How to control black spot on tomato plants

Early blight, caused by Alteranaria solani, is one of the most important diseases in tomato crops. Managing it requires the integration of measures that include the use of fungicides applied preventively or as soon as the first symptoms appear. 

Tomato cultivation is high risk, due to the variety of environments in which it is grown and because it is a plant very susceptible to attack by pests and diseases and demands on inputs and services, which increases the investment of financial resources for the its production. Alternariasis is one of the most common fungal diseases in vegetables worldwide. In tomatoes, one of the most important is black spot or alternaria spot caused by the fungus Alternaria Solani (Ell & Martin) Jones & Grout.

Losses caused by early blight on tomato plants range from 5% to 78%.

The disease can attack the plant at any age and can cause, under ideal temperature and humidity conditions, destruction of the leaves, rendering the seedlings unusable for planting. It occurs in regions where potatoes and tomatoes are grown, and its distribution is widespread, with only its incidence varying. During the hottest months (25ºC to 30ºC) and with high humidity it becomes a serious problem for tomato plants.

On leaves, symptoms are expressed through necrotic leaf lesions, circular or not, dark brown, with characteristic concentric rings and well-defined edges. Lesions occur singly or in groups, and may or may not have a chlorotic halo. Lesions on stems can appear on adult plants, and are characterized by being dark brown, elongated, depressed, and may or may not present concentric halos. In young plants, cankers can form on the seedling neck, which culminates in their tipping over and death.

In fruits, symptoms occur at the insertion point of the peduncle, with necrotic spots that originate from the connection region between the calyx and the fruit. The spots are usually brown to black, firm, depressed and generally have distinct concentric rings.

The occurrence of severe epidemics of the disease is associated with temperatures in the range of 25ºC to 32ºC and high humidity. The fungus A.solani It survives between crops in infected crop residues and susceptible solanaceae, and can also survive in agricultural equipment, stakes and boxes used to store the product or even in seeds. In addition to these forms of survival, there is the possibility that the pathogen remains viable in the soil in the form of mycelium, spores or chlamydospores. Conidia are characterized by being highly resistant to low levels of humidity and can remain viable for up to two years under these conditions.

Cultural control methods, such as seeds of good origin, crop rotation, elimination of crop residues, planting far from old crops, avoiding lowland and humid areas, can be recommended. Chemical control must be carried out with preventive fungicides indicated for the crop.

Tomato varieties resistant to A.solani are being studied, as it is the best way to reduce the use of fungicides.

At the Ministry of Agriculture, Livestock and Supply (Mapa) there are 127 products approved for the control of early blight on tomatoes. However, before opting to apply fungicides, the area must be monitored, especially environmental conditions, as this is the fundamental factor for the expansion and infection of the pathogen. A.solani on site.

New mixtures of chemical groups are currently being tested to control black spot, which has shown good control efficiency.

Among the fungicides for controlling early blight, Boscalid, Metiran+Pyraclostrobin, Boscalid, Flutriafol, Metiram and Difeconazole stand out, applied preventively or at the onset of the first symptoms of the disease. With this measure, it becomes more difficult for the pathogen to cause a serious epidemic in the producing area.

Weekly applications of copper hydroxide, preventatively or at low pressure for black spot, result in good disease control efficiency. It is a product that offers advantages to the producer, as it is of medium toxicity, in addition to offering control of other diseases, mainly those caused by bacteria.

It should not be forgotten that when producing tomatoes you are working with a product that will most of the time be consumed in natura. Therefore, great care must be taken when choosing and using chemical products, always respecting withdrawal periods, application intervals and recommended doses.

The tomato

Due to its countless uses in cooking, tomatoes (Lycopersicon esculentum Mill.) is one of the most important vegetables in the world. In 2011, global tomato production was approximately 38 million tons, with the United States being the largest producer, Brazil occupying fifth place with a production of 1,78 million tons. The average global per capita consumption of tomatoes is 5,8 kg/year, and in Brazil this average reaches 6,4 kg/year (Seab-PR).

Flávia de Oliveira Marzarotto, Campos Gerais Agricultural Experimental Station (EEACG)

Article published in issue 79 of Cultivar Hortaliças e Frutas