Destructive power of septoria in tomato

Septoria or septoria spot is an important disease of table tomatoes during the rainy season, occurring in almost all producing regions in Brazil and the world. Under conditions of constant rainfall, common in the summer, the disease becomes limiting to cultivation in most staked tomato producing regions. In recent years, this disease has also become quite destructive to industrial tomato crops, irrigated via central pivot, in the Central-West region of the country. In this situation, it is necessary to use fungicides to control it, which increases production costs. The identification and, consequently, the adequate management of septoria have been difficult, especially in creeping tomatoes due to it being confused with other foliar diseases, mainly early blight and bacterial spot. Septoria causes losses due to the progressive destruction of foliage, which reduces the leaf area responsible for photosynthesis, causing a negative impact on fruit production and exposing them to sunburn.

Symptoms

The initial symptoms are observed on older leaves, generally at the time of the formation of the first cluster, through numerous circular and elliptical spots, 2mm to 3mm in diameter, with darkened edges and a straw-colored center in which punctures can be seen. dark colors corresponding to the pathogen's fructifications (Figure 1). When climatic conditions are very favorable and the cultivar is very susceptible, the lesions can reach 5mm or more in diameter, making them more easily confused with early blight lesions (Figure 2). Generally, the presence of a narrow yellow halo surrounding the lesions is observed. The spots often coalesce and cause blight, intense burning of the lower leaves ("skirt burn") and defoliation of the plants (Figure 3 A and B). Severe attacks also cause lesions on stems, peduncle and calyx, and in these organs the lesions are generally smaller and darker. Fruits are rarely affected and stem and calyx lesions usually do not present pycnidia.

Causative agent and favorable conditions

Tomato septoria leaf spot is caused by the fungus Septoria lycopersici. Spores are formed within dark pycnidia, which can be seen with the naked eye in more advanced lesions. The spores are released from the pycnidia in hyaline cirri (masses), bound together by a mucilaginous substance.

The main sources of pathogen inoculum are seeds, tigueras (volunteer plants), crop residues, cuttings already used in previous crops (in staked tomatoes) and other solanaceous species such as eggplant, jiló and invasive nightshades. In conditions of high humidity, conidia are released from the pycnidia. These are spread by water splashes, mainly caused by rain and sprinkler irrigation. Spread in the field can also be done by workers, implements, insects and birds, moving through moist plants. After the conidium germinates, the fungus penetrates the plant through the stomata and the initial symptoms appear in around six days. The optimum temperatures for disease epidemics to occur are between 20ºC and 25ºC.^oC. Thus, long periods of mild temperatures, high relative humidity, abundant rainfall or sprinkler irrigation constitute favorable conditions for the development of the disease. The incidence is more serious in crops grown during the rainy period of the year (table tomatoes), however, severe attacks have occurred in industrial tomatoes grown in the dry period of the year, irrigated via central pivot, mainly in circumstances where irrigation and leaf wetness are exaggerated.

Control measures

Despite the existence of good sources of tomato germplasm, there are no commercial cultivars or hybrids that present good levels of resistance.

The main way of introducing septoria into new fields/production areas is transmission via seed. Therefore, seed health is a fundamental prerequisite to prevent the pathogen from entering the crop. Furthermore, care must be taken from seedling production to harvest; preventive application with agrochemicals registered by the Ministry of Agriculture (Table 1).

In epidemics occurring under field conditions, one of the few control measures available has been the use of contact or systemic fungicides. This strategy, however, may be inefficient under highly favorable temperature and precipitation conditions or when the disease is already present in highly susceptible hybrid crops.

Other auxiliary measures in managing the disease are: avoiding frequent irrigation, when using sprinklers, rotating crops, destroying crop residues immediately after harvest, avoiding new plantings close to older or infected crops or in areas already infested with volunteer plants. of tomatoes, apply balanced fertilization and allow good ventilation between the plants.