity and quality, both by reducing the photosynthetic area of the plant and by exposing the fruits to the sun, which reduces the color of the fruits, highly valued by the processing industries. of tomato. In some regions or growing seasons, losses due to disease can reach 100% of production, due to the death of plants.
Symptoms
Septoria can occur at any stage of tomato development, but symptoms usually appear on lower leaves shortly after fruiting begins. Symptoms occur mainly on leaves, but can also appear on petioles, stems and flowers of the plant, rarely on fruits. Initially they appear on the underside of the leaves in the form of small water-soaked spots with a more or less circular to elliptical shape, measuring 2mm to 3mm in diameter. As the disease develops, the lesions acquire a grayish-brown color in the center with darkened edges and a narrow yellowish halo around them, reaching up to 5 mm in diameter.
Figure 1 - Symptoms of septoria in tomato. Typical lesions surrounded by a yellowish halo and burning of the lower leaves
In severe attacks, the lesions coalesce, the leaves turn yellow, dry and fall. The lesions located on the petioles and stem are dark and without the presence of a yellowish halo. Over time, the disease can progress upwards in the plant, starting in the lower leaves towards the youngest leaves in the upper part, causing severe defoliation of the plant. New injuries caused by Septoria lycopersici can be confused with those of other diseases such as bacterial spot (xanthomonas spp.) and black spot (Alternaria spp.), which makes identification difficult. Fruits produced on severely defoliated plants are reduced in size and burn due to direct exposure to sunlight.
Figure 2 - Tomato plants severely attacked by septoria with severe defoliation (skirt blight)
Lesions on the leaves are formed, on average, six days after infection. Ten to 14 days after infection, small black dots can be seen in the center of the lesions, which correspond to the pathogen's reproductive structures, known as pycnidia. The presence of pycnidia in the center of the lesions is a striking feature for identifying septoria in tomato. In conditions of high relative humidity, conidiophores (reproductive structures) and conidia (spores) of the fungus are formed inside the pycnidia. In the presence of humidity, the pycnidia of the fungus absorb water and form a mucilaginous mass of conidia (cirri), which are released and disseminated to form new lesions.
Figure 3 - Identification of septoria by visualizing small black dots in the center of the lesions (pycnidia) (A), where the pathogen's conidia are produced (B)
Epidemiology
Although the pathogen is not a soil inhabitant, it can persist from one growing season to another in crop residues from diseased plants, associated with other solanaceous plants such as potato and eggplant or with weed hosts such as Solanum carolinense L. (horse nettle), Solanum nigrum L. (nightshade) and datura stramonium L. (stramonium), in addition to contaminated seeds, which constitute sources of initial inoculum for subsequent crops.
The pathogen's conidia can be spread over long distances through contaminated seeds or to other parts of the plant or neighboring plants by the impact of water drops, whether from rain or sprinkler irrigation, associated with strong winds. The pathogen can also be spread through workers during cultural treatments, insects (beetles), implements and agricultural tools.
Rain strongly contributes to the production, release and dispersion of the pathogen. When associated with strong winds, dispersion reaches greater distances.
In conditions of high relative humidity and favorable temperatures of 15°C to 27°C, optimum of 25°C, conidia germinate in the presence of free water on the leaf in approximately 48 hours, penetrate through the stomata and colonize the leaf. After six days, lesions develop. Prolonged periods of temperatures between 20°C and 25°C, accompanied by rain and/or dew, in the presence of large quantities of inoculum, are extremely favorable to the progress of epidemics.
How to control
Control of septoria is commonly carried out with the foliar application of contact and systemic fungicides, often already used to control other diseases. Contact fungicides are less efficient than systemic fungicides because they are easily removed by rainwater or sprinkler irrigation. Currently, there are many fungicides registered with the Ministry of Agriculture, Livestock and Supply (Mapa) to control the disease, such as cuprics, triazoles, isophthalonitrile, dithiocarbamates and strubilurins. Applications should be started immediately after the first symptoms appear and it is recommended to repeat them at intervals of seven to 14 days.
There are currently no commercial tomato cultivars or hybrids available commercially with satisfactory levels of resistance. This is attributed to the difficulty in transferring resistance factors, generally quantitative, from wild species to advanced tomato lines.
Other control methods used preventively with the aim of reducing sources of initial inoculum are crop rotation with non-solanaceous plants, for example: grasses, destruction or removal of crop residues from tomatoes and other nightshades or host weeds immediately after harvest. , in addition to planting pathogen-free seeds and seedlings. If seedlings are infected in the nursery, elimination is recommended. Avoid sprinkler irrigation, as it provides conditions favorable to infection. If this is not possible, carry out them in the morning, to allow the leaves to dry before nightfall. Avoid cultural treatments in the field when the leaves are wet. Balanced fertilization also contributes to reducing the disease, as it allows the plant to be more resistant to the disease.
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